1mnesia_frag_hash(3)        Erlang Module Definition        mnesia_frag_hash(3)
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NAME

6       mnesia_frag_hash - Defines mnesia_frag_hash callback behavior
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DESCRIPTION

9       This module defines a callback behavior for user-defined hash functions
10       of fragmented tables.
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12       Which module that is selected to implement the mnesia_frag_hash  behav‐
13       ior  for  a  particular fragmented table is specified together with the
14       other frag_properties. The hash_module defines  the  module  name.  The
15       hash_state defines the initial hash state.
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17       This module implements dynamic hashing, which is a kind of hashing that
18       grows nicely when new fragments are added. It is well suited for  scal‐
19       able hash tables.
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EXPORTS

22       init_state(Tab, State) -> NewState | abort(Reason)
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24              Types:
25
26                 Tab = atom()
27                 State = term()
28                 NewState = term()
29                 Reason = term()
30
31              Starts when a fragmented table is created with the function mne‐
32              sia:create_table/2 or when a normal (unfragmented) table is con‐
33              verted to be a fragmented table with mnesia:change_table_frag/2.
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35              Notice that the function add_frag/2 is started one time for each
36              of the other fragments (except number 1) as a part of the  table
37              creation procedure.
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39              State is the initial value of the hash_state frag_property. New‐
40              State is stored as hash_state among the other frag_properties.
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42       add_frag(State)  ->  {NewState,   IterFrags,   AdditionalLockFrags}   |
43       abort(Reason)
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45              Types:
46
47                 State = term()
48                 NewState = term()
49                 IterFrags = [integer()]
50                 AdditionalLockFrags = [integer()]
51                 Reason = term()
52
53              To  scale well, it is a good idea to ensure that the records are
54              evenly distributed over all fragments, including the new one.
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56              NewState is stored as hash_state among  the  other  frag_proper‐
57              ties.
58
59              As  a  part  of the add_frag procedure, Mnesia iterates over all
60              fragments corresponding to  the  IterFrags  numbers  and  starts
61              key_to_frag_number(NewState,RecordKey)  for  each record. If the
62              new fragment differs from the old fragment, the record is  moved
63              to the new fragment.
64
65              As  the  add_frag  procedure  is a part of a schema transaction,
66              Mnesia acquires write locks on the  affected  tables.  That  is,
67              both  the  fragments corresponding to IterFrags and those corre‐
68              sponding to AdditionalLockFrags.
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70       del_frag(State)  ->  {NewState,   IterFrags,   AdditionalLockFrags}   |
71       abort(Reason)
72
73              Types:
74
75                 State = term()
76                 NewState = term()
77                 IterFrags = [integer()]
78                 AdditionalLockFrags = [integer()]
79                 Reason = term()
80
81              NewState  is  stored  as hash_state among the other frag_proper‐
82              ties.
83
84              As a part of the del_frag procedure, Mnesia  iterates  over  all
85              fragments  corresponding  to  the  IterFrags  numbers and starts
86              key_to_frag_number(NewState,RecordKey) for each record.  If  the
87              new  fragment differs from the old fragment, the record is moved
88              to the new fragment.
89
90              Notice that all records in the last fragment must  be  moved  to
91              another fragment, as the entire fragment is deleted.
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93              As  the  del_frag  procedure  is a part of a schema transaction,
94              Mnesia acquires write locks on the  affected  tables.  That  is,
95              both  the  fragments corresponding to IterFrags and those corre‐
96              sponding to AdditionalLockFrags.
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98       key_to_frag_number(State, Key) -> FragNum | abort(Reason)
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100              Types:
101
102                 FragNum = integer()
103                 Reason = term()
104
105              Starts whenever Mnesia needs to determine which fragment a  cer‐
106              tain  record belongs to. It is typically started at read, write,
107              and delete.
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109       match_spec_to_frag_numbers(State, MatchSpec) -> FragNums  |  abort(Rea‐
110       son)
111
112              Types:
113
114                 MatcSpec = ets_select_match_spec()
115                 FragNums = [FragNum]
116                 FragNum = integer()
117                 Reason = term()
118
119              This function is called whenever Mnesia needs to determine which
120              fragments that need to be searched for a MatchSpec. It is  typi‐
121              cally called by select and match_object.
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SEE ALSO

124       mnesia(3)
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128Ericsson AB                       mnesia 4.20              mnesia_frag_hash(3)
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