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.TH mnesia_frag_hash 3 "mnesia 4.4.3" "Ericsson AB" "ERLANG MODULE DEFINITION"
.SH MODULE
mnesia_frag_hash \- Defines mnesia_frag_hash callback behaviour
.SH DESCRIPTION
.LP
The module \fImnesia_frag_hash\fR defines a callback behaviour for user defined hash functions of fragmented tables\&.
.LP
Which module that is selected to implement the \fImnesia_frag_hash\fR behaviour for a particular fragmented table is specified together with the other \fIfrag_properties\fR\&. The \fIhash_module\fR defines the module name\&. The \fIhash_state\fR defines the initial hash state\&.
.LP
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
.SH EXPORTS
.LP
.B
init_state(Tab, State) -> NewState | abort(Reason)
.br
.RS
.TP
Types
Tab = atom()
.br
State = term()
.br
NewState = term()
.br
Reason = term()
.br
.RE
.RS
.LP
This function is invoked when a fragmented table is created with \fImnesia:create_table/2\fR or when a normal (un-fragmented) table is converted to be a fragmented table with \fImnesia:change_table_frag/2\fR\&.
.LP
Note that the \fIadd_frag/2\fR 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\&.
.LP
\fIState\fR is the initial value of the \fIhash_state\fR\fIfrag_property\fR\&. The \fINewState\fR will be stored as \fIhash_state\fR among the other \fIfrag_properties\fR\&.
.RE
.LP
.B
add_frag(State) -> {NewState, IterFrags, AdditionalLockFrags} | abort(Reason)
.br
.RS
.TP
Types
State = term()
.br
NewState = term()
.br
IterFrags = [integer()]
.br
AdditionalLockFrags = [integer()]
.br
Reason = term()
.br
.RE
.RS
.LP
In order to scale well, it is a good idea ensure that the records are evenly distributed over all fragments including the new one\&.
.LP
The \fINewState\fR will be stored as \fIhash_state\fR among the other \fIfrag_properties\fR\&.
.LP
As a part of the \fIadd_frag\fR procedure, Mnesia will iterate over all fragments corresponding to the \fIIterFrags\fR numbers and invoke \fIkey_to_frag_number(NewState, RecordKey)\fR for each record\&. If the new fragment differs from the old fragment, the record will be moved to the new fragment\&.
.LP
As the \fIadd_frag\fR 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 \fIIterFrags\fR and those corresponding to \fIAdditionalLockFrags\fR\&.
.RE
.LP
.B
del_frag(State) -> {NewState, IterFrags, AdditionalLockFrags} | abort(Reason)
.br
.RS
.TP
Types
State = term()
.br
NewState = term()
.br
IterFrags = [integer()]
.br
AdditionalLockFrags = [integer()]
.br
Reason = term()
.br
.RE
.RS
.LP
The \fINewState\fR will be stored as \fIhash_state\fR among the other \fIfrag_properties\fR\&.
.LP
As a part of the \fIdel_frag\fR procedure, Mnesia will iterate over all fragments corresponding to the \fIIterFrags\fR numbers and invoke \fIkey_to_frag_number(NewState, RecordKey)\fR for each record\&. If the new fragment differs from the old fragment, the record will be moved to the new fragment\&.
.LP
Note that all records in the last fragment must be moved to another fragment as the entire fragment will be deleted\&.
.LP
As the \fIdel_frag\fR 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 \fIIterFrags\fR and those corresponding to \fIAdditionalLockFrags\fR\&.
.RE
.LP
.B
key_to_frag_number(State, Key) -> FragNum | abort(Reason)
.br
.RS
.TP
Types
FragNum = integer()()
.br
Reason = term()
.br
.RE
.RS
.LP
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\&.
.RE
.LP
.B
match_spec_to_frag_numbers(State, MatchSpec) -> FragNums | abort(Reason)
.br
.RS
.TP
Types
MatcSpec = ets_select_match_spec()
.br
FragNums = [FragNum]
.br
FragNum = integer()
.br
Reason = term()
.br
.RE
.RS
.LP
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\&.
.RE
.SH SEE ALSO
.LP
mnesia(3)
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