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<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>Håkan 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>
