path: root/lib/mnesia/doc/src/mnesia.xml

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<!DOCTYPE erlref SYSTEM "erlref.dtd">

<erlref>
  <header>
    <copyright>
      <year>1996</year><year>2010</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      The contents of this file are subject to the Erlang Public License,
      Version 1.1, (the "License"); you may not use this file except in
      compliance with the License. You should have received a copy of the
      Erlang Public License along with this software. If not, it can be
      retrieved online at http://www.erlang.org/.

      Software distributed under the License is distributed on an "AS IS"
      basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See
      the License for the specific language governing rights and limitations
      under the License.

    </legalnotice>

    <title>mnesia</title>
    <prepared>Claes Wikstr&ouml;m and H&aring;kan Mattsson</prepared>
    <responsible></responsible>
    <docno></docno>
    <approved></approved>
    <checked></checked>
    <date></date>
    <rev></rev>
    <file></file>
  </header>
  <module>mnesia</module>
  <modulesummary>A Distributed Telecommunications DBMS </modulesummary>
  <description>
    <p><c>Mnesia</c> is a distributed DataBase Management System (DBMS),
      appropriate for telecommunications applications and other Erlang
      applications which require continuous operation and  exhibit soft
      real-time properties.
      </p>
    <p>Listed below are some of the most important and attractive capabilities, Mnesia provides:
      </p>
    <list type="bulleted">
      <item>
        <p>A relational/object hybrid data model which is
          suitable for telecommunications applications.
          </p>
      </item>
      <item>
        <p>A specifically designed  DBMS query language, QLC (as an add-on library). 
          </p>
      </item>
      <item>
        <p>Persistence. Tables may be coherently kept on disc as
          well as in main memory.
          </p>
      </item>
      <item>
        <p>Replication. Tables may be replicated at several nodes.
          </p>
      </item>
      <item>
        <p>Atomic transactions. A series of table manipulation
          operations can be grouped into a single atomic
          transaction.
          </p>
      </item>
      <item>
        <p>Location transparency. Programs can be written without
          knowledge of the actual location of data.
          </p>
      </item>
      <item>
        <p>Extremely fast real time data searches.
          </p>
      </item>
      <item>
        <p>Schema manipulation routines. It is possible to
          reconfigure the DBMS at runtime without stopping the
          system.
          </p>
      </item>
    </list>
    <p>This Reference Manual describes the Mnesia API. This includes
      functions used to define and manipulate Mnesia tables.
      </p>
    <p>All functions documented in these pages can be used in any
      combination with queries using the list comprehension notation. The
      query notation is described in the QLC's man page.
      </p>
    <p>Data in Mnesia is organized as a set of tables.  Each table
      has a name which must be an atom. Each table is made up of
      Erlang records.  The user is responsible for the record
      definitions.  Each table also has a set of properties. Below
      are some of the properties that are associated with each
      table:
      </p>
    <list type="bulleted">
      <item>
        <p><c>type</c>. Each table can either have 'set',
          'ordered_set' or 'bag' semantics. Note: currently 'ordered_set' 
          is not supported for 'disc_only_copies'. If a table is of type
          'set' it means that each key leads to either one or zero
          records.           <br></br>
If a new item is inserted with the same key as
          an existing record, the old record is overwritten. On the
          other hand, if a table is of type 'bag', each key can map to
          several records. However, all records in type bag tables are
          unique, only the keys may be duplicated.
          </p>
      </item>
      <item>
        <p><c>record_name</c>. All records stored in a table must
          have the same name. You may say that the records must be
          instances of the same record type.
          </p>
      </item>
      <item>
        <p><c>ram_copies</c> A table can be replicated on a number
          of Erlang nodes. The <c>ram_copies</c> property specifies a
          list of Erlang nodes where RAM copies are kept.  These
          copies can be dumped to disc at regular intervals.  However,
          updates to these copies are not written to disc on a
          transaction basis.
          </p>
      </item>
      <item>
        <p><c>disc_copies</c> The <c>disc_copies</c> property
          specifies a list of Erlang nodes where the table is kept in
          RAM as well as on disc. All updates of the table are
          performed on the actual table and are also logged to disc.
          If a table is of type <c>disc_copies</c> at a certain node,
          it means that the entire table is resident in RAM memory as
          well as on disc. Each transaction performed on the table is
          appended to a LOG file as well as written into the RAM
          table.
          </p>
      </item>
      <item>
        <p><c>disc_only_copies</c> Some, or all, table replicas
          can be kept on disc only. These replicas are considerably
          slower than the RAM based replicas.
          </p>
      </item>
      <item>
        <p><c>index</c> This is a list of attribute names, or
          integers, which specify the tuple positions on which
          Mnesia shall build and maintain an extra index table.
          </p>
      </item>
      <item>
        <p><c>local_content</c> When an application requires 
          tables whose contents is local to each node,
          <c>local_content</c> tables may be used. The name of the
          table is known to all Mnesia nodes, but its contents is
          unique on each node. This means that access to such a table
          must be done locally.  Set the <c>local_content</c> field to
          <c>true</c> if you want to enable the <c>local_content</c>
          behavior. The default is <c>false</c>.
          </p>
      </item>
      <item>
        <p><c>snmp</c> Each (set based) Mnesia table can be
          automatically turned into an SNMP ordered table as well.
          This property specifies the types of the SNMP keys.
          </p>
      </item>
      <item>
        <p><c>attributes</c>. The names of the attributes for the
          records that are inserted in the table.
          </p>
      </item>
    </list>
    <p>See <c>mnesia:create_table/2</c> about the complete set of
      table properties and their details.
      </p>
    <p>This document uses a table of persons to illustrate various
      examples. The following record definition is assumed:
      </p>
    <code type="none">
-record(person, {name,
                 age = 0,
                 address = unknown,
                 salary = 0,
                 children = []}),
    </code>
    <p>The first attribute of the record is the primary key, or key
      for short.
      </p>
    <p>The function descriptions are sorted in alphabetic order. <em>Hint:</em>
      start to read about <c>mnesia:create_table/2</c>,
      <c>mnesia:lock/2</c> and <c>mnesia:activity/4</c> before you continue on
      and learn about the rest.
      </p>
    <p>Writing or deleting in transaction context creates a local copy
      of each modified record during the transaction. During iteration,
      i.e. <c>mnesia:fold[lr]/4</c> <c>mnesia:next/2</c> <c>mnesia:prev/2</c> 
      <c>mnesia:snmp_get_next_index/2</c>, mnesia will compensate for 
      every written or deleted record, which may reduce the
      performance. If possible avoid writing or deleting records in
      the same transaction before iterating over the table.
    </p>
  </description>
  <funcs>
    <func>
      <name>abort(Reason) -> transaction abort </name>
      <fsummary>Abort the current transaction.</fsummary>
      <desc>
        <p>Makes the transaction silently 
          return the tuple <c>{aborted, Reason}</c>.
          The abortion of a Mnesia transaction means that
          an exception will be thrown to an enclosing <c>catch</c>.
          Thus, the expression <c>catch mnesia:abort(x)</c> does
          not abort the transaction. </p>
      </desc>
    </func>
    <func>
      <name>activate_checkpoint(Args) -> {ok,Name,Nodes} | {error,Reason}</name>
      <fsummary>Activate a checkpoint.</fsummary>
      <desc>
        <p>A checkpoint is a consistent view of the system. 
          A checkpoint can be activated on a set of tables.
          This checkpoint can then be traversed and will
          present a view of the system as it existed at the time when
          the checkpoint was activated,  even if the tables are being or have been
          manipulated. 
          </p>
        <p><c>Args</c> is a list of the following tuples: 
          </p>
        <list type="bulleted">
          <item>
            <p><c>{name,Name}</c>. <c>Name</c> of checkpoint. Each
              checkpoint must have a name which is unique to the
              associated nodes. The name can be reused only once the
              checkpoint has been deactivated. By default, a name
              which is probably unique is generated.
              </p>
          </item>
          <item>
            <p><c>{max,MaxTabs}</c><c>MaxTabs</c> is a list of
              tables that should be included in the checkpoint. The
              default is []. For these tables, the redundancy will be
              maximized and checkpoint information will be retained together
              with all replicas. The checkpoint becomes more fault
              tolerant if the tables have several replicas. When a new
              replica is added by means of the schema manipulation
              function <c>mnesia:add_table_copy/3</c>, a retainer will 
              also be attached automatically.
              </p>
          </item>
          <item>
            <p><c>{min,MinTabs}</c>. <c>MinTabs</c> is a list of
              tables that should be included in the checkpoint. The
              default is []. For these tables, the redundancy will be
              minimized and the checkpoint information will only be retained
              with one replica, preferably on the local node.
              </p>
          </item>
          <item>
            <p><c>{allow_remote,Bool}</c>. <c>false</c> means that
              all retainers must be local. The checkpoint cannot be
              activated if a table does not reside locally.
              <c>true</c> allows retainers to be allocated on any
              node. Default is set to <c>true</c>.
              </p>
          </item>
          <item>
            <p><c>{ram_overrides_dump,Bool} </c> Only applicable
              for <c>ram_copies</c>.  <c>Bool</c> allows you to choose
              to backup the table state as it is in RAM, or as it is on
              disc. <c>true</c> means that the latest committed
              records in RAM should be included in the checkpoint.
              These are the  records that the application accesses.
              <c>false</c> means that the records dumped to DAT files
              should be included in the checkpoint. These are the
              records that will be loaded at startup. Default is
              <c>false</c>.   
              </p>
          </item>
        </list>
        <p>Returns <c>{ok,Name,Nodes}</c> or <c>{error,Reason}</c>.
          <c>Name</c> is the (possibly generated) name of the
          checkpoint. <c>Nodes</c> are the nodes that
          are involved in the checkpoint. Only nodes that keep a
          checkpoint retainer know about the checkpoint.
          </p>
      </desc>
    </func>
    <func>
      <name>activity(AccessContext, Fun [, Args]) -> ResultOfFun | exit(Reason)</name>
      <fsummary>Execute <c>Fun</c>in <c>AccessContext</c>.</fsummary>
      <desc>
        <p>Invokes <c>mnesia:activity(AccessContext, Fun, Args, AccessMod)</c> where <c>AccessMod</c> is the default
          access callback module obtained by
          <c>mnesia:system_info(access_module)</c>. <c>Args</c>
          defaults to the empty list <c>[]</c>.</p>
      </desc>
    </func>
    <func>
      <name>activity(AccessContext, Fun, Args, AccessMod) -> ResultOfFun | exit(Reason)</name>
      <fsummary>Execute <c>Fun</c>in <c>AccessContext</c>.</fsummary>
      <desc>
        <p>This function executes the functional object <c>Fun</c>
          with the arguments <c>Args</c>. 
          </p>
        <p>The code which executes inside the activity can
          consist of a series of table manipulation functions, which is
          performed in a <c>AccessContext</c>. Currently, the following
          access contexts are supported:
          </p>
        <taglist>
          <tag><c>transaction</c></tag>
          <item>
            <p>Short for <c>{transaction, infinity}</c></p>
          </item>
          <tag><c>{transaction, Retries}</c></tag>
          <item>
            <p>Invokes <c>mnesia:transaction(Fun, Args, Retries)</c>. Note that the result from the <c>Fun</c> is
              returned if the transaction was successful (atomic),
              otherwise the function exits with an abort reason.
              </p>
          </item>
          <tag><c>sync_transaction</c></tag>
          <item>
            <p>Short for <c>{sync_transaction, infinity}</c></p>
          </item>
          <tag><c>{sync_transaction, Retries}</c></tag>
          <item>
            <p>Invokes <c>mnesia:sync_transaction(Fun, Args, Retries)</c>. Note that the result from the <c>Fun</c> is
              returned if the transaction was successful (atomic),
              otherwise the function exits with an abort reason.
              </p>
          </item>
          <tag><c>async_dirty</c></tag>
          <item>
            <p>Invokes <c>mnesia:async_dirty(Fun, Args)</c>.
              </p>
          </item>
          <tag><c>sync_dirty</c></tag>
          <item>
            <p>Invokes <c>mnesia:sync_dirty(Fun, Args)</c>.
              </p>
          </item>
          <tag><c>ets</c></tag>
          <item>
            <p>Invokes <c>mnesia:ets(Fun, Args)</c>.
              </p>
          </item>
        </taglist>
        <p>This function (<c>mnesia:activity/4</c>) differs in an
          important aspect from the <c>mnesia:transaction</c>,
          <c>mnesia:sync_transaction</c>,
          <c>mnesia:async_dirty</c>, <c>mnesia:sync_dirty</c> and
          <c>mnesia:ets</c> functions. The <c>AccessMod</c> argument
          is the name of a callback module which implements the
          <c>mnesia_access</c> behavior.
          </p>
        <p>Mnesia will forward calls to the following functions:
          </p>
        <list type="bulleted">
          <item>
            <p>mnesia:lock/2 (read_lock_table/1, write_lock_table/1)</p>
          </item>
          <item>
            <p>mnesia:write/3 (write/1, s_write/1)</p>
          </item>
          <item>
            <p>mnesia:delete/3 (delete/1, s_delete/1)</p>
          </item>
          <item>
            <p>mnesia:delete_object/3 (delete_object/1, s_delete_object/1)</p>
          </item>
          <item>
            <p>mnesia:read/3 (read/1, wread/1)</p>
          </item>
          <item>
            <p>mnesia:match_object/3 (match_object/1)</p>
          </item>
          <item>
            <p>mnesia:all_keys/1</p>
          </item>
          <item>
            <p>mnesia:first/1</p>
          </item>
          <item>
            <p>mnesia:last/1</p>
          </item>
          <item>
            <p>mnesia:prev/2</p>
          </item>
          <item>
            <p>mnesia:next/2</p>
          </item>
          <item>
            <p>mnesia:index_match_object/4 (index_match_object/2)</p>
          </item>
          <item>
            <p>mnesia:index_read/3</p>
          </item>
          <item>
            <p>mnesia:table_info/2</p>
          </item>
        </list>
        <p>to the corresponding:
          </p>
        <list type="bulleted">
          <item>
            <p>AccessMod:lock(ActivityId, Opaque, LockItem, LockKind)</p>
          </item>
          <item>
            <p>AccessMod:write(ActivityId, Opaque, Tab, Rec, LockKind)</p>
          </item>
          <item>
            <p>AccessMod:delete(ActivityId, Opaque, Tab, Key, LockKind)</p>
          </item>
          <item>
            <p>AccessMod:delete_object(ActivityId, Opaque, Tab, RecXS, LockKind)</p>
          </item>
          <item>
            <p>AccessMod:read(ActivityId, Opaque, Tab, Key, LockKind)</p>
          </item>
          <item>
            <p>AccessMod:match_object(ActivityId, Opaque, Tab, Pattern, LockKind)</p>
          </item>
          <item>
            <p>AccessMod:all_keys(ActivityId, Opaque, Tab, LockKind)</p>
          </item>
          <item>
            <p>AccessMod:first(ActivityId, Opaque, Tab)</p>
          </item>
          <item>
            <p>AccessMod:last(ActivityId, Opaque, Tab)</p>
          </item>
          <item>
            <p>AccessMod:prev(ActivityId, Opaque, Tab, Key)</p>
          </item>
          <item>
            <p>AccessMod:next(ActivityId, Opaque, Tab, Key)</p>
          </item>
          <item>
            <p>AccessMod:index_match_object(ActivityId, Opaque, Tab, Pattern, Attr, LockKind)</p>
          </item>
          <item>
            <p>AccessMod:index_read(ActivityId, Opaque, Tab, SecondaryKey, Attr, LockKind)</p>
          </item>
          <item>
            <p>AccessMod:table_info(ActivityId, Opaque, Tab, InfoItem)</p>
          </item>
        </list>
        <p>where <c>ActivityId</c> is a record which represents the
          identity of the enclosing Mnesia activity. The first field
          (obtained with <c>element(1, ActivityId)</c> contains an
          atom which may be interpreted as the type of the activity:
          <c>'ets'</c>, <c>'async_dirty'</c>, <c>'sync_dirty'</c> or
          <c>'tid'</c>. <c>'tid'</c> means that the activity is a
          transaction. The structure of the rest of the identity
          record is internal to Mnesia.
