NAME

       mnesia_frag_hash - Defines mnesia_frag_hash callback behaviour

DESCRIPTION

       The  module  mnesia_frag_hash  defines a callback behaviour for user defined hash functions of fragmented
       tables.

       Which module that is selected to implement the mnesia_frag_hash behaviour  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.

       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

EXPORTS

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

              Types:

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

              This  function  is invoked when a fragmented table is created with mnesia:create_table/2 or when a
              normal   (un-fragmented)   table   is    converted    to    be    a    fragmented    table    with
              mnesia:change_table_frag/2.

              Note that the add_frag/2 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.

              State  is  the  initial  value  of  the  hash_state  frag_property. The NewState will be 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()

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

              The NewState will be stored as hash_state among the other frag_properties.

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

              As  the  add_frag 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 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()

              The NewState will be stored as hash_state among the other frag_properties.

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

              Note that all records in the last fragment must  be  moved  to  another  fragment  as  the  entire
              fragment will be deleted.

              As  the  del_frag 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 IterFrags and those corresponding
              to AdditionalLockFrags.

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

              Types:

                 FragNum = integer()()
                 Reason = term()

              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.

       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  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.

SEE ALSO

       mnesia(3erl)

Ericsson AB                                        mnesia 4.11                            mnesia_frag_hash(3erl)