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<header>
<copyright>
<year>1996</year><year>2011</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
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Erlang Public License along with this software. If not, it can be
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Software distributed under the License is distributed on an "AS IS"
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<title>mnesia</title>
<prepared>Claes Wikström and Håkan 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>majority</c> This attribute can be either <c>true</c> or
<c>false</c> (default is <c>false</c>). When <c>true</c>, a majority
of the table replicas must be available for an update to succeed.
Majority checking can be enabled on tables with mission-critical data,
where it is vital to avoid inconsistencies due to network splits.
</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>change_table_majority(Tab, Majority) -> {aborted, R} | {atomic, ok}</name>
<fsummary>Change the majority check setting for the table.</fsummary>
<desc>
<p><c>Majority</c> must be a boolean; the default is <c>false</c>.
When <c>true</c>, a majority of the table's replicas must be available
for an update to succeed. When used on fragmented tables, <c>Tab</c>
must be the name base table. Directly changing the majority setting on
individual fragments is not allowed.</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>{majority, Flag}</c>, where <c>Flag</c> must be a boolean.
If <c>true</c>, any (non-dirty) update to the table will abort unless
a majority of the table's replicas are available for the commit.
When used on a fragmented table, all fragments will be given
the same majority setting.
</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>.</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 like
<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 like
<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.</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 like
<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 like
<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 like
<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.</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. If majority checking
is active on the table, it will be checked as soon as a write lock is
attempted. This can be used to quickly abort if the majority condition
isn't met.
</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">
MatchHead = #person{name='$1', sex=male, age='$2', _='_'},
Guard = {'>', '$2', 30},
Result = '$1',
mnesia:select(Tab,[{MatchHead, [Guard], [Result]}]),
</code>
</desc>
</func>
<func>
<name>select(Tab, MatchSpec, NObjects, Lock) -> transaction abort | {[Object],Cont} | '$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>