          </p>
        <p><c>Opaque</c> is an opaque data structure which is internal
          to Mnesia.</p>
      </desc>
    </func>
    <func>
      <name>add_table_copy(Tab, Node, Type) -> {aborted, R} | {atomic, ok}</name>
      <fsummary>Copy a table to a remote node.</fsummary>
      <desc>
        <p>This function makes another copy of a table  at the
          node <c>Node</c>. The <c>Type</c> argument must be
          either of the atoms <c>ram_copies</c>, <c>disc_copies</c>,
          or
          <c>disc_only_copies</c>. For example, the following call
          ensures that a disc replica of the <c>person</c> table  also
          exists at node <c>Node</c>.</p>
        <code type="none">
mnesia:add_table_copy(person, Node, disc_copies)
        </code>
        <p>This function can also be used to add a replica of the
          table named <c>schema</c>.</p>
      </desc>
    </func>
    <func>
      <name>add_table_index(Tab, AttrName) -> {aborted, R} | {atomic, ok}</name>
      <fsummary>Create an index for a table. </fsummary>
      <desc>
        <p>Table indices can and should be used whenever the user
          wants to frequently use some other field than the key field
          to look up records. If this other field has an index
          associated with it, these lookups can occur in constant time
          and space.  For example, if our application wishes to use
          the age field of persons to efficiently find all person with
          a specific age, it might be a good idea to have an index on
          the age field. This can be accomplished with the following
          call:</p>
        <code type="none">
mnesia:add_table_index(person, age)
        </code>
        <p>Indices do not come free, they occupy space which is 
          proportional to the size of the table. They also cause insertions
          into the table to execute slightly slower. </p>
      </desc>
    </func>
    <func>
      <name>all_keys(Tab) -> KeyList | transaction abort</name>
      <fsummary>Return all keys in a table.</fsummary>
      <desc>
        <p>This function returns a list of all keys in the table
          named <c>Tab</c>. The semantics of this function is context
          sensitive. See <c>mnesia:activity/4</c> for more information. In
          transaction context it acquires a read lock on the entire
          table.</p>
      </desc>
    </func>
    <func>
      <name>async_dirty(Fun, [, Args])  -> ResultOfFun | exit(Reason)</name>
      <fsummary>Call the Fun in a context which is not protected by a transaction.</fsummary>
      <desc>
        <p>Call the <c>Fun</c> in a context which is not protected
          by a transaction. The Mnesia function calls performed in the
          <c>Fun</c> are mapped to the corresponding dirty
          functions. This still involves logging, replication and
          subscriptions, but there is no locking, local transaction
          storage, or commit protocols involved. Checkpoint retainers
          and indices are updated, but they will be updated dirty. As
          for normal mnesia:dirty_* operations, the operations are
          performed semi-asynchronously. See
          <c>mnesia:activity/4</c> and the Mnesia User's Guide for
          more details.
          </p>
        <p>It is possible to manipulate the Mnesia tables without
          using transactions. This has some serious disadvantages, but
          is considerably faster since the transaction manager is not
          involved and no locks are set. A dirty operation does,
          however, guarantee a certain level of consistency and it is
          not possible for the dirty operations to return garbled
          records. All dirty operations provide location transparency
          to the programmer and a program does not have to be aware of
          the whereabouts of a certain table in order to function. 
          </p>
        <p><em>Note:</em>It is more than 10 times more efficient to read records dirty
          than within a transaction.
          </p>
        <p>Depending on the application, it may be a good idea to use
          the dirty functions for certain operations. Almost all
          Mnesia functions which can be called within transactions
          have a dirty equivalent which is much more
          efficient. However, it must be noted that it is possible for
          the database to be left in an inconsistent state if dirty
          operations are used to update it. Dirty operations should
          only be used for performance reasons when it is absolutely
          necessary.  </p>
        <p><em>Note:</em> Calling (nesting) a <c>mnesia:[a]sync_dirty</c>
          inside a transaction context will inherit the transaction semantics.
          </p>
      </desc>
    </func>
    <func>
      <name>backup(Opaque [, BackupMod]) -> ok | {error,Reason}</name>
      <fsummary>Back up all tables in the database.</fsummary>
      <desc>
        <p>Activates a new checkpoint covering all Mnesia tables,
          including the schema, with maximum degree of redundancy and
          performs a backup using <c>backup_checkpoint/2/3</c>.  The
          default value of the backup callback module <c>BackupMod</c>
          is obtained by <c>mnesia:system_info(backup_module)</c>.</p>
      </desc>
    </func>
    <func>
      <name>backup_checkpoint(Name, Opaque [, BackupMod]) ->  ok | {error,Reason}</name>
      <fsummary>Back up all tables in a checkpoint.</fsummary>
      <desc>
        <p>The tables are backed up to external media using the backup
          module <c>BackupMod</c>. Tables with the local contents
          property is being backed up as they exist on the current
          node.  <c>BackupMod</c> is the default backup callback
          module obtained by
          <c>mnesia:system_info(backup_module)</c>. See the User's
          Guide about the exact callback interface (the
          <c>mnesia_backup behavior</c>).</p>
      </desc>
    </func>
    <func>
      <name>change_config(Config, Value) -> {error, Reason} | {ok, ReturnValue}</name>
      <fsummary>Change a configuration parameter.</fsummary>
      <desc>
        <p>The <c>Config</c> should be an atom of the following 
          configuration parameters: </p>
        <taglist>
          <tag><c>extra_db_nodes</c></tag>
          <item>
            <p><c>Value</c> is a list of nodes which Mnesia should try to connect to.
              The <c>ReturnValue</c> will be those nodes in 
              <c>Value</c> that Mnesia are connected to.
                            <br></br>
Note: This function shall only be used to connect to newly started ram nodes
              (N.D.R.S.N.) with an empty schema. If for example it is used after the network 
              have been partitioned it may lead to inconsistent tables.
                            <br></br>
Note: Mnesia may be connected to other nodes than those
              returned in <c>ReturnValue</c>.</p>
          </item>
          <tag><c>dc_dump_limit</c></tag>
          <item>
            <p><c>Value</c> is a number. See description in 
              <c>Configuration Parameters</c> below.
              The <c>ReturnValue</c> is the new value. Note this configuration parameter
              is not persistent, it will be lost when mnesia stopped.</p>
          </item>
        </taglist>
      </desc>
    </func>
    <func>
      <name>change_table_access_mode(Tab, AccessMode) -> {aborted, R} | {atomic, ok}</name>
      <fsummary>Change the access mode for the table.</fsummary>
      <desc>
        <p>The <c>AcccessMode</c> is by default the atom
          <c>read_write</c> but it may also be set to the atom
          <c>read_only</c>. If the <c>AccessMode</c> is set to
          <c>read_only</c>, it means that it is not possible to perform
          updates to the table. At startup Mnesia always loads
          <c>read_only</c> tables locally regardless of when and if
          Mnesia was terminated on other nodes.</p>
      </desc>
    </func>
    <func>
      <name>change_table_copy_type(Tab, Node, To) -> {aborted, R} | {atomic, ok}</name>
      <fsummary>Change the storage type of a table.</fsummary>
      <desc>
        <p>For example:</p>
        <code type="none">
mnesia:change_table_copy_type(person, node(), disc_copies)
        </code>
        <p>Transforms our <c>person</c> table from a RAM table into
          a disc based table at <c>Node</c>. 
          </p>
        <p>This function can also be used to change the storage type of
          the table named <c>schema</c>. The schema table can only
          have <c>ram_copies</c> or <c>disc_copies</c> as the storage type. If the
          storage type of the schema is <c>ram_copies</c>, no other table
          can be disc resident on that node.</p>
      </desc>
    </func>
    <func>
      <name>change_table_load_order(Tab, LoadOrder) -> {aborted, R} | {atomic, ok}</name>
      <fsummary>Change the load order priority for the table.</fsummary>
      <desc>
        <p>The <c>LoadOrder</c> priority is by default <c>0</c> (zero)
          but may be set to any integer. The tables with the highest
          <c>LoadOrder</c> priority will be loaded first at startup.</p>
      </desc>
    </func>
    <func>
      <name>clear_table(Tab) -> {aborted, R} | {atomic, ok}</name>
      <fsummary>Deletes all entries in a table.</fsummary>
      <desc>
        <p>Deletes all entries in the table <c>Tab</c>.</p>
      </desc>
    </func>
    <func>
      <name>create_schema(DiscNodes) -> ok | {error,Reason}</name>
      <fsummary>Create a brand new schema on the specified nodes.</fsummary>
      <desc>
        <p>Creates a new database on disc. Various files are
          created in the local Mnesia directory of each node. Note
          that the directory must be unique for each node. Two nodes
          may never share the same directory. If possible, use a local
          disc device in order to improve performance.</p>
        <p><c>mnesia:create_schema/1</c> fails if any of the
          Erlang nodes given as <c>DiscNodes</c> are not alive, if
          Mnesia is running on anyone of the nodes, or if anyone of
          the nodes already has a schema.  Use
          <c>mnesia:delete_schema/1</c> to get rid of old faulty
          schemas.
          </p>
        <p><em>Note:</em> Only nodes with disc should be
          included in <c>DiscNodes</c>. Disc-less nodes, that is nodes
          where all tables including the schema only resides in RAM,
          may not be included.</p>
      </desc>
    </func>
    <func>
      <name>create_table(Name, TabDef) -> {atomic, ok} | {aborted, Reason}</name>
      <fsummary>Create a Mnesia table called <c>Name</c>with properties as described by the argument <c>TabDef</c>.</fsummary>
      <desc>
        <p>This function creates a Mnesia table called <c>Name</c>
          according to the
          argument <c>TabDef</c>. This list must be a list of
          <c>{Item, Value}</c> tuples, where the following values are
          allowed:</p>
        <list type="bulleted">
          <item>
            <p><c>{access_mode, Atom}</c>.  The access mode  is by
              default the atom <c>read_write</c> but it may also be
              set to the atom <c>read_only</c>. If the
              <c>AccessMode</c> is set to <c>read_only</c>, it means
              that it is not possible to perform updates to the table.
              </p>
            <p>At startup Mnesia always loads <c>read_only</c> tables
              locally regardless of when and if Mnesia was terminated
              on other nodes. This argument returns the access mode of
              the table. The access mode may either be read_only or
              read_write.
              </p>
          </item>
          <item>
            <p><c>{attributes, AtomList}</c> a list of the
              attribute names for the records that are supposed to
              populate the table. The default value is <c>[key, val]</c>. The table must have at least one extra
              attribute in addition to the  key.
              </p>
            <p>When accessing single attributes in a record, it is not
              necessary, or even recommended, to hard code any
              attribute names as atoms. Use the construct
              <c>record_info(fields, RecordName)</c> instead. It can be
              used for records of type <c>RecordName</c></p>
          </item>
          <item>
            <p><c>{disc_copies, Nodelist}</c>, where
              <c>Nodelist</c> is a list of the nodes where this table
              is supposed to have  disc copies. If a table replica is
              of type <c>disc_copies</c>, all write operations on this
              particular replica of the table are written to disc as
              well as to the RAM copy of the table.
              </p>
            <p>It is possible
              to have a replicated table of type   <c>disc_copies</c>
              on one node, and another type on another node.  The
              default value is <c>[]</c></p>
          </item>
          <item>
            <p><c>{disc_only_copies, Nodelist}</c>, where
              <c>Nodelist</c> is a list of the nodes where this table
              is supposed to  have <c>disc_only_copies</c>. A disc only
              table replica is kept on disc  only and unlike the other
              replica types, the contents of the replica will not
              reside in RAM. These replicas are considerably slower
              than replicas held in RAM.
              </p>
          </item>
          <item>
            <p><c>{index, Intlist}</c>, where
              <c>Intlist</c> is a list of attribute names (atoms) or
              record fields for which Mnesia shall build and maintain
              an extra index table. The <c>qlc</c> query compiler may
              or may not utilize any additional indices while
              processing queries on a table. 
              </p>
          </item>
          <item>
            <p><c>{load_order, Integer}</c>. The load order
              priority is by default <c>0</c> (zero) but may be set to
              any integer. The tables with the highest load order
              priority will be loaded first at startup.
              </p>
          </item>
          <item>
            <p><c>{ram_copies, Nodelist}</c>, where
              <c>Nodelist</c> is a list of the nodes where this table
              is supposed to  have RAM copies. A table replica of type
              <c>ram_copies</c> is obviously not written to disc on a
              per transaction basis. It is possible to  dump
              <c>ram_copies</c> replicas to disc with the function
              <c>mnesia:dump_tables(Tabs)</c>. The default value for
              this attribute is <c>[node()]</c>.
              </p>
          </item>
          <item>
            <p><c>{record_name, Name}</c>, where <c>Name</c> must
              be an atom.  All records, stored in the table, must have
              this name as the first element. It defaults to the same
              name as the name of the table.
              </p>
          </item>
          <item>
            <p><c>{snmp, SnmpStruct}</c>. See
              <c>mnesia:snmp_open_table/2</c> for a description of
              <c>SnmpStruct</c>. If this attribute is present in the
              <c>ArgList</c> to <c>mnesia:create_table/2</c>, the
              table is immediately accessible by means of the Simple
              Network Management  Protocol (SNMP). This means that
              applications which use SNMP to manipulate and control
              the system can be designed easily, since Mnesia provides
              a direct mapping between the logical tables that make up
              an SNMP control application and the physical data which
              makes up a Mnesia table.
