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Linux 2.6 - man page for mnesia_frag_hash (linux section 3erl)

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  frag-
       ments 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_ta-
	      ble/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 dis-
	      tributed 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  corre-
	      sponding 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 corre-
	      sponding 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 cer-
	      tain 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 4.4.17 		   mnesia_frag_hash(3erl)


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