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<?xml version="1.0" encoding="latin1" ?>
<!DOCTYPE erlref SYSTEM "erlref.dtd">

<erlref>
  <header>
    <copyright>
      <year>2002</year>
      <year>2007</year>
      <holder>Ericsson AB, All Rights Reserved</holder>
    </copyright>
    <legalnotice>
  The contents of this file are subject to the Erlang Public License,
  Version 1.1, (the "License"); you may not use this file except in
  compliance with the License. You should have received a copy of the
  Erlang Public License along with this software. If not, it can be
  retrieved online at http://www.erlang.org/.

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

  The Initial Developer of the Original Code is Ericsson AB.
    </legalnotice>

    <title>mnesia_frag_hash</title>
    <prepared>H&aring;kan Mattsson</prepared>
    <responsible></responsible>
    <docno></docno>
    <approved></approved>
    <checked></checked>
    <date>2002-09-10</date>
    <rev>A</rev>
    <file>mnesia_frag_hash.sgml</file>
  </header>
  <module>mnesia_frag_hash</module>
  <modulesummary>Defines mnesia_frag_hash callback behaviour</modulesummary>
  <description>
    <p>The module <c>mnesia_frag_hash</c> defines a callback
      behaviour for user defined hash functions of fragmented tables.</p>
    <p>Which module that is selected to implement the <c>mnesia_frag_hash</c>
      behaviour for a particular fragmented table is specified together
      with the other <c>frag_properties</c>. The <c>hash_module</c> defines
      the module name. The <c>hash_state</c> defines the initial hash state.</p>
    <p>It implements dynamic hashing which is a kind of hashing
      that grows nicely when new fragments are added. It is well
      suited for scalable hash tables</p>
  </description>
  <funcs>
    <func>
      <name>init_state(Tab, State) -> NewState | abort(Reason)</name>
      <fsummary>Initiate the hash state for a new table</fsummary>
      <type>
        <v>Tab = atom()</v>
        <v>State = term()</v>
        <v>NewState = term()</v>
        <v>Reason = term()</v>
      </type>
      <desc>
        <p>This function is invoked when a fragmented table is
          created with <c>mnesia:create_table/2</c> or when a
          normal (un-fragmented) table is converted to be a
          fragmented table with <c>mnesia:change_table_frag/2</c>.</p>
        <p>Note that the <c>add_frag/2</c> function will be invoked
          one time each for the rest of the fragments (all but number 1)
          as a part of the table creation procedure.</p>
        <p><c>State</c> is the initial value of the <c>hash_state</c><c>frag_property</c>. The <c>NewState</c> will be stored as
          <c>hash_state</c> among the other <c>frag_properties</c>.
          </p>
      </desc>
    </func>
    <func>
      <name>add_frag(State) -> {NewState, IterFrags, AdditionalLockFrags} | abort(Reason)</name>
      <fsummary>This function is invoked when a new fragment is added to a fragmented table</fsummary>
      <type>
        <v>State = term()</v>
        <v>NewState = term()</v>
        <v>IterFrags = [integer()]</v>
        <v>AdditionalLockFrags = [integer()]</v>
        <v>Reason = term()</v>
      </type>
      <desc>
        <p>In order to scale well, it is a good idea ensure that the
          records are evenly distributed over all fragments including
          the new one.</p>
        <p>The <c>NewState</c> will be stored as <c>hash_state</c> among the
          other <c>frag_properties</c>. 
          </p>
        <p>As a part of the <c>add_frag</c> procedure, Mnesia will iterate
          over all fragments corresponding to the <c>IterFrags</c> numbers
          and invoke <c>key_to_frag_number(NewState,RecordKey)</c> for
          each record. If the new fragment differs from the old
          fragment, the record will be moved to the new fragment.</p>
        <p>As the <c>add_frag</c> procedure is a part of a schema
          transaction Mnesia will acquire a write locks on the
          affected tables. That is both the fragments corresponding
          to <c>IterFrags</c> and those corresponding to
          <c>AdditionalLockFrags</c>.</p>
      </desc>
    </func>
    <func>
      <name>del_frag(State) ->  {NewState, IterFrags, AdditionalLockFrags} | abort(Reason)</name>
      <fsummary>This function is invoked when a fragment is deleted from a fragmented table</fsummary>
      <type>
        <v>State = term()</v>
        <v>NewState = term()</v>
        <v>IterFrags = [integer()]</v>
        <v>AdditionalLockFrags = [integer()]</v>
        <v>Reason = term()</v>
      </type>
      <desc>
        <p>The <c>NewState</c> will be stored as <c>hash_state</c> among the
          other <c>frag_properties</c>. 
          </p>
        <p>As a part of the <c>del_frag</c> procedure, Mnesia will iterate
          over all fragments corresponding to the <c>IterFrags</c> numbers
          and invoke <c>key_to_frag_number(NewState,RecordKey)</c> for
          each record. If the new fragment differs from the old
          fragment, the record will be moved to the new fragment.</p>
        <p>Note that all records in the last fragment must be moved to
          another fragment as the entire fragment will be deleted.</p>
        <p>As the <c>del_frag</c> procedure is a part of a schema
          transaction Mnesia will acquire a write locks on the
          affected tables. That is both the fragments corresponding
          to <c>IterFrags</c> and those corresponding to
          <c>AdditionalLockFrags</c>.</p>
      </desc>
    </func>
    <func>
      <name>key_to_frag_number(State, Key) -> FragNum | abort(Reason)</name>
      <fsummary>Resolves the key of a record into a fragment number</fsummary>
      <type>
        <v>FragNum = integer()()</v>
        <v>Reason = term()</v>
      </type>
      <desc>
        <p>This function is invoked whenever Mnesia needs to determine
          which fragment a certain record belongs to. It is typically
          invoked at read, write and delete.</p>
      </desc>
    </func>
    <func>
      <name>match_spec_to_frag_numbers(State, MatchSpec) -> FragNums | abort(Reason)</name>
      <fsummary>Resolves a MatchSpec into a list of fragment numbers</fsummary>
      <type>
        <v>MatcSpec = ets_select_match_spec()</v>
        <v>FragNums = [FragNum]</v>
        <v>FragNum = integer()</v>
        <v>Reason = term()</v>
      </type>
      <desc>
        <p>This function is invoked whenever Mnesia needs to determine
          which fragments that needs to be searched for a MatchSpec.
          It is typically invoked at select and match_object.</p>
      </desc>
    </func>
  </funcs>

  <section>
    <title>See Also</title>
    <p>mnesia(3)
      </p>
  </section>
  
</erlref>