              </p>
          </item>
          <item>
            <p><c>{type, Type}</c>, where <c>Type</c> must be
              either of the atoms <c>set</c>, <c>ordered_set</c> or
              <c>bag</c>. The default value is <c>set</c>. In a
              <c>set</c> all records have unique keys and in a
              <c>bag</c> several records may have the same key, but
              the record content is unique. If a non-unique record is
              stored the old, conflicting record(s) will simply be
              overwritten. Note: currently 'ordered_set' 
              is not supported for 'disc_only_copies'.
              </p>
          </item>
          <item>
            <p><c>{local_content, Bool}</c>, where <c>Bool</c> must be
              either <c>true</c> or <c>false</c>. The default value is <c>false</c>.\011      </p>
          </item>
        </list>
        <p>For example, the following call creates the <c>person</c> table
          previously defined and replicates it on 2 nodes: 
          </p>
        <code type="none">
mnesia:create_table(person, 
    [{ram_copies, [N1, N2]},
     {attributes, record_info(fields,person)}]).
        </code>
        <p>If it was required that Mnesia   build and maintain an extra index
          table on the <c>address</c> attribute of all the <c>person</c>
          records that are inserted in the table, the following code would be issued:
          </p>
        <code type="none">
mnesia:create_table(person,
    [{ram_copies, [N1, N2]},
     {index, [address]},
     {attributes, record_info(fields,person)}]).
        </code>
        <p>The specification of <c>index</c> and <c>attributes</c> may be
          hard coded as <c>{index, [2]}</c> and 
          <c>{attributes, [name, age, address, salary, children]}</c> 
          respectively.
          </p>
        <p><c>mnesia:create_table/2</c> writes records into the
          <c>schema</c> table. This function, as well as all other
          schema manipulation functions, are implemented with the
          normal transaction management system. This guarantees that
          schema updates are performed on all nodes in an atomic
          manner.</p>
      </desc>
    </func>
    <func>
      <name>deactivate_checkpoint(Name) -> ok | {error, Reason}</name>
      <fsummary>Deactivate a checkpoint.</fsummary>
      <desc>
        <p>The checkpoint is automatically deactivated when some of
          the tables involved have no retainer attached to them. This may
          happen when nodes go down or when a replica is deleted.
          Checkpoints will also be deactivated with this function.
          <c>Name</c> is the name of an active checkpoint.</p>
      </desc>
    </func>
    <func>
      <name>del_table_copy(Tab, Node) -> {aborted, R} | {atomic, ok}</name>
      <fsummary>Delete the replica of table <c>Tab</c>at node <c>Node</c>.</fsummary>
      <desc>
        <p>Deletes the replica of table <c>Tab</c> at node <c>Node</c>.
          When the last replica is deleted with this
          function, the table disappears entirely.
          </p>
        <p>This function may also be used to delete a replica of
          the table named <c>schema</c>. Then the mnesia node will be removed.
          Note: Mnesia must be stopped on the node first.</p>
      </desc>
    </func>
    <func>
      <name>del_table_index(Tab, AttrName) -> {aborted, R} | {atomic, ok}</name>
      <fsummary>Delete an index in a table. </fsummary>
      <desc>
        <p>This function deletes the index on attribute with name
          <c>AttrName</c> in a table.</p>
      </desc>
    </func>
    <func>
      <name>delete({Tab, Key}) -> transaction abort | ok </name>
      <fsummary>Delete all records in table <c>Tab</c>with the key <c>Key</c>.</fsummary>
      <desc>
        <p>Invokes <c>mnesia:delete(Tab, Key, write)</c></p>
      </desc>
    </func>
    <func>
      <name>delete(Tab, Key, LockKind) -> transaction abort | ok </name>
      <fsummary>Delete all records in table <c>Tab</c>with the key <c>Key</c>.</fsummary>
      <desc>
        <p>Deletes all records in table <c>Tab</c> with the key
          <c>Key</c>.  
          </p>
        <p>The semantics of this function is context sensitive. See
          <c>mnesia:activity/4</c> for more information. In transaction
          context it acquires a lock of type <c>LockKind</c> in the
          record.  Currently the lock types <c>write</c> and
          <c>sticky_write</c> are supported.</p>
      </desc>
    </func>
    <func>
      <name>delete_object(Record) -> transaction abort | ok </name>
      <fsummary>Delete a record</fsummary>
      <desc>
        <p>Invokes <c>mnesia:delete_object(Tab, Record, write)</c> where
          <c>Tab</c> is <c>element(1, Record)</c>.</p>
      </desc>
    </func>
    <func>
      <name>delete_object(Tab, Record, LockKind) -> transaction abort | ok </name>
      <fsummary>Delete a record</fsummary>
      <desc>
        <p>If a table is of type <c>bag</c>, we may sometimes
          want to delete only some of the records with a certain
          key. This can be done with the <c>delete_object/3</c>
          function.  A complete record must be supplied to this
          function.  
          </p>
        <p>The semantics of this function is context sensitive. See
          <c>mnesia:activity/4</c> for more information. In transaction
          context it acquires a lock of type <c>LockKind</c> on the
          record. Currently the lock types <c>write</c> and
          <c>sticky_write</c> are supported.</p>
      </desc>
    </func>
    <func>
      <name>delete_schema(DiscNodes) -> ok | {error,Reason}</name>
      <fsummary>Delete the schema on the given nodes</fsummary>
      <desc>
        <p>Deletes a database created with
          <c>mnesia:create_schema/1</c>.
          <c>mnesia:delete_schema/1</c> fails if any of the Erlang
          nodes given as <c>DiscNodes</c> is not alive, or if Mnesia
          is running on any of the nodes.
          </p>
        <p>After the database has been deleted, it may still be
          possible to start Mnesia as a disc-less node. This depends on
          how the configuration parameter <c>schema_location</c> is set.
          </p>
        <warning>
          <p>This function must be used with extreme
            caution since it makes  existing persistent data
            obsolete. Think twice before using it.  </p>
        </warning>
      </desc>
    </func>
    <func>
      <name>delete_table(Tab) -> {aborted, Reason} | {atomic, ok} </name>
      <fsummary>Delete permanently all replicas of table <c>Tab</c>.</fsummary>
      <desc>
        <p>Permanently deletes all replicas of table <c>Tab</c>.</p>
      </desc>
    </func>
    <func>
      <name>dirty_all_keys(Tab) -> KeyList | exit({aborted, Reason}).</name>
      <fsummary>Dirty search for all record keys in table.</fsummary>
      <desc>
        <p>This is the dirty equivalent of the
          <c>mnesia:all_keys/1</c> function.</p>
      </desc>
    </func>
    <func>
      <name>dirty_delete({Tab, Key}) -> ok | exit({aborted, Reason}) </name>
      <fsummary>Dirty delete of a record.</fsummary>
      <desc>
        <p>Invokes <c>mnesia:dirty_delete(Tab, Key)</c>.</p>
      </desc>
    </func>
    <func>
      <name>dirty_delete(Tab, Key) -> ok | exit({aborted, Reason}) </name>
      <fsummary>Dirty delete of a record. </fsummary>
      <desc>
        <p>This is the dirty equivalent of the 
          <c>mnesia:delete/3</c> function.</p>
      </desc>
    </func>
    <func>
      <name>dirty_delete_object(Record) </name>
      <fsummary>Dirty delete of a record.</fsummary>
      <desc>
        <p>Invokes <c>mnesia:dirty_delete_object(Tab, Record)</c>
          where <c>Tab</c> is <c>element(1, Record)</c>.</p>
      </desc>
    </func>
    <func>
      <name>dirty_delete_object(Tab, Record) </name>
      <fsummary>Dirty delete of a record. </fsummary>
      <desc>
        <p>This is the dirty equivalent of the
          <c>mnesia:delete_object/3</c> function.</p>
      </desc>
    </func>
    <func>
      <name>dirty_first(Tab) ->  Key | exit({aborted, Reason}) </name>
      <fsummary>Return the key for the first record in a table.</fsummary>
      <desc>
        <p>Records in <c>set</c> or <c>bag</c> tables are not ordered. 
          However, there 
          is an ordering of the records which is not known 
          to the user. Accordingly, it is possible to traverse a table by means
          of this function in conjunction with the <c>mnesia:dirty_next/2</c> 
          function.
          </p>
        <p>If there are no records at all in the table, this function
          returns the atom <c>'$end_of_table'</c>. For this reason, it
          is highly undesirable, but not disallowed, to use this atom
          as the key for any user records.</p>
      </desc>
    </func>
    <func>
      <name>dirty_index_match_object(Pattern, Pos)</name>
      <fsummary>Dirty pattern match using index.</fsummary>
      <desc>
        <p>Invokes <c>mnesia:dirty_index_match_object(Tab, Pattern, Pos)</c> where <c>Tab</c> is <c>element(1, Pattern)</c>.</p>
      </desc>
    </func>
    <func>
      <name>dirty_index_match_object(Tab, Pattern, Pos)</name>
      <fsummary>Dirty pattern match using index.</fsummary>
      <desc>
        <p>This is the dirty equivalent of the
          <c>mnesia:index_match_object/4</c> function.</p>
      </desc>
    </func>
    <func>
      <name>dirty_index_read(Tab, SecondaryKey, Pos)</name>
      <fsummary>Dirty read using index.</fsummary>
      <desc>
        <p>This is the dirty equivalent of the
          <c>mnesia:index_read/3</c> function.</p>
      </desc>
    </func>
    <func>
      <name>dirty_last(Tab) ->  Key | exit({aborted, Reason}) </name>
      <fsummary>Return the key for the last record in a table.</fsummary>
      <desc>
        <p>This function works exactly 
          <c>mnesia:dirty_first/1</c> but returns the last object in
          Erlang  term  order for the <c>ordered_set</c> table type. For
          all other table types, <c>mnesia:dirty_first/1</c> and 
          <c>mnesia:dirty_last/1</c> are synonyms.</p>
      </desc>
    </func>
    <func>
      <name>dirty_match_object(Pattern) -> RecordList | exit({aborted, Reason}).</name>
      <fsummary>Dirty pattern match pattern.</fsummary>
      <desc>
        <p>Invokes <c>mnesia:dirty_match_object(Tab, Pattern)</c>
          where <c>Tab</c> is <c>element(1, Pattern)</c>.</p>
      </desc>
    </func>
    <func>
      <name>dirty_match_object(Tab, Pattern) -> RecordList | exit({aborted, Reason}).</name>
      <fsummary>Dirty pattern match pattern.</fsummary>
      <desc>
        <p>This is the dirty equivalent of the
          <c>mnesia:match_object/3</c> function.</p>
      </desc>
    </func>
    <func>
      <name>dirty_next(Tab, Key) -> Key | exit({aborted, Reason}) </name>
      <fsummary>Return the next key in a table. </fsummary>
      <desc>
        <p>This function makes it possible to traverse a table
          and perform operations on all records in the table. When
          the end of the table is reached, the special key
          <c>'$end_of_table'</c> is returned. Otherwise, the function
          returns a key which can be used to read the actual record.The
          behavior is undefined if another Erlang process performs write
          operations on the table while it is being traversed with the
          <c>mnesia:dirty_next/2</c> function.</p>
      </desc>
    </func>
    <func>
      <name>dirty_prev(Tab, Key) -> Key | exit({aborted, Reason}) </name>
      <fsummary>Return the previous key in a table. </fsummary>
      <desc>
        <p>This function works exactly 
          <c>mnesia:dirty_next/2</c> but returns the previous object in
          Erlang term  order for the ordered_set table type. For
          all other table types, <c>mnesia:dirty_next/2</c> and 
          <c>mnesia:dirty_prev/2</c> are synonyms.\011  </p>
      </desc>
    </func>
    <func>
      <name>dirty_read({Tab, Key}) -> ValueList | exit({aborted, Reason}</name>
      <fsummary>Dirty read of records.</fsummary>
      <desc>
        <p>Invokes <c>mnesia:dirty_read(Tab, Key)</c>.</p>
      </desc>
    </func>
    <func>
      <name>dirty_read(Tab, Key) -> ValueList | exit({aborted, Reason}</name>
      <fsummary>Dirty read of records.</fsummary>
      <desc>
        <p>This is the dirty equivalent of the
          <c>mnesia:read/3</c> function.</p>
      </desc>
    </func>
    <func>
      <name>dirty_select(Tab, MatchSpec) -> ValueList | exit({aborted, Reason}</name>
      <fsummary>Dirty match the objects in <c>Tab</c>against <c>MatchSpec</c>.</fsummary>
      <desc>
        <p>This is the dirty equivalent of the
          <c>mnesia:select/2</c> function.</p>
      </desc>
    </func>
    <func>
      <name>dirty_slot(Tab, Slot) -> RecordList | exit({aborted, Reason})</name>
      <fsummary>Return the list of records that are associated with  Slot in a table.</fsummary>
      <desc>
        <p>This function can be used to traverse a table in a
          manner similar to the <c>mnesia:dirty_next/2</c> function.
          A table has a number of slots which range from 0 (zero) to some
          unknown upper bound.  The function
          <c>mnesia:dirty_slot/2</c> returns the special atom
          <c>'$end_of_table'</c> when the end of the table is reached.
          The behavior of this function is undefined if a write
          operation is performed on the table while it is being
          traversed.</p>
      </desc>
    </func>
    <func>
      <name>dirty_update_counter({Tab, Key}, Incr) -> NewVal | exit({aborted, Reason})</name>
      <fsummary>Dirty update of a counter record.</fsummary>
      <desc>
        <p>Invokes <c>mnesia:dirty_update_counter(Tab, Key, Incr)</c>.</p>
      </desc>
    </func>
    <func>
      <name>dirty_update_counter(Tab, Key, Incr) -> NewVal | exit({aborted, Reason})</name>
      <fsummary>Dirty update of a counter record.</fsummary>
      <desc>
        <p>There are no special counter records in Mnesia. However,
          records of the form <c>{Tab, Key, Integer}</c> can be used
          as (possibly disc resident) counters, when <c>Tab</c> is a
          <c>set</c>. This function updates a counter with a
          positive or negative number. However, counters can never become less
          than zero. There are two significant differences between
          this function and the action of first reading the record,
          performing the arithmetics, and then writing the record:</p>
        <list type="bulleted">
          <item>It is much more efficient</item>
          <item><c>mnesia:dirty_update_counter/3</c> is
           performed as an atomic operation despite the fact that it is not
           protected by a transaction.</item>
        </list>
        <p>If two processes perform <c>mnesia:dirty_update_counter/3</c>
          simultaneously, both updates will take effect without the
          risk of losing one of the updates. The new value
          <c>NewVal</c> of the counter is returned.</p>
        <p>If <c>Key</c> don't exits, a new record is created with the value
          <c>Incr</c> if it is larger than 0, otherwise it is set to 0.</p>
      </desc>
    </func>
    <func>
      <name>dirty_write(Record) -> ok | exit({aborted, Reason})</name>
      <fsummary>Dirty write of a record.</fsummary>
      <desc>
        <p>Invokes <c>mnesia:dirty_write(Tab, Record)</c>
          where <c>Tab</c> is <c>element(1, Record)</c>.</p>
      </desc>
    </func>
    <func>
      <name>dirty_write(Tab, Record) -> ok | exit({aborted, Reason})</name>
      <fsummary>Dirty write of a record.</fsummary>
      <desc>
        <p>This is the dirty equivalent of <c>mnesia:write/3</c>.</p>
      </desc>
    </func>
    <func>
      <name>dump_log() -> dumped</name>
      <fsummary>Perform a user initiated dump of the local log file.</fsummary>
      <desc>
        <p>Performs a user initiated dump of the local log file.
