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  <header>
    <copyright>
      <year>2002</year>
      <year>2021</year>
      <holder>Ericsson AB, All Rights Reserved</holder>
    </copyright>
    <legalnotice>
  Licensed under the Apache License, Version 2.0 (the "License");
  you may not use this file except in compliance with the License.
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    </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 since="">mnesia_frag_hash</module>
  <modulesummary>Defines mnesia_frag_hash callback behavior</modulesummary>
  <description>
    <p>This module defines a callback behavior for user-defined hash
      functions of fragmented tables.</p>
    <p>Which module that is selected to implement the <c>mnesia_frag_hash</c>
      behavior 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>This module 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 since="">init_state(Tab, State) -> NewState | abort(Reason)</name>
      <fsummary>Initiates 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>Starts when a fragmented table is
          created with the function <c>mnesia:create_table/2</c> or
          when a normal (unfragmented) table is converted to be a
          fragmented table with <c>mnesia:change_table_frag/2</c>.</p>
        <p>Notice that the function <c>add_frag/2</c> is started
          one time for each of the other fragments (except 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>. <c>NewState</c> is stored as
          <c>hash_state</c> among the other <c>frag_properties</c>.</p>
      </desc>
    </func>
    <func>
      <name since="">add_frag(State) -> {NewState, IterFrags, AdditionalLockFrags} | abort(Reason)</name>
      <fsummary>Starts 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>To scale well, it is a good idea to ensure that the
          records are evenly distributed over all fragments, including
          the new one.</p>
        <p><c>NewState</c> is 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 iterates
          over all fragments corresponding to the <c>IterFrags</c> numbers
          and starts <c>key_to_frag_number(NewState,RecordKey)</c> for
          each record. If the new fragment differs from the old
          fragment, the record is moved to the new fragment.</p>
        <p>As the <c>add_frag</c> procedure is a part of a schema
          transaction, Mnesia acquires 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 since="">del_frag(State) ->  {NewState, IterFrags, AdditionalLockFrags} | abort(Reason)</name>
      <fsummary>Starts 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><c>NewState</c> is 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 iterates
          over all fragments corresponding to the <c>IterFrags</c> numbers
          and starts <c>key_to_frag_number(NewState,RecordKey)</c> for
          each record. If the new fragment differs from the old
          fragment, the record is moved to the new fragment.</p>
        <p>Notice that all records in the last fragment must be moved to
          another fragment, as the entire fragment is deleted.</p>
        <p>As the <c>del_frag</c> procedure is a part of a schema
          transaction, Mnesia acquires 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 since="">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>Starts whenever Mnesia needs to determine
          which fragment a certain record belongs to. It is typically
          started at <c>read</c>, <c>write</c>, and <c>delete</c>.</p>
      </desc>
    </func>
    <func>
      <name since="">match_spec_to_frag_numbers(State, MatchSpec) -> FragNums | abort(Reason)</name>
      <fsummary>Resolves a <c>MatchSpec</c> 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 called whenever Mnesia needs to determine
          which fragments that need to be searched for a <c>MatchSpec</c>.
          It is typically called by <c>select</c> and
          <c>match_object</c>.</p>
      </desc>
    </func>
  </funcs>

  <section>
    <title>See Also</title>
    <p><seeerl marker="mnesia:mnesia">mnesia(3)</seeerl></p>
  </section>
  
</erlref>

