# 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)

### AUTHORS

Håkan Mattsson - support@erlang.ericsson.se

mnesia 4.3.1
Copyright © 1991-2006 Ericsson AB