          This is usually not necessary since Mnesia, by default,
          manages this automatically.</p>
      </desc>
    </func>
    <func>
      <name>dump_tables(TabList) -> {atomic, ok} | {aborted, Reason}</name>
      <fsummary>Dump all RAM tables to disc.</fsummary>
      <desc>
        <p>This function dumps a set of <c>ram_copies</c> tables
          to disc. The next time the system is started, these tables
          are initiated with the data found in the files that are the
          result of this dump. None of the tables may have disc
          resident replicas.</p>
      </desc>
    </func>
    <func>
      <name>dump_to_textfile(Filename) </name>
      <fsummary>Dump local tables into a text file.</fsummary>
      <desc>
        <p>Dumps all local tables of a mnesia system into a text file
          which can then be edited (by means of a normal text editor)
          and then later be reloaded with
          <c>mnesia:load_textfile/1</c>. Only use this function for
          educational purposes. Use other functions to deal with real
          backups.</p>
      </desc>
    </func>
    <func>
      <name>error_description(Error) -> String </name>
      <fsummary>Return a string describing a particular Mnesia error.</fsummary>
      <desc>
        <p>All Mnesia transactions, including all the schema
          update functions, either return the value <c>{atomic, Val}</c> or the tuple <c>{aborted, Reason}</c>. The
          <c>Reason</c> can be either of the following atoms. The
          <c>error_description/1</c> function returns a descriptive
          string which describes the error.
          </p>
        <list type="bulleted">
          <item>
            <p><c>nested_transaction</c>. Nested transactions are
              not allowed in this context.
              </p>
          </item>
          <item>
            <p><c>badarg</c>. Bad or invalid argument, possibly
              bad type.
              </p>
          </item>
          <item>
            <p><c>no_transaction</c>. Operation not allowed
              outside transactions.
              </p>
          </item>
          <item>
            <p><c>combine_error</c>. Table options  were illegally
              combined.
              </p>
          </item>
          <item>
            <p><c>bad_index</c>. Index already exists or was out
              of bounds.
              </p>
          </item>
          <item>
            <p><c>already_exists</c>. Schema option is already set.
              </p>
          </item>
          <item>
            <p><c>index_exists</c>. Some operations cannot  be performed on 
              tabs with index.
              </p>
          </item>
          <item>
            <p><c>no_exists</c>. Tried to perform operation on
              non-existing, or not alive, item.
              </p>
          </item>
          <item>
            <p><c>system_limit</c>. Some system_limit was exhausted.
              </p>
          </item>
          <item>
            <p><c>mnesia_down</c>.  A transaction involving
              records at some remote  node which died while
              transaction was executing. Record(s) are no longer
              available elsewhere in the network.
              </p>
          </item>
          <item>
            <p><c>not_a_db_node</c>. A node which does not exist
              in  the schema was mentioned.
              </p>
          </item>
          <item>
            <p><c>bad_type</c>. Bad type on some arguments.
              </p>
          </item>
          <item>
            <p><c>node_not_running</c>. Node not running.
              </p>
          </item>
          <item>
            <p><c>truncated_binary_file</c>. Truncated binary in file.
              </p>
          </item>
          <item>
            <p><c>active</c>. Some delete operations require that 
              all active records are removed.
              </p>
          </item>
          <item>
            <p><c>illegal</c>. Operation not supported on record.
              </p>
          </item>
        </list>
        <p>The <c>Error</c> may be <c>Reason</c>,
          <c>{error, Reason}</c>, or <c>{aborted, Reason}</c>. The
          <c>Reason</c> may be an atom or a tuple with <c>Reason</c>
          as an atom in the first field.</p>
      </desc>
    </func>
    <func>
      <name>ets(Fun, [, Args]) -> ResultOfFun | exit(Reason)</name>
      <fsummary>Call the Fun in a raw context which is not protected by a transaction.</fsummary>
      <desc>
        <p>Call the <c>Fun</c> in a raw context which is not protected by
          a transaction. The Mnesia function call is performed in the
          <c>Fun</c> are performed directly on the local <c>ets</c> tables on
          the assumption that the local storage type is
          <c>ram_copies</c> and the tables are not replicated to other
          nodes. Subscriptions are not triggered and checkpoints are
          not updated, but it is extremely fast.  This function can
          also be applied to <c>disc_copies</c> tables if all
          operations are read only.  See <c>mnesia:activity/4</c>
          and the Mnesia User's Guide for more details.</p>
        <p><em>Note:</em> Calling (nesting) a <c>mnesia:ets</c>
          inside a transaction context will inherit the transaction semantics.</p>
      </desc>
    </func>
    <func>
      <name>first(Tab) ->  Key | transaction abort </name>
      <fsummary>Return the key for the first record in a table.</fsummary>
      <desc>
        <p>Records in <c>set</c> or <c>bag</c> tables are not ordered. 
          However, there 
          is an ordering of the records which is not known 
          to the user. Accordingly, it is possible to traverse a table by means
          of this function in conjunction with the <c>mnesia:next/2</c> 
          function.
          </p>
        <p>If there are no records at all in the table, this function
          returns the atom <c>'$end_of_table'</c>. For this reason, it
          is highly undesirable, but not disallowed, to use this atom
          as the key for any user records.</p>
      </desc>
    </func>
    <func>
      <name>foldl(Function, Acc, Table) -> NewAcc | transaction abort </name>
      <fsummary>Call Function for each record in Table </fsummary>
      <desc>
        <p>Iterates over the table <c>Table</c> and calls 
          <c>Function(Record, NewAcc)</c> for each <c>Record</c> in the table.
          The term returned from <c>Function</c> will be used as the second 
          argument in the next call to the <c>Function</c>. 
          </p>
        <p><c>foldl</c> returns the same term as the last call to 
          <c>Function</c> returned.</p>
      </desc>
    </func>
    <func>
      <name>foldr(Function, Acc, Table) -> NewAcc | transaction abort </name>
      <fsummary>Call Function for each record in Table  </fsummary>
      <desc>
        <p>This function works exactly as 
          <c>foldl/3</c> but iterates the table in the opposite order 
          for the <c>ordered_set</c> table type. For
          all other table types, <c>foldr/3</c> and 
          <c>foldl/3</c> are synonyms.</p>
      </desc>
    </func>
    <func>
      <name>force_load_table(Tab) -> yes | ErrorDescription </name>
      <fsummary>Force a table to be loaded into the system </fsummary>
      <desc>
        <p>The Mnesia algorithm for table load might lead to a
          situation where a table cannot be loaded. This situation
          occurs when a node is started and Mnesia concludes, or
          suspects, that another copy of the table was active after
          this local copy became inactive due to a system crash.
          </p>
        <p>If this situation is not acceptable, this function can be
          used to override the strategy of the Mnesia table load
          algorithm. This could lead to a situation where some
          transaction effects are lost with a inconsistent database as
          result, but for some applications high availability is more
          important than consistent data.</p>
      </desc>
    </func>
    <func>
      <name>index_match_object(Pattern, Pos) -> transaction abort | ObjList</name>
      <fsummary>Match records and utilizes index information.</fsummary>
      <desc>
        <p>Invokes <c>mnesia:index_match_object(Tab, Pattern, Pos, read)</c> where <c>Tab</c> is <c>element(1, Pattern)</c>.</p>
      </desc>
    </func>
    <func>
      <name>index_match_object(Tab, Pattern, Pos, LockKind) -> transaction abort | ObjList</name>
      <fsummary>Match records and utilizes index information.</fsummary>
      <desc>
        <p>In a manner similar to the <c>mnesia:index_read/3</c>
          function, we can also utilize any index information when we
          try to match records. This function takes a pattern which
          obeys the same rules as the <c>mnesia:match_object/3</c>
          function with the exception that this function requires the
          following conditions:
          </p>
        <list type="bulleted">
          <item>
            <p>The table <c>Tab</c> must have an index on
              position <c>Pos</c>.
              </p>
          </item>
          <item>
            <p>The element in position <c>Pos</c> in
              <c>Pattern</c> must be bound. <c>Pos</c> may either be
              an integer (#record.Field), or an attribute name.</p>
          </item>
        </list>
        <p>The two index search functions described here are
          automatically invoked when searching tables with <c>qlc</c>
          list comprehensions and also when using the low level
          <c>mnesia:[dirty_]match_object</c> functions.
          </p>
        <p></p>
        <p>The semantics of this function is context sensitive. See
          <c>mnesia:activity/4</c> for more information. In transaction
          context it acquires a lock of type <c>LockKind</c> on the
          entire table or on a single record. Currently, the lock type
          <c>read</c> is supported.
          </p>
      </desc>
    </func>
    <func>
      <name>index_read(Tab, SecondaryKey, Pos) -> transaction abort | RecordList </name>
      <fsummary>Read records via index table. </fsummary>
      <desc>
        <p>Assume there is an index on position <c>Pos</c> for a
          certain record type. This function can be used to read the
          records without knowing the actual key for the record. For
          example, with an index in position 1 of the <c>person</c>
          table, the call <c>mnesia:index_read(person, 36, #person.age)</c> returns a list of all persons with age
          equal to 36.  <c>Pos</c> may also be an attribute name
          (atom), but if the notation <c>mnesia:index_read(person, 36, age)</c> is used, the field position will be searched for in
          runtime, for each call.
          </p>
        <p>The semantics of this function is context sensitive. See
          <c>mnesia:activity/4</c> for more information. In transaction
          context it acquires a read lock on the entire table.</p>
      </desc>
    </func>
    <func>
      <name>info() -> ok </name>
      <fsummary>Print some information about the system on the tty.</fsummary>
      <desc>
        <p>Prints some information about the system on the tty.
          This function may be used even if Mnesia is not started.
          However, more information will be displayed if Mnesia is
          started.</p>
      </desc>
    </func>
    <func>
      <name>install_fallback(Opaque) -> ok | {error,Reason}</name>
      <fsummary>Install a backup as fallback.</fsummary>
      <desc>
        <p>Invokes <c>mnesia:install_fallback(Opaque, Args)</c> where
          <c>Args</c> is <c>[{scope, global}]</c>.</p>
      </desc>
    </func>
    <func>
      <name>install_fallback(Opaque), BackupMod) -> ok | {error,Reason}</name>
      <fsummary>Install a backup as fallback.</fsummary>
      <desc>
        <p>Invokes <c>mnesia:install_fallback(Opaque, Args)</c> where
          <c>Args</c> is <c>[{scope, global}, {module, BackupMod}]</c>.</p>
      </desc>
    </func>
    <func>
      <name>install_fallback(Opaque, Args) -> ok | {error,Reason}</name>
      <fsummary>Install a backup as fallback.</fsummary>
      <desc>
        <p>This function is used to install a backup as fallback.  The
          fallback will be used to restore the database at the next
          start-up. Installation of fallbacks requires Erlang to be up
          and running on all the involved nodes, but it does not
          matter if Mnesia is running or not. The installation of the
          fallback will fail if the local node is not one of the disc
          resident nodes in the backup.
          </p>
        <p><c>Args</c> is a list of the following tuples: 
          </p>
        <list type="bulleted">
          <item>
            <p><c>{module, BackupMod}</c>. 
              All accesses of the backup media is performed via a
              callback module named <c>BackupMod</c>. The
              <c>Opaque</c> argument is forwarded to the callback
              module which may interpret it as it wish. The default
              callback module is called <c>mnesia_backup</c> and it
              interprets the <c>Opaque</c> argument as a local
              filename. The default for this module is also
              configurable via the <c>-mnesia mnesia_backup</c>
              configuration parameter.  </p>
          </item>
          <item>
            <p><c>{scope, Scope}</c>
              The <c>Scope</c> of a fallback may either be
              <c>global</c> for the entire database or <c>local</c>
              for one node. By default, the installation of a fallback
              is a global operation which either is performed all
              nodes with disc resident schema or none. Which nodes
              that are disc resident or not, is determined from the
              schema info in the backup.</p>
            <p>If the <c>Scope</c> of the operation is <c>local</c>
              the fallback will only be installed on the local node.</p>
          </item>
          <item>
            <p><c>{mnesia_dir, AlternateDir}</c> 
              This argument is only valid if the scope of the
              installation is <c>local</c>. Normally the installation
              of a fallback is targeted towards the Mnesia directory
              as configured with the <c>-mnesia dir</c> configuration
              parameter. But by explicitly supplying an
              <c>AlternateDir</c> the fallback will be installed there
              regardless of the Mnesia directory configuration
              parameter setting. After installation of a fallback on
              an alternate Mnesia directory that directory is fully
              prepared for usage as an active Mnesia directory.
