Data.Acid

AcidState container using a transaction log on disk.

To see how it all fits together, have a look at these example http://mirror.seize.it/acid-state/examples/.

Documentation

data AcidState st

State container offering full ACID (Atomicity, Consistency, Isolation and Durability) guarantees.

Atomicity

State changes are all-or-nothing. This is what you'd expect of any state variable in Haskell and AcidState doesn't change that.

Consistency

No event or set of events will break your data invariants.

Isolation

Transactions cannot interfere with each other even when issued in parallel.

Durability

Successful transaction are guaranteed to survive unexpected system shutdowns (both those caused by hardware and software).

openLocalState

Arguments

:: (Typeable st, IsAcidic st)

=> st

Initial state value. This value is only used if no checkpoint is found.

-> IO (AcidState st)

Create an AcidState given an initial value.

This will create or resume a log found in the "state/[typeOf state]/" directory.

openLocalStateFrom

Arguments

:: IsAcidic st

=> FilePath

Location of the checkpoint and transaction files.

-> st

Initial state value. This value is only used if no checkpoint is found.

-> IO (AcidState st)

Create an AcidState given a log directory and an initial value.

This will create or resume a log found in directory. Running two AcidState's from the same directory is an error but will not result in dataloss.

closeAcidState :: AcidState st -> IO ()

Close an AcidState and associated resources. Any subsequent usage of the AcidState will throw an exception.

createCheckpoint :: AcidState st -> IO ()

Take a snapshot of the state and save it to disk. Creating checkpoints makes it faster to resume AcidStates and you're free to create them as often or seldom as fits your needs. Transactions can run concurrently with this call.

This call will not return until the operation has succeeded.

update :: UpdateEvent event => AcidState (EventState event) -> event -> IO (EventResult event)

Issue an Update event and wait for its result. Once this call returns, you are guaranteed that the changes to the state are durable. Events may be issued in parallel.

It's a run-time error to issue events that aren't supported by the AcidState.

query :: QueryEvent event => AcidState (EventState event) -> event -> IO (EventResult event)

Issue a Query event and wait for its result. Events may be issued in parallel.

type EventResult ev = MethodResult ev

Events return the same thing as Methods. The exact type of EventResult depends on the event.

type EventState ev = MethodState ev

class Method ev => UpdateEvent ev

All UpdateEvents are also Methods.

class Method ev => QueryEvent ev

All QueryEvents are also Methods.

data Update st a

Context monad for Update events.

Instances

Monad (Update st) => MonadState st (Update st)

Monad (Update st)

Functor (Update st)

Functor (Update st) => Applicative (Update st)

data Query st a

Context monad for Query events.

Instances

Monad (Query st) => MonadReader st (Query st)

Monad (Query st)

Functor (Query st)

Functor (Query st) => Applicative (Query st)

class SafeCopy st => IsAcidic st

makeAcidic :: Name -> [Name] -> Q [Dec]

Create the control structures required for acid states using Template Haskell.

This code:

myUpdate :: Argument -> Update State Result
myUpdate arg = ...

myQuery :: Argument -> Query State Result
myQuery arg = ...

$(makeAcidic ''State ['myUpdate, 'myQuery])

will make State an instance of IsAcidic and provide the following events:

data MyUpdate = MyUpdate Argument
data MyQuery  = MyQuery Argument

runQuery :: Query st a -> Update st a

Run a query in the Update Monad.