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NAME

       mnesia_frag_hash - Defines mnesia_frag_hash callback behavior

DESCRIPTION

       This  module  defines  a  callback  behavior for user-defined hash functions of fragmented
       tables.

       Which module that is selected to implement the mnesia_frag_hash behavior for a  particular
       fragmented  table  is  specified  together with the other frag_properties. The hash_module
       defines the module name. The hash_state defines the initial hash state.

       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.

EXPORTS

       init_state(Tab, State) -> NewState | abort(Reason)

              Types:

                 Tab = atom()
                 State = term()
                 NewState = term()
                 Reason = term()

              Starts  when  a fragmented table is created with the function mnesia:create_table/2
              or when a normal (unfragmented) table is converted to be a  fragmented  table  with
              mnesia:change_table_frag/2.

              Notice  that  the  function  add_frag/2  is  started one time for each of the other
              fragments (except number 1) as a part of the table creation procedure.

              State is the initial value of the hash_state frag_property. NewState is  stored  as
              hash_state among the other frag_properties.

       add_frag(State) -> {NewState, IterFrags, AdditionalLockFrags} | abort(Reason)

              Types:

                 State = term()
                 NewState = term()
                 IterFrags = [integer()]
                 AdditionalLockFrags = [integer()]
                 Reason = term()

              To  scale well, it is a good idea to ensure that the records are evenly distributed
              over all fragments, including the new one.

              NewState is stored as hash_state among the other frag_properties.

              As  a  part  of  the  add_frag  procedure,  Mnesia  iterates  over  all   fragments
              corresponding       to       the       IterFrags       numbers      and      starts
              key_to_frag_number(NewState,RecordKey) for each record. If the new fragment differs
              from the old fragment, the record is moved to the new fragment.

              As  the add_frag procedure is a part of a schema transaction, Mnesia acquires write
              locks on the  affected  tables.  That  is,  both  the  fragments  corresponding  to
              IterFrags and those corresponding to AdditionalLockFrags.

       del_frag(State) -> {NewState, IterFrags, AdditionalLockFrags} | abort(Reason)

              Types:

                 State = term()
                 NewState = term()
                 IterFrags = [integer()]
                 AdditionalLockFrags = [integer()]
                 Reason = term()

              NewState is stored as hash_state among the other frag_properties.

              As   a  part  of  the  del_frag  procedure,  Mnesia  iterates  over  all  fragments
              corresponding      to      the       IterFrags       numbers       and       starts
              key_to_frag_number(NewState,RecordKey) for each record. If the new fragment differs
              from the old fragment, the record is moved to the new fragment.

              Notice that all records in the last fragment must be moved to another fragment,  as
              the entire fragment is deleted.

              As  the del_frag procedure is a part of a schema transaction, Mnesia acquires write
              locks on the  affected  tables.  That  is,  both  the  fragments  corresponding  to
              IterFrags and those corresponding to AdditionalLockFrags.

       key_to_frag_number(State, Key) -> FragNum | abort(Reason)

              Types:

                 FragNum = integer()
                 Reason = term()

              Starts  whenever  Mnesia needs to determine which fragment a certain record belongs
              to. It is typically started at read, write, and delete.

       match_spec_to_frag_numbers(State, MatchSpec) -> FragNums | abort(Reason)

              Types:

                 MatcSpec = ets_select_match_spec()
                 FragNums = [FragNum]
                 FragNum = integer()
                 Reason = term()

              This function is called whenever Mnesia needs to  determine  which  fragments  that
              need  to  be  searched  for  a  MatchSpec.  It  is  typically  called by select and
              match_object.

SEE ALSO

       mnesia(3erl)