              </p>
            <p>This is a somewhat dangerous feature which must be
              used with care. By unintentional mixing of directories
              you may easily end up with a inconsistent database, if
              the same backup is installed on more than one directory.</p>
          </item>
        </list>
      </desc>
    </func>
    <func>
      <name>is_transaction() ->  boolean </name>
      <fsummary>Check if code is running in a transaction.</fsummary>
      <desc>
        <p>When this function is executed inside a transaction context
	  it returns <c>true</c>, otherwise <c>false</c>.</p>
      </desc>
    </func>
    <func>
      <name>last(Tab) ->  Key | transaction abort </name>
      <fsummary>Return the key for the last record in a table.</fsummary>
      <desc>
        <p>This function works exactly 
          <c>mnesia:first/1</c> but returns the last object in
          Erlang  term  order for the <c>ordered_set</c> table type. For
          all other table types, <c>mnesia:first/1</c> and 
          <c>mnesia:last/1</c> are synonyms.</p>
      </desc>
    </func>
    <func>
      <name>load_textfile(Filename)</name>
      <fsummary>Load tables from a text file.</fsummary>
      <desc>
        <p>Loads a series of definitions and data found in the
          text file (generated with <c>mnesia:dump_to_textfile/1</c>)
          into Mnesia. This function also starts Mnesia and possibly
          creates a new schema. This function is intended for
          educational purposes only and using other functions to deal
          with real backups, is recommended.</p>
      </desc>
    </func>
    <func>
      <name>lock(LockItem, LockKind) -> Nodes | ok | transaction abort</name>
      <fsummary>Explicit grab lock.</fsummary>
      <desc>
        <p>Write locks are normally acquired on all nodes where a
          replica of the table resides (and is active). Read locks are
          acquired on one node (the local node if  a local
          replica exists). Most of the context sensitive access functions
          acquire an implicit lock if they are invoked in a
          transaction context. The granularity of a lock may either
          be a single record or an entire table.
	</p>
	<p>The normal usage is to call the function without checking
	  the return value since it exits if it fails and the
	  transaction is restarted by the transaction manager.  It
	  returns all the locked nodes if a write lock is acquired, and
	  <c>ok</c> if it was a read lock.
	</p>
        <p>This function <c>mnesia:lock/2</c> is intended to support
          explicit locking on tables but also intended for situations
          when locks need to be acquired regardless of how tables are
          replicated. Currently, two <c>LockKind</c>'s are supported:
          </p>
        <taglist>
          <tag><c>write</c></tag>
          <item>
            <p>Write locks are exclusive, which means that if one
              transaction manages to acquire a write lock on an item,
              no other transaction may acquire any kind of lock on the
              same item. 
              </p>
          </item>
          <tag><c>read</c></tag>
          <item>
            <p>Read locks may be shared, which means that if one
              transaction manages to acquire a read lock on an item,
              other transactions may also acquire a read lock on the
              same item. However, if someone has a read lock no one can
              acquire a write lock at the same item. If some one has a
              write lock no one can acquire  a read lock nor
              a write lock at the same item.</p>
          </item>
        </taglist>
        <p>Conflicting lock requests are automatically queued if there
          is no risk of a deadlock. Otherwise the transaction must be
          aborted and executed again. Mnesia does this automatically
          as long as the upper limit of maximum <c>retries</c> is not
          reached. See <c>mnesia:transaction/3</c> for the details.
          </p>
        <p>For the sake of completeness sticky write locks will also 
          be described here even if a sticky write lock is not
          supported by this particular function:
          </p>
        <taglist>
          <tag><c>sticky_write</c></tag>
          <item>
            <p>Sticky write locks are a mechanism which can be used
              to optimize write lock acquisition. If your application
              uses replicated tables mainly for fault tolerance (as
              opposed to read access optimization purpose), sticky
              locks may be the best option available. 
              </p>
            <p>When a sticky write lock is acquired, all nodes will be
              informed which node is locked. Subsequently,
              sticky lock requests from the same node will be
              performed as  a local operation without any
              communication with other nodes. The sticky lock
              lingers on the node even after the transaction has
              ended. See the Mnesia User's Guide for more information.</p>
          </item>
        </taglist>
        <p>Currently, two kinds of <c>LockItem</c>'s are supported by
          this function:
          </p>
        <taglist>
          <tag><c>{table, Tab}</c></tag>
          <item>
            <p>This acquires a lock of type <c>LockKind</c> on the
              entire table <c>Tab</c>.
              </p>
          </item>
          <tag><c>{global, GlobalKey, Nodes}</c></tag>
          <item>
            <p>This acquires a lock of type <c>LockKind</c> on the
              global resource <c>GlobalKey</c>. The lock is acquired
              on all active nodes in the <c>Nodes</c> list. </p>
          </item>
        </taglist>
        <p>Locks are released when the outermost transaction ends.
          </p>
        <p>The semantics of this function is context sensitive. See
          <c>mnesia:activity/4</c> for more information. In transaction
          context it acquires locks otherwise it just ignores the
          request.</p>
      </desc>
    </func>
    <func>
      <name>match_object(Pattern) ->transaction abort | RecList </name>
      <fsummary>Match <c>Pattern</c>for records.   </fsummary>
      <desc>
        <p>Invokes <c>mnesia:match_object(Tab, Pattern, read)</c> where
          <c>Tab</c> is <c>element(1, Pattern)</c>.</p>
      </desc>
    </func>
    <func>
      <name>match_object(Tab, Pattern, LockKind) ->transaction abort | RecList </name>
      <fsummary>Match <c>Pattern</c>for records. </fsummary>
      <desc>
        <p>This function takes a pattern with 'don't care' variables
          denoted as a '_' parameter.  This function returns a list of
          records which matched the pattern.  Since the second element
          of a record in a table is considered to be the key for the
          record, the performance of this function depends on whether
          this key is bound or not. 
          </p>
        <p>For example, the call <c>mnesia:match_object(person, {person, '_', 36, '_', '_'}, read)</c> returns a list of all person records with an 
          age field of thirty-six (36).
          </p>
        <p>The function <c>mnesia:match_object/3</c>
          automatically uses indices if these exist. However, no
          heuristics are performed in order to select the best
          index.
          </p>
        <p>The semantics of this function is context sensitive. See
          <c>mnesia:activity/4</c> for more information. In transaction
          context it acquires a lock of type <c>LockKind</c> on the
          entire table or a single record. Currently, the lock type
          <c>read</c> is supported.</p>
      </desc>
    </func>
    <func>
      <name>move_table_copy(Tab, From, To) -> {aborted, Reason} | {atomic, ok}</name>
      <fsummary>Move the copy of table <c>Tab</c>from node<c>From</c>to node <c>To</c>.</fsummary>
      <desc>
        <p>Moves the copy of table <c>Tab</c> from node
          <c>From</c> to node <c>To</c>.
          </p>
        <p>The storage type is preserved. For example, a RAM table
          moved from one node remains a RAM on the new node.  It is
          still possible for other transactions to read and write in
          the table while it is being moved.
          </p>
        <p>This function cannot be used on <c>local_content</c> tables.</p>
      </desc>
    </func>
    <func>
      <name>next(Tab, Key) -> Key | transaction abort </name>
      <fsummary>Return the next key in a table. </fsummary>
      <desc>
        <p>This function makes it possible to traverse a table
          and perform operations on all records in the table. When
          the end of the table is reached, the special key
          <c>'$end_of_table'</c> is returned. Otherwise, the function
          returns a key which can be used to read the actual record.</p>
      </desc>
    </func>
    <func>
      <name>prev(Tab, Key) -> Key | transaction abort </name>
      <fsummary>Return the previous key in a table. </fsummary>
      <desc>
        <p>This function works exactly 
          <c>mnesia:next/2</c> but returns the previous object in
          Erlang term  order for the ordered_set table type. For
          all other table types, <c>mnesia:next/2</c> and 
          <c>mnesia:prev/2</c> are synonyms.\011  </p>
      </desc>
    </func>
    <func>
      <name>read({Tab, Key}) -> transaction abort | RecordList </name>
      <fsummary>Read records(s) with a given key. </fsummary>
      <desc>
        <p>Invokes <c>mnesia:read(Tab, Key, read)</c>.</p>
      </desc>
    </func>
    <func>
      <name>read(Tab, Key) -> transaction abort | RecordList </name>
      <fsummary>Read records(s) with a given key. </fsummary>
      <desc>
        <p>Invokes <c>mnesia:read(Tab, Key, read)</c>.</p>
      </desc>
    </func>
    <func>
      <name>read(Tab, Key, LockKind) -> transaction abort | RecordList </name>
      <fsummary>Read records(s) with a given key. </fsummary>
      <desc>
        <p>This function reads all records from table <c>Tab</c> with
          key <c>Key</c>. This function has the same semantics
          regardless of the location of <c>Tab</c>. If the table is
          of type <c>bag</c>, the <c>mnesia:read(Tab, Key)</c> can
          return an arbitrarily long list. If the table is of type
          <c>set</c>, the list is either of length 1, or <c>[]</c>.
          </p>
        <p>The semantics of this function is context sensitive. See
          <c>mnesia:activity/4</c> for more information. In transaction
          context it acquires a lock of type
          <c>LockKind</c>. Currently, the lock types <c>read</c>,
          <c>write</c> and <c>sticky_write</c> are supported.
          </p>
        <p>If the user wants to update the record it is more efficient to 
          use  <c>write/sticky_write</c> as the LockKind.
          </p>
      </desc>
    </func>
    <func>
      <name>read_lock_table(Tab) -> ok | transaction abort</name>
      <fsummary>Set a read lock on an entire table.</fsummary>
      <desc>
        <p>Invokes <c>mnesia:lock({table, Tab}, read)</c>.</p>
      </desc>
    </func>
    <func>
      <name>report_event(Event) -> ok</name>
      <fsummary>Report a user event to Mnesia's event handler.</fsummary>
      <desc>
        <p>When tracing a system of Mnesia applications it is useful
          to be able to interleave Mnesia's own events with
          application related events that give information about the
          application context. 
          </p>
        <p>Whenever the application begins a
          new and demanding Mnesia task, or if it is entering a new
          interesting phase in its execution, it may be a good idea to
          use <c>mnesia:report_event/1</c>. The <c>Event</c> may be
          any term and generates a <c>{mnesia_user, Event}</c> event
          for any processes that  subscribe to Mnesia system
          events.</p>
      </desc>
    </func>
    <func>
      <name>restore(Opaque, Args) -> {atomic, RestoredTabs} |{aborted, Reason}</name>
      <fsummary>Online restore of backup.</fsummary>
      <desc>
        <p>With this function, tables may be restored online from a
          backup without restarting Mnesia.  <c>Opaque</c> is forwarded
          to the backup module.  <c>Args</c> is a list of the following
          tuples:
          </p>
        <list type="bulleted">
          <item>
            <p><c>{module,BackupMod}</c> The backup module
              <c>BackupMod</c> will be used to access the backup
              media. If omitted, the default backup module will be
              used. 
              </p>
          </item>
          <item><c>{skip_tables, TabList}</c> Where <c>TabList</c>
           is a list of tables which should not be read from the
           backup.
          </item>
          <item><c>{clear_tables, TabList}</c> Where
          <c>TabList</c> is a list of tables which should be
           cleared, before the records from the backup are inserted,
           ie. all records in the tables are deleted before the
           tables are restored.  Schema information about the tables
           is not cleared or read from backup.
          </item>
          <item><c>{keep_tables, TabList}</c> Where <c>TabList</c>
           is a list of tables which should be not be cleared, before
           the records from the backup are inserted, i.e. the records
           in the backup will be added to the records in the table.
           Schema information about the tables is not cleared or read
           from backup.
          </item>
          <item><c>{recreate_tables, TabList}</c> Where
          <c>TabList</c> is a list of tables which should be
           re-created, before the records from the backup are
           inserted. The tables are first deleted and then created with
           the schema information from the backup. All the nodes in the
           backup needs to be up and running.
          </item>
          <item><c>{default_op, Operation}</c> Where <c>Operation</c> is
           one of the following operations <c>skip_tables</c>,
          <c>clear_tables</c>, <c>keep_tables</c> or
          <c>recreate_tables</c>. The default operation specifies
           which operation should be used on tables from the backup
           which are not specified in any of the lists above.  If
           omitted, the operation <c>clear_tables</c> will be used.
          </item>
        </list>
        <p>The affected tables are write locked during the
          restoration, but regardless of the lock conflicts caused by
          this, the applications can continue to do their work while
          the restoration is being performed. The restoration is
          performed as one single transaction. 
          </p>
        <p>If the database is
          huge, it may not be possible to restore it online. In such 
          cases, the old database must be restored by installing a
          fallback and then restart.</p>
      </desc>
    </func>
    <func>
      <name>s_delete({Tab, Key}) -> ok | transaction abort </name>
      <fsummary>Set sticky lock and delete records.</fsummary>
      <desc>
        <p>Invokes <c>mnesia:delete(Tab, Key, sticky_write)</c></p>
      </desc>
    </func>
    <func>
      <name>s_delete_object(Record) -> ok | transaction abort </name>
      <fsummary>Set sticky lock and delete record.</fsummary>
      <desc>
        <p>Invokes <c>mnesia:delete_object(Tab, Record, sticky_write)</c> where <c>Tab</c> is <c>element(1, Record)</c>.</p>
      </desc>
    </func>
    <func>
      <name>s_write(Record) -> ok | transaction abort </name>
      <fsummary>Write <c>Record</c>and sets stick lock.</fsummary>
      <desc>
        <p>Invokes <c>mnesia:write(Tab, Record, sticky_write)</c>
          where <c>Tab</c> is <c>element(1, Record)</c>.</p>
      </desc>
    </func>
    <func>
      <name>schema() -> ok </name>
      <fsummary>Print information about all table definitions on the tty. </fsummary>
      <desc>
        <p>Prints information about all table definitions on the tty.</p>
      </desc>
    </func>
    <func>
      <name>schema(Tab) -> ok </name>
      <fsummary>Print information about one table definition on the tty.</fsummary>
      <desc>
        <p>Prints information about one table definition on the tty.</p>
      </desc>
    </func>
    <func>
      <name>select(Tab, MatchSpec [, Lock]) ->  transaction abort | [Object] </name>
      <fsummary>Match the objects in <c>Tab</c>against <c>MatchSpec</c>.</fsummary>
      <desc>
        <p>Matches the objects in the table <c>Tab</c> using a
          match_spec as described in the ERTS Users Guide. Optionally a lock
          <c>read</c> or <c>write</c> can be given as the third
          argument, default is <c>read</c>.  The return value depends
          on the <c>MatchSpec</c>.</p>
        <p><em>Note:</em> for best performance <c>select</c> should
          be used before any modifying operations are done on that table
          in the same transaction, i.e. don't use <c>write</c> or <c>delete</c>
          before a <c>select</c>.</p>
        <p>In its simplest forms the match_spec's look like this:</p>
        <list type="bulleted">
          <item>MatchSpec = [MatchFunction]</item>
          <item>MatchFunction = {MatchHead, [Guard], [Result]}</item>
          <item>MatchHead = tuple() | record()</item>
          <item>Guard =  {"Guardtest name", ...}</item>
          <item>Result = "Term construct"</item>
        </list>
        <p>See the ERTS Users Guide and <c>ets</c> documentation for a
          complete description of the select.</p>
        <p>For example to find the names of all male persons with an age over 30 in table
          Tab do:</p>
        <code type="none">
\011  MatchHead = #person{name='$1', sex=male, age='$2', _='_'},
\011  Guard = {'>', '$2', 30},
\011  Result = '$1',
\011  mnesia:select(Tab,[{MatchHead, [Guard], [Result]}]),
        </code>
      </desc>
    </func>
    <func>
      <name>select(Tab, MatchSpec, NObjects, Lock) ->  transaction abort | {[Object],Cont} | '$end_of_table'</name>
      <fsummary>Match the objects in <c>Tab</c>against <c>MatchSpec</c>.</fsummary>
      <desc>
        <p>Matches the objects in the table <c>Tab</c> using a
          match_spec as described in ERTS users guide, and returns
          a chunk of terms and a continuation, the wanted number 
          of returned terms is specified by the <c>NObjects</c> argument.
