mnesia_frag_hash(3erl)					     Erlang Module Definition					    mnesia_frag_hash(3erl)

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_proper-
	      ties .

       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 typi-
	      cally invoked at select and match_object.

SEE ALSO

       mnesia(3erl)

Ericsson AB							   mnesia 4.4.17					    mnesia_frag_hash(3erl)