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

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  <header>
    <copyright>
      <year>2002</year>
      <year>2018</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.
  You may obtain a copy of the License at
 
      http://www.apache.org/licenses/LICENSE-2.0

  Unless required by applicable law or agreed to in writing, software
  distributed under the License is distributed on an "AS IS" BASIS,
  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  See the License for the specific language governing permissions 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 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>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>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>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>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>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><seealso marker="mnesia:mnesia">mnesia(3)</seealso></p>
  </section>
  
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