          The lock argument can be <c>read</c> or <c>write</c>.
          The continuation should be used as argument to <c>mnesia:select/1</c>,
          if more or all answers are needed.</p>
        <p><em>Note:</em> for best performance <c>select</c> should
          be used before any modifying operations are done on that
          table in the same transaction, i.e. don't use
          <c>mnesia:write</c> or <c>mnesia:delete</c> before a
          <c>mnesia:select</c>. For efficiency the <c>NObjects</c> is
          a recommendation only and the result may contain anything
          from an empty list to all available results.  </p>
      </desc>
    </func>
    <func>
      <name>select(Cont) ->  transaction abort | {[Object],Cont} | '$end_of_table'</name>
      <fsummary>Continues selecting objects.   </fsummary>
      <desc>
        <p>Selects more objects with the match specification initiated
          by <c>mnesia:select/4</c>.
          </p>
        <p><em>Note:</em> Any modifying operations, i.e. <c>mnesia:write</c>
          or <c>mnesia:delete</c>, that are done between the <c>mnesia:select/4</c>
          and <c>mnesia:select/1</c> calls will not be visible in the result.</p>
      </desc>
    </func>
    <func>
      <name>set_debug_level(Level) -> OldLevel</name>
      <fsummary>Change the internal debug level of Mnesia</fsummary>
      <desc>
        <p>Changes the internal debug level of Mnesia. See the
          chapter about configuration parameters for details.</p>
      </desc>
    </func>
    <func>
      <name>set_master_nodes(MasterNodes) -> ok | {error, Reason} </name>
      <fsummary>Set the master nodes for all tables</fsummary>
      <desc>
        <p>For each table Mnesia will determine its replica nodes
          (<c>TabNodes</c>) and invoke <c>mnesia:set_master_nodes(Tab, TabMasterNodes)</c> where <c>TabMasterNodes</c> is the
          intersection of <c>MasterNodes</c> and <c>TabNodes</c>. See
          <c>mnesia:set_master_nodes/2</c> about the semantics.</p>
      </desc>
    </func>
    <func>
      <name>set_master_nodes(Tab, MasterNodes) -> ok | {error, Reason} </name>
      <fsummary>Set the master nodes for a table</fsummary>
      <desc>
        <p>If the application detects that there has been a
          communication failure (in a potentially partitioned network) which
          may have caused an inconsistent database, it may use the
          function <c>mnesia:set_master_nodes(Tab, MasterNodes)</c> to
          define from which nodes  each table will be loaded.
          At startup Mnesia's normal table load algorithm will be
          bypassed and the table will be loaded from one of the master
          nodes defined for the table, regardless of when and if Mnesia
          was terminated on other nodes. The <c>MasterNodes</c> may only
          contain nodes where the table has a replica and if the
          <c>MasterNodes</c> list is empty, the master node recovery
          mechanism for the particular table will be reset and the
          normal load mechanism will be used at next restart.
          </p>
        <p>The master node setting is always local and it may be
          changed regardless of whether Mnesia is started or not.
          </p>
        <p>The database may also become inconsistent if the
          <c>max_wait_for_decision</c> configuration parameter is used
          or if <c>mnesia:force_load_table/1</c> is used.</p>
      </desc>
    </func>
    <func>
      <name>snmp_close_table(Tab) -> {aborted, R} | {atomic, ok}</name>
      <fsummary>Remove the possibility for SNMP to manipulate the table.</fsummary>
      <desc>
        <p>Removes the possibility for SNMP to manipulate the
          table.</p>
      </desc>
    </func>
    <func>
      <name>snmp_get_mnesia_key(Tab, RowIndex) -> {ok, Key} | undefined</name>
      <fsummary>Get the corresponding Mnesia key from an SNMP index.</fsummary>
      <type>
        <v>Tab ::= atom()</v>
        <v>RowIndex ::= [integer()]</v>
        <v>Key ::= key() | {key(), key(), ...}</v>
        <v>key() ::= integer() | string() | [integer()]</v>
      </type>
      <desc>
        <p>Transforms an SNMP index to the corresponding Mnesia key.
          If the SNMP table has multiple keys, the key is a tuple of
          the key columns.</p>
      </desc>
    </func>
    <func>
      <name>snmp_get_next_index(Tab, RowIndex) -> {ok, NextIndex} | endOfTable</name>
      <fsummary>Get the index of the next lexicographical row.</fsummary>
      <type>
        <v>Tab ::= atom()</v>
        <v>RowIndex ::= [integer()]</v>
        <v>NextIndex ::= [integer()]</v>
      </type>
      <desc>
        <p>The <c>RowIndex</c> may specify a non-existing row.
          Specifically, it might be the empty list. Returns the index
          of the next lexicographical row. If <c>RowIndex</c> is the
          empty list, this function will return the index of the first row
          in the table.</p>
      </desc>
    </func>
    <func>
      <name>snmp_get_row(Tab, RowIndex) -> {ok, Row} | undefined</name>
      <fsummary>Retrieve a row indexed by an SNMP index.</fsummary>
      <type>
        <v>Tab ::= atom()</v>
        <v>RowIndex ::= [integer()]</v>
        <v>Row ::= record(Tab)</v>
      </type>
      <desc>
        <p>Makes it possible to read a row by its SNMP index. This
          index is specified as an SNMP OBJECT IDENTIFIER, a list of
          integers.</p>
      </desc>
    </func>
    <func>
      <name>snmp_open_table(Tab, SnmpStruct) -> {aborted, R} | {atomic, ok}</name>
      <fsummary>Organize a Mnesia table  as an SNMP table.</fsummary>
      <type>
        <v>Tab ::= atom()</v>
        <v>SnmpStruct ::= [{key, type()}]</v>
        <v>type() ::= type_spec() | {type_spec(), type_spec(), ...}</v>
        <v>type_spec() ::= fix_string | string | integer</v>
      </type>
      <desc>
        <p>It is possible to establish a direct one to one mapping
          between Mnesia tables and SNMP tables. Many
          telecommunication applications are controlled and monitored
          by the SNMP protocol. This connection between Mnesia and
          SNMP makes it simple and convenient to achieve this.
          </p>
        <p>The <c>SnmpStruct</c> argument is a list of SNMP
          information. Currently, the only information needed is
          information about the key types in the table.  It is not
          possible to handle multiple keys in Mnesia, but many SNMP
          tables have multiple keys. Therefore, the following
          convention is used: if a table has multiple keys, these must
          always be stored as a tuple of the keys.  Information about
          the key types is specified as a tuple of atoms describing
          the types. The only significant type is
          <c>fix_string</c>. This means that a string has fixed
          size. For example:
          </p>
        <code type="none">
mnesia:snmp_open_table(person, [{key, string}])
        </code>
        <p>causes the <c>person</c> table to be ordered as an SNMP
          table.
          </p>
        <p>Consider the following schema for a table of company
          employees. Each employee is identified by department number
          and name. The other table column stores the telephone number:
          </p>
        <code type="none">
mnesia:create_table(employee,
    [{snmp, [{key, {integer, string}}]},
     {attributes, record_info(fields, employees)}]),
        </code>
        <p>The corresponding SNMP table would have three columns;
          <c>department</c>, <c>name</c> and <c>telno</c>.
          </p>
        <p>It is possible to have table columns that are not visible
          through the SNMP protocol. These columns must be the last
          columns of the table. In the previous example, the SNMP
          table could have columns <c>department</c> and <c>name</c>
          only. The application could then use the <c>telno</c> column
          internally, but it would not be visible to the SNMP
          managers.
          </p>
        <p>In a table monitored by SNMP, all elements must be
          integers, strings, or lists of integers.
          </p>
        <p>When a table is SNMP ordered, modifications are more
          expensive than usual, O(logN).  And more memory is used.
          </p>
        <p><em>Note:</em>Only the lexicographical SNMP ordering is
          implemented in Mnesia, not the actual SNMP monitoring.</p>
      </desc>
    </func>
    <func>
      <name>start() -> ok | {error, Reason} </name>
      <fsummary>Start a local Mnesia system.</fsummary>
      <desc>
        <p>The start-up procedure for a set of Mnesia nodes is a
          fairly complicated operation. A Mnesia system consists of a set
          of nodes, with Mnesia  started locally on all
          participating nodes. Normally, each node has a directory where
          all the Mnesia files are written. This directory will be
          referred to as the Mnesia directory. Mnesia may also be
          started on disc-less nodes. See <c>mnesia:create_schema/1</c>
          and the Mnesia User's Guide for more information about disc-less
          nodes.
          </p>
        <p>The set of nodes which makes up a Mnesia system is kept in
          a schema and it is possible to add and remove Mnesia nodes
          from the schema. The initial schema is normally created on
          disc with the function <c>mnesia:create_schema/1</c>. On
          disc-less nodes, a tiny default schema is generated each time
          Mnesia is started. During the start-up procedure, Mnesia
          will exchange schema information between the nodes in order
          to verify that the table definitions are compatible.
          </p>
        <p>Each schema has a unique cookie which may be regarded as a
          unique schema identifier. The cookie must be the same on all
          nodes where Mnesia is supposed to run. See the Mnesia
          User's Guide for more information about these details.
          </p>
        <p>The schema file, as well as all other files which Mnesia
          needs, are kept in the Mnesia directory. The command line
          option <c>-mnesia dir Dir</c> can be used to specify the
          location of this directory to the Mnesia system. If no such
          command line option is found, the name of the directory
          defaults to <c>Mnesia.Node</c>.
          </p>
        <p><c>application:start(mnesia)</c> may also be used.</p>
      </desc>
    </func>
    <func>
      <name>stop() -> stopped </name>
      <fsummary>Stop Mnesia locally.</fsummary>
      <desc>
        <p>Stops Mnesia locally on the current node.
          </p>
        <p><c>application:stop(mnesia)</c> may also be used.</p>
      </desc>
    </func>
    <func>
      <name>subscribe(EventCategory)</name>
      <fsummary>Subscribe to events of type <c>EventCategory</c>.</fsummary>
      <desc>
        <p>Ensures that a copy of all events of type
          <c>EventCategory</c> are sent to the caller.  The event
          types available are described in the Mnesia User's Guide.</p>
      </desc>
    </func>
    <func>
      <name>sync_dirty(Fun, [, Args]) -> ResultOfFun | exit(Reason) </name>
      <fsummary>Call the Fun in a context which is not protected by a transaction.</fsummary>
      <desc>
        <p>Call the <c>Fun</c> in a context which is not protected
          by a transaction. The Mnesia function calls performed in the
          <c>Fun</c> are mapped to the corresponding dirty functions.
          It is performed in almost the same context as
          <c>mnesia:async_dirty/1,2</c>. The difference is that the
          operations are performed synchronously. The caller waits for
          the updates to be performed on all active replicas before
          the <c>Fun</c> returns. See <c>mnesia:activity/4</c> and the
          Mnesia User's Guide for more details.</p>
      </desc>
    </func>
    <func>
      <name>sync_transaction(Fun, [[, Args], Retries]) -> {aborted, Reason} | {atomic, ResultOfFun} </name>
      <fsummary>Synchronously execute a transaction.</fsummary>
      <desc>
        <p>This function waits until data have been committed and
          logged to disk (if disk is used) on every involved node before
          it returns, otherwise it behaves as 
          <c>mnesia:transaction/[1,2,3]</c>.</p>
        <p>This functionality can be used to avoid that one process may overload 
          a database on another node.</p>
      </desc>
    </func>
    <func>
      <name>system_info(InfoKey) -> Info | exit({aborted, Reason})</name>
      <fsummary>Return information about the Mnesia system</fsummary>
      <desc>
        <p>Returns information about the Mnesia system, such as
          transaction statistics, db_nodes, and configuration parameters.
          Valid keys are:</p>
        <list type="bulleted">
          <item>
            <p><c>all</c>. This argument returns a list of all
              local system information. Each element is a
              <c>{InfoKey, InfoVal}</c> tuples.<em>Note:</em> New <c>InfoKey</c>'s may
              be added and old undocumented <c>InfoKey</c>'s may be removed without 
              notice.</p>
          </item>
          <item>
            <p><c>access_module</c>. This argument returns the name of 
              the module which is configured to be the activity access
              callback module.
              </p>
          </item>
          <item>
            <p><c>auto_repair</c>. This argument returns
              <c>true</c> or <c>false</c> to indicate if Mnesia is
              configured to invoke the auto repair facility on corrupted
              disc files. 
              </p>
          </item>
          <item>
            <p><c>backup_module</c>. This argument returns the name of 
              the module which is configured to be the backup
              callback module.
              </p>
          </item>
          <item>
            <p><c>checkpoints</c>. This argument
              returns a list of the names of the
              checkpoints currently active on this node.
              </p>
          </item>
          <item>
            <p><c>event_module</c>. This argument returns the name of 
              the module which is the event handler callback module.
              </p>
          </item>
          <item>
            <p><c>db_nodes</c>. This argument returns
              the nodes which make up the persistent database. Disc
              less nodes will only be included in the list of nodes if
              they explicitly has been added to the schema, e.g. with
              <c>mnesia:add_table_copy/3</c>. The function can be
              invoked even if Mnesia is not yet running. 
              </p>
          </item>
          <item>
            <p><c>debug</c>. This argument returns the current
              debug level of Mnesia.  
              </p>
          </item>
          <item>
            <p><c>directory</c>. This argument returns the name of
              the Mnesia directory. It can be invoked even if Mnesia is
              not yet running.
              </p>
          </item>
          <item>
            <p><c>dump_log_load_regulation</c>. This argument
              returns a boolean which tells whether Mnesia is
              configured to load regulate the dumper process or not.
              This feature is temporary and will disappear in future
              releases.
              </p>
          </item>
          <item>
            <p><c>dump_log_time_threshold</c>. This argument
              returns the time threshold for transaction log dumps in
              milliseconds. 
              </p>
          </item>
          <item>
            <p><c>dump_log_update_in_place</c>. This argument
              returns a boolean which tells whether Mnesia is
              configured to perform the updates in the dets files
              directly or if the updates should be performed in a copy
              of the dets files.
