# mnesia_frag_hash

### MODULE

mnesia_frag_hash

### MODULE SUMMARY

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(3)

mnesia 4.4.3
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