              </p>
          </item>
          <item>
            <p><c>dump_log_write_threshold</c>. This argument
              returns the write threshold for transaction log dumps as
              the  number of writes to the transaction log.
              </p>
          </item>
          <item>
            <p><c>extra_db_nodes</c>. This argument returns a list
              of extra db_nodes to be contacted at start-up.
              </p>
          </item>
          <item>
            <p><c>fallback_activated</c>. This argument returns
              true if a fallback is activated, otherwise false. 
              </p>
          </item>
          <item>
            <p><c>held_locks</c>. This argument returns a list of
              all locks held by the local Mnesia lock manager.
              </p>
          </item>
          <item>
            <p><c>is_running</c>. This argument returns <c>yes</c>
              or <c>no</c> to indicate if Mnesia is running. It may
              also return <c>starting</c> or <c>stopping</c>.  Can be
              invoked even if Mnesia is not yet running.
              </p>
          </item>
          <item>
            <p><c>local_tables</c>. This argument returns a list
              of all tables which are configured to reside locally.
              </p>
          </item>
          <item>
            <p><c>lock_queue</c>. This argument returns a list of
              all transactions that are queued for execution by the
              local lock manager.
              </p>
          </item>
          <item>
            <p><c>log_version</c>. This argument returns the
              version number of the Mnesia transaction log format.
              </p>
          </item>
          <item>
            <p><c>master_node_tables</c>. This argument returns a
              list of all tables with at least one master node.
              </p>
          </item>
          <item>
            <p><c>protocol_version</c>. This argument
              returns the version number
              of the Mnesia inter-process communication protocol.
              </p>
          </item>
          <item>
            <p><c>running_db_nodes</c>. This argument returns a
              list of nodes where Mnesia currently is running. This
              function can be invoked even if Mnesia is not yet
              running, but it will then have slightly different
              semantics. If Mnesia is down on the local node, the
              function will return those other <c>db_nodes</c> and
              <c>extra_db_nodes</c> that for the moment are up and
              running. If Mnesia is started, the function will return
              those nodes that Mnesia on the local node is fully
              connected to. Only those nodes that Mnesia has exchanged
              schema information with are included as
              <c>running_db_nodes</c>. After the merge of schemas, the
              local Mnesia system is fully operable and applications
              may perform access of remote replicas. Before the schema
              merge Mnesia will only operate locally. Sometimes there
              may be more nodes included in the
              <c>running_db_nodes</c> list than all <c>db_nodes</c>
              and <c>extra_db_nodes</c> together.
              </p>
          </item>
          <item>
            <p><c>schema_location</c>. This argument returns the
              initial schema location.
              </p>
          </item>
          <item>
            <p><c>subscribers</c>. This argument returns a list of
              local processes currently subscribing to system events.
              </p>
          </item>
          <item>
            <p><c>tables</c>. This argument returns a list of all
              locally known tables.
              </p>
          </item>
          <item>
            <p><c>transactions</c>. This argument returns a list
              of all currently active local transactions.
              </p>
          </item>
          <item>
            <p><c>transaction_failures</c>. This argument returns
              a number which indicates how many transactions have
              failed since Mnesia was started.
              </p>
          </item>
          <item>
            <p><c>transaction_commits</c>. This argument returns a
              number which indicates how many transactions have
              terminated successfully since Mnesia was started.
              </p>
          </item>
          <item>
            <p><c>transaction_restarts</c>. This argument returns
              a number which indicates how many transactions have been
              restarted since Mnesia was started.
              </p>
          </item>
          <item>
            <p><c>transaction_log_writes</c>. This argument
              returns a number which indicates the number of write
              operation that have been performed to the transaction
              log since start-up.
              </p>
          </item>
          <item>
            <p><c>use_dir</c>. This argument returns a boolean
              which indicates whether the Mnesia directory is used or
              not. Can be invoked even if Mnesia is not yet running.  
              </p>
          </item>
          <item>
            <p><c>version</c>. This argument returns the current
              version number of Mnesia.
              </p>
          </item>
        </list>
      </desc>
    </func>
    <func>
      <name>table(Tab [,[Option]]) -> QueryHandle </name>
      <fsummary>Return a QLC query handle.</fsummary>
      <desc>
        <p>          <marker id="qlc_table"></marker>
Returns a QLC (Query List Comprehension) query handle, see
          <seealso marker="stdlib:qlc">qlc(3)</seealso>.The module <c>qlc</c> implements a query language, it
          can use mnesia tables as sources of data. Calling
          <c>mnesia:table/1,2</c> is the means to make the <c>mnesia</c>
          table <c>Tab</c> usable to QLC.</p>
        <p>The list of Options may contain mnesia options or QLC
          options, the following options are recognized by Mnesia:
          <c>{traverse, SelectMethod},{lock, Lock},{n_objects,Number}</c>, any other option is forwarded
          to QLC. The <c>lock</c> option may be <c>read</c> or
          <c>write</c>, default is <c>read</c>.  The option
          <c>n_objects</c> specifies (roughly) the number of objects
          returned from mnesia to QLC. Queries to remote tables may
          need a larger chunks to reduce network overhead, default
          <c>100</c> objects at a time are returned.  The option
          <c>traverse</c> determines the method to traverse the whole
          table (if needed), the default method is <c>select</c>:</p>
        <list type="bulleted">
          <item>
            <p><c>select</c>. The table is traversed by calling
              <c>mnesia:select/4</c> and <c>mnesia:select/1</c>. The
              match specification (the second argument of <c>select/3</c>) 
              is assembled by QLC: simple filters are
              translated into equivalent match specifications while
              more complicated filters have to be applied to all
              objects returned by <c>select/3</c> given a match
              specification that matches all objects.</p>
          </item>
          <item>
            <p><c>{select, MatchSpec}</c>. As for <c>select</c>
              the table is traversed by calling <c>mnesia:select/3</c> and
              <c>mnesia:select/1</c>. The difference is that the match
              specification is explicitly given. This is how to state
              match specifications that cannot easily be expressed
              within the syntax provided by QLC.</p>
          </item>
        </list>
      </desc>
    </func>
    <func>
      <name>table_info(Tab, InfoKey) -> Info | exit({aborted, Reason})</name>
      <fsummary>Return local information about table.</fsummary>
      <desc>
        <p>The <c>table_info/2</c> function takes two arguments.
          The first is the name of a Mnesia table, the second is one of
          the following keys:
          </p>
        <list type="bulleted">
          <item>
            <p><c>all</c>. This argument returns a list of all
              local table information. Each element is a <c>{InfoKey, ItemVal}</c> tuples. <em>Note:</em> New <c>InfoItem</c>'s may be
              added and old undocumented <c>InfoItem</c>'s may be removed without 
              notice.</p>
          </item>
          <item>
            <p><c>access_mode</c>. This argument returns the
              access mode of the table. The access mode may either be
              read_only or read_write.
              </p>
          </item>
          <item>
            <p><c>arity</c>. This argument returns the arity of
              records in the table as specified in the schema. 
              </p>
          </item>
          <item>
            <p><c>attributes</c>. This argument returns the table
              attribute names which are specified in the schema. 
              </p>
          </item>
          <item>
            <p><c>checkpoints</c>. This argument returns the names
              of the currently active checkpoints which involves this
              table on this node.
              </p>
          </item>
          <item>
            <p><c>cookie</c>. This argument returns a table cookie
              which is a unique system generated identifier for the
              table. The cookie is used internally to ensure that two
              different table definitions using the same table name
              cannot accidentally be intermixed. The cookie is
              generated when the table is initially created.
              </p>
          </item>
          <item>
            <p><c>disc_copies</c>. This argument returns the nodes
              where a disc_copy of the table resides according to the
              schema. 
              </p>
          </item>
          <item>
            <p><c>disc_only_copies </c>. This argument returns the
              nodes where a disc_only_copy of the table resides
              according to the schema. 
              </p>
          </item>
          <item>
            <p><c>index</c>. This argument returns the list of
              index position integers for the table. 
              </p>
          </item>
          <item>
            <p><c>load_node</c>. This argument returns the name of
              the node that Mnesia loaded the table from. The
              structure of the returned value is unspecified but may
              be useful for debugging purposes.
              </p>
          </item>
          <item>
            <p><c>load_order</c>. This argument returns the load
              order priority of the table. It is an integer and
              defaults to <c>0</c> (zero).
              </p>
          </item>
          <item>
            <p><c>load_reason</c>. This argument returns the
              reason of why Mnesia decided to load the table. The
              structure of the returned value is unspecified but may
              be useful for debugging purposes.
              </p>
          </item>
          <item>
            <p><c>local_content</c>. This argument returns
              <c>true</c> or <c>false</c> to indicate whether the
              table is configured to have locally unique content on
              each node. 
              </p>
          </item>
          <item>
            <p><c>master_nodes</c>. This argument returns the
              master nodes of a table.
              </p>
          </item>
          <item>
            <p><c>memory</c>. This argument returns the number of
              words allocated to the table on this node. 
              </p>
          </item>
          <item>
            <p><c>ram_copies</c>. This argument returns the nodes
              where a ram_copy of the table resides according to the
              schema. 
              </p>
          </item>
          <item>
            <p><c>record_name</c>. This argument returns the
              record name, common for all records in the table
              </p>
          </item>
          <item>
            <p><c>size</c>. This argument returns the number of
              records inserted in the table. 
              </p>
          </item>
          <item>
            <p><c>snmp</c>. This argument returns the SNMP struct.
              <c>[]</c>meaning that the table currently has no SNMP
              properties.
              </p>
          </item>
          <item>
            <p><c>storage_type</c>.This argument returns the local
              storage type of the table. It can be <c>disc_copies</c>,
              <c>ram_copies</c>, <c>disc_only_copies</c>, or the atom
              <c>unknown</c>. <c>unknown</c> is returned for all
              tables which only reside remotely.
              </p>
          </item>
          <item>
            <p><c>subscribers</c>. This argument returns a list
              of local processes currently subscribing to local table
              events which involve this table on this node.
              </p>
          </item>
          <item>
            <p><c>type</c>. This argument returns the table type,
              which is either <c>bag</c>, <c>set</c> or <c>ordered_set</c>..
              </p>
          </item>
          <item>
            <p><c>user_properties</c>. This argument returns the
              user associated table properties of the table. It is a
              list of the stored property records.
              </p>
          </item>
          <item>
            <p><c>version</c>. This argument returns the current
              version of the table definition. The table version is
              incremented when the table definition is changed. The
              table definition may be incremented directly when the
              table definition has been changed in a schema
              transaction, or when a committed table definition is
              merged with table definitions from other nodes during
              start-up.
              </p>
          </item>
          <item>
            <p><c>where_to_read</c>.This argument returns the node
              where the table can be read. If the value <c>nowhere</c>
              is returned, the table is not loaded, or it resides at a
              remote node which is not running.             
              </p>
          </item>
          <item>
            <p><c>where_to_write</c>. This argument returns a list
              of the nodes that currently hold an active replica of
              the table. 
              </p>
          </item>
          <item>
            <p><c>wild_pattern</c>. This argument returns a
              structure which can be given to the various match
              functions for a certain table. A record tuple is where all
              record fields have the value <c>'_'</c>.
              </p>
          </item>
        </list>
      </desc>
    </func>
    <func>
      <name>transaction(Fun [[, Args], Retries]) -> {aborted, Reason} | {atomic, ResultOfFun}</name>
      <fsummary>Execute a transaction.</fsummary>
      <desc>
        <p>This function executes the functional object <c>Fun</c>
          with arguments <c>Args</c> as a transaction.
          </p>
        <p>The code which executes inside the transaction
          can consist of a series of table manipulation functions.
          If something goes wrong inside the transaction as a result of a
          user error or a certain table not being available, the
          entire transaction is aborted and the function 
          <c>transaction/1</c> returns the tuple 
          <c>{aborted, Reason}</c>.
          </p>
        <p>If all is well, <c>{atomic, ResultOfFun}</c> is returned where
          <c>ResultOfFun</c> is the value of the last expression in
          <c>Fun</c>.
          </p>
        <p>A function which adds a family to the database can be
          written as follows if we have a structure <c>{family, Father, Mother, ChildrenList}</c>:
          </p>
        <code type="none">
add_family({family, F, M, Children}) ->
    ChildOids = lists:map(fun oid/1, Children),
    Trans = fun() ->      
        mnesia:write(F#person{children = ChildOids}, 
        mnesia:write(M#person{children = ChildOids},
        Write = fun(Child) -> mnesia:write(Child) end,
        lists:foreach(Write, Children)
    end,
    mnesia:transaction(Trans).

oid(Rec) -> {element(1, Rec), element(2, Rec)}.
        </code>
        <p>This code adds a set of people to the database. Running this code
          within one transaction will ensure that either the whole
          family is added to the database, or the  whole transaction
          aborts. For example, if the last child is badly formatted,
          or the executing process terminates due to an
          <c>'EXIT'</c> signal while executing the family code, the
          transaction aborts. Accordingly, the situation where half a
          family is added can never occur.
          </p>
        <p>It is also useful to update the database within a transaction
          if several processes concurrently update the same records.
          For example, the function <c>raise(Name, Amount)</c>, which 
          adds <c>Amount</c> to the salary field of a person, should
          be implemented as follows:
          </p>
        <code type="none">
raise(Name, Amount) ->
    mnesia:transaction(fun() ->
        case mnesia:wread({person, Name}) of
            [P] ->
                Salary = Amount + P#person.salary,
                P2 = P#person{salary = Salary},
                mnesia:write(P2);
            _ ->
                mnesia:abort("No such person")
        end
    end).
        </code>
        <p>When this function executes within a transaction, 
          several processes running on different nodes can concurrently
          execute the <c>raise/2</c> function without interfering 
          with each other. 
          </p>
        <p>Since Mnesia detects deadlocks, a transaction can be
          restarted any number of times.  This function will attempt a restart as specified in
          <c>Retries</c>. <c>Retries</c> must
          be an integer greater than 0 or the atom <c>infinity</c>. Default is
          <c>infinity</c>.</p>
      </desc>
    </func>
    <func>
      <name>transform_table(Tab, Fun, NewAttributeList, NewRecordName) ->  {aborted, R} | {atomic, ok} </name>
      <fsummary>Change format on all records in table. <c>Tab</c></fsummary>
      <desc>
        <p>This function applies the argument <c>Fun</c> to all
          records in the table. <c>Fun</c> is a function which takes a
          record of the old type and returns a transformed record of the
          new type. The <c>Fun</c> argument can also be the atom
          <c>ignore</c>, it indicates that only the meta data about the table will
          be updated. Usage of <c>ignore</c> is not recommended but included
          as a possibility for the user do to his own transform.
          <c>NewAttributeList</c> and <c>NewRecordName</c>
          specifies the attributes and the new record type of converted
          table. Table name will always remain unchanged, if the
          record_name is changed only the mnesia functions which
          uses table identifiers will work, e.g. <c>mnesia:write/3</c>
          will work but <c>mnesia:write/1</c> will not.</p>
      </desc>
    </func>
    <func>
      <name>transform_table(Tab, Fun, NewAttributeList) ->  {aborted, R} | {atomic, ok} </name>
      <fsummary>Change format on all records in table. <c>Tab</c></fsummary>
      <desc>
        <p>Invokes <c>mnesia:transform_table(Tab, Fun, NewAttributeList, RecName)</c>
          where <c>RecName</c> is <c>mnesia:table_info(Tab, record_name)</c>.</p>
      </desc>
    </func>
    <func>
      <name>traverse_backup(Source, [SourceMod,] Target, [TargetMod,] Fun, Acc) -> {ok, LastAcc} | {error, Reason}</name>
      <fsummary>Traversal of a backup.</fsummary>
      <desc>
        <p>With this function it is possible to iterate over a backup,
          either for the purpose of transforming it into a new backup,
          or just reading it. The arguments are explained briefly
          below. See the Mnesia User's Guide for additional
          details.
          </p>
        <list type="bulleted">
          <item><c>SourceMod</c> and <c>TargetMod</c> are the names of
           the modules which actually access the backup
           media.
          </item>
          <item><c>Source</c> and <c>Target</c> are opaque data used
           exclusively by the modules <c>SourceMod</c> and
          <c>TargetMod</c> for the purpose of initializing the
           backup media. 
          </item>
          <item><c>Acc</c> is an initial accumulator value.
          </item>
          <item><c>Fun(BackupItems, Acc)</c> is applied to each item in
           the backup. The Fun must return a tuple
          <c>{BackupItems,NewAcc}</c>, where <c>BackupItems</c> is
           a list of valid backup items, and <c>NewAcc</c> is a new
           accumulator value. The returned backup items are written
           in the target backup. 
          </item>
          <item><c>LastAcc</c> is the last accumulator value. This is
           the last <c>NewAcc</c> value that was returned by <c>Fun</c>.
          </item>
        </list>
      </desc>
    </func>
    <func>
      <name>uninstall_fallback() -> ok | {error,Reason}</name>
      <fsummary>Uninstall a fallback.</fsummary>
      <desc>
        <p>Invokes <c>mnesia:uninstall_fallback([{scope, global}])</c>.</p>
      </desc>
    </func>
    <func>
      <name>uninstall_fallback(Args) -> ok | {error,Reason}</name>
      <fsummary>Uninstall a fallback.</fsummary>
      <desc>
        <p>This function is used to de-install a fallback before it
          has been used to restore the database. This is normally a
          distributed operation that is either performed on all
          nodes with disc resident schema or none.  Uninstallation of
          fallbacks requires Erlang to be up and running on all
          involved nodes, but it does not matter if Mnesia is running
          or not. Which nodes that are considered as disc-resident
          nodes is determined from the schema info in the local
          fallback.
          </p>
        <p><c>Args</c> is a list of the following tuples: 
          </p>
        <list type="bulleted">
          <item>
            <p><c>{module, BackupMod}</c>. 
              See <c>mnesia:install_fallback/2</c> about the
              semantics.</p>
          </item>
          <item>
            <p><c>{scope, Scope}</c>
              See <c>mnesia:install_fallback/2</c> about the
              semantics.</p>
          </item>
          <item>
            <p><c>{mnesia_dir, AlternateDir}</c> 
              See <c>mnesia:install_fallback/2</c> about the
              semantics.</p>
          </item>
        </list>
      </desc>
    </func>
    <func>
      <name>unsubscribe(EventCategory)</name>
      <fsummary>Subscribe to events of type <c>EventCategory</c>.</fsummary>
      <desc>
        <p>Stops sending events of type
          <c>EventCategory</c> to the caller.</p>
      </desc>
    </func>
    <func>
      <name>wait_for_tables(TabList,Timeout) -> ok | {timeout, BadTabList} | {error, Reason} </name>
      <fsummary>Wait for tables to be accessible.</fsummary>
      <desc>
        <p>Some applications need to wait for certain tables to
          be accessible in order to do useful work.
          <c>mnesia:wait_for_tables/2</c> hangs until all tables in the
          <c>TabList</c> are accessible, or until <c>timeout</c> is
          reached.</p>
      </desc>
    </func>
    <func>
      <name>wread({Tab, Key}) -> transaction abort | RecordList </name>
      <fsummary>Read records with given key.</fsummary>
      <desc>
        <p>Invoke <c>mnesia:read(Tab, Key, write)</c>.</p>
      </desc>
    </func>
    <func>
      <name>write(Record) -> transaction abort | ok </name>
      <fsummary>Writes a record into the database.</fsummary>
      <desc>
        <p>Invoke <c>mnesia:write(Tab, Record, write)</c> where
          <c>Tab</c> is <c>element(1, Record)</c>.</p>
      </desc>
    </func>
    <func>
      <name>write(Tab, Record, LockKind) -> transaction abort | ok </name>
      <fsummary>Write a record into the database.</fsummary>
      <desc>
        <p>Writes the record <c>Record</c> to the table <c>Tab</c>.
          </p>
        <p>The function returns <c>ok</c>, or aborts if an error
          occurs. For example, the transaction aborts if no
          <c>person</c> table exists.
          </p>
        <p>The semantics of this function is context sensitive. See
          <c>mnesia:activity/4</c> for more information. In transaction
          context it acquires a lock of type <c>LockKind</c>. The
          following lock types are supported: <c>write</c> and
          <c>sticky_write</c>.</p>
      </desc>
    </func>
    <func>
      <name>write_lock_table(Tab) -> ok | transaction abort</name>
      <fsummary>Set write lock on an entire table.</fsummary>
      <desc>
        <p>Invokes <c>mnesia:lock({table, Tab}, write)</c>.</p>
      </desc>
    </func>
  </funcs>

  <section>
    <title>Configuration Parameters</title>
    <p>Mnesia reads the following application configuration
      parameters:</p>
    <list type="bulleted">
      <item>
        <p><c>-mnesia access_module Module</c>. The 
          name of the Mnesia activity access callback module. The default is
          <c>mnesia</c>.
          </p>
      </item>
      <item>
        <p><c>-mnesia auto_repair true | false</c>. This flag controls
          whether Mnesia will try to automatically repair
          files that have not been properly closed. The default is 
          <c>true</c>.
          </p>
      </item>
      <item>
        <p><c>-mnesia backup_module Module</c>. The 
          name of the Mnesia backup callback module. The default is
          <c>mnesia_backup</c>.
          </p>
      </item>
      <item>
        <p><c>-mnesia debug Level</c>
          Controls the debug level of Mnesia.
          Possible values are:</p>
        <taglist>
          <tag><c>none</c></tag>
          <item>
            <p>No trace outputs at all. This is the default setting.
              </p>
          </item>
          <tag><c>verbose</c></tag>
          <item>
            <p>Activates tracing of important debug events. These
              debug events generate <c>{mnesia_info, Format, Args}</c>
              system events. Processes may subscribe to these events with
              <c>mnesia:subscribe/1</c>. The events are always sent to Mnesia's
              event handler. 
              </p>
          </item>
          <tag><c>debug</c></tag>
          <item>
            <p>Activates all events at the verbose level plus full
              trace of all debug events. These debug events generate
              <c>{mnesia_info, Format, Args}</c> system events. Processes may
              subscribe to these events with <c>mnesia:subscribe/1</c>. The
              events are always sent to the Mnesia event handler. On this
              debug level, the Mnesia event handler starts subscribing to
              updates in the schema table. 
              </p>
          </item>
          <tag><c>trace</c></tag>
          <item>
            <p>Activates all events at the level debug. On this
              debug level, the Mnesia event handler starts subscribing to
              updates on all Mnesia tables. This level is only intended
              for debugging  small toy systems since many large
              events may be generated.
              </p>
          </item>
          <tag><c>false</c></tag>
          <item>
            <p>An alias for none.
              </p>
          </item>
          <tag><c>true</c></tag>
          <item>
            <p>An alias for debug.
              </p>
          </item>
        </taglist>
      </item>
      <item>
        <p><c>-mnesia core_dir Directory</c>. The name of the
          directory where Mnesia core files is stored or
          false. Setting it implies that also ram only nodes, will
          generate a core file if a crash occurs. </p>
      </item>
      <item>
        <p><c>-mnesia dc_dump_limit Number</c>. 
          Controls how often <c>disc_copies</c> tables are dumped from memory.
          Tables are dumped when 
          <c>filesize(Log) > (filesize(Tab)/Dc_dump_limit)</c>.
          Lower values reduces cpu overhead but increases disk space and
          startup times. The default is 4.</p>
      </item>
      <item>
        <p><c>-mnesia dir Directory</c>. The name of the directory
          where all Mnesia data is stored. The name of the directory must
          be unique for the current node. Two nodes may, under no
          circumstances, share the same Mnesia directory. The results are
          totally unpredictable.</p>
      </item>
      <item>
        <p><c>-mnesia dump_log_load_regulation true | false</c>.
          Controls if the log dumps should be performed as fast as
          possible or if the dumper should do its own load
          regulation. This feature is temporary and will disappear in a
          future release. The default is <c>false</c>.
          </p>
      </item>
      <item>
        <p><c>-mnesia dump_log_update_in_place true | false</c>.
          Controls if log dumps are performed on a copy of
          the original data file, or if  the log dump is 
          performed on the original data file. The default is <c>true</c></p>
      </item>
      <item>
        <p><c>-mnesia dump_log_write_threshold Max</c>, where
          <c>Max</c> is an integer which specifies the maximum number of writes
          allowed to the transaction log before a new dump of the log
          is performed. It defaults to 100 log writes.
          </p>
      </item>
      <item>
        <p><c>-mnesia dump_log_time_threshold Max</c>,
          where <c>Max</c> is an integer which
          specifies the dump log interval in milliseconds. It defaults
          to 3 minutes. If a dump has not been performed within
          <c>dump_log_time_threshold</c> milliseconds, then a new dump is
          performed regardless of how many writes have been
          performed. 
          </p>
      </item>
      <item>
        <p><c>-mnesia event_module Module</c>. The 
          name of the Mnesia event handler callback module. The default is
          <c>mnesia_event</c>.
          </p>
      </item>
      <item>
        <p><c>-mnesia extra_db_nodes Nodes</c> specifies a list of
          nodes, in addition to the ones found in the schema, with which
          Mnesia should also establish contact. The default value
          is the empty list <c>[]</c>. 
          </p>
      </item>
      <item>
        <p><c>-mnesia fallback_error_function {UserModule, UserFunc}</c>
          specifies a user supplied callback function
          which will be called if a fallback is installed and mnesia 
          goes down on another node. Mnesia will call the function
          with one argument the name of the dying node, e.g.
          <c>UserModule:UserFunc(DyingNode)</c>.
          Mnesia should be restarted or else
          the database could be inconsistent.
          The default behaviour is to terminate mnesia.
          </p>
      </item>
      <item>
        <p><c>-mnesia max_wait_for_decision Timeout</c>. Specifies
          how long Mnesia will wait for other nodes to share their
          knowledge regarding the outcome of an unclear transaction.  By
          default the <c>Timeout</c> is set to the atom
          <c>infinity</c>, which implies that if Mnesia upon startup
          encounters a "heavyweight transaction" whose outcome is
          unclear, the local Mnesia will wait until Mnesia is started
          on some (in worst cases all) of the other nodes that were
          involved in the interrupted transaction. This is a very rare
          situation, but when/if it happens, Mnesia does not guess if
          the transaction on the other nodes was committed or aborted.
          Mnesia will wait until it knows the outcome and then act
          accordingly.
          </p>
        <p>If <c>Timeout</c> is set to an integer value in
          milliseconds, Mnesia will force "heavyweight transactions"
          to be finished, even if the outcome of the transaction for
          the moment is unclear. After <c>Timeout</c> milliseconds,
          Mnesia  will commit/abort the transaction and continue with
          the startup. This may lead to a situation where the
          transaction is committed on some nodes and aborted on other
          nodes. If the transaction was a schema transaction, the
          inconsistency may be fatal. 
          </p>
      </item>
      <item>
        <p><c>-mnesia no_table_loaders NUMBER</c> specifies the number of
          parallel table loaders during start. More loaders can be good if the
          network latency is high or if many tables contains few records.
          The default value is <c>2</c>.
          </p>
      </item>
      <item>
        <p><c>-mnesia send_compressed Level</c> specifies the level of
          compression to be used when copying a table from the local node to
          another one. The default level is 0.
          </p>
        <p><c>Level</c> must be an integer in the interval [0, 9], with 0
          representing no compression and 9 representing maximum compression.
          Before setting it to a non-zero value, make sure the remote nodes
          understand this configuration.
          </p>
      </item>
      <item>
        <p><c>-mnesia schema_location Loc</c> controls where
          Mnesia will look for its schema. The parameter
          <c>Loc</c> may be one of the following atoms: </p>
        <taglist>
          <tag><c>disc</c></tag>
          <item>
            <p>Mandatory disc. The schema is assumed to be located
              in the Mnesia directory. If the schema cannot be found,
              Mnesia refuses to start. This is the old behavior.
              </p>
          </item>
          <tag><c>ram</c></tag>
          <item>
            <p>Mandatory RAM. The schema resides in RAM
              only. At start-up, a tiny new schema is generated. This
              default schema just contains the definition of the schema
              table and  only resides on the local node. Since no other
              nodes are found in the default schema, the configuration
              parameter  <c>extra_db_nodes</c> must be used in
              order to let the 
              node share its table definitions with other nodes. (The
              <c>extra_db_nodes</c> parameter may also be used on disc based nodes.)
              </p>
          </item>
          <tag><c>opt_disc</c></tag>
          <item>
            <p>Optional disc. The schema may reside either on disc
              or in RAM. If the schema is found on disc, Mnesia starts as a
              disc based node and the storage type of the schema table is
              <c>disc_copies</c>. If no schema is found on disc, Mnesia starts
              as a disc-less node and the storage type of the schema table is
              <c>ram_copies</c>. The default value for the application parameter
              is <c>opt_disc</c>.  
              </p>
          </item>
        </taglist>
      </item>
    </list>
    <p>First the SASL application parameters are checked, then
      the command line flags are checked, and finally, the default
      value is chosen.
      </p>
  </section>

  <section>
    <title>See Also</title>
    <p>mnesia_registry(3), mnesia_session(3), qlc(3),
      dets(3), ets(3), disk_log(3), application(3) 
      </p>
  </section>
  
</erlref>