Data.State.Acid.Local

AcidState container using a transaction log on disk. The term 'Event' is loosely used for transactions with ACID guarantees. 'Method' is loosely used for state operations without ACID guarantees (see Data.State.Acid.Core).

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. This includes power outages,

Isolation

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

Durability

Successful transaction are guaranteed to survive system failure (both hardware and software).

data Event st where

We distinguish between events that modify the state and those that do not.

UpdateEvents are executed in a MonadState context and have to be serialized to disk before they are considered durable.

QueryEvents are executed in a MonadReader context and obviously do not have to be serialized to disk.

Constructors

UpdateEvent :: UpdateEvent ev => (ev -> Update st (EventResult ev)) -> Event st

QueryEvent :: QueryEvent ev => (ev -> Query st (EventResult ev)) -> Event st

type EventResult ev = MethodResult ev

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

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

MonadState st (Update st)

Monad (Update st)

data Query st a

Context monad for Query events.

Instances

MonadReader st (Query st)

Monad (Query st)

mkAcidState

Arguments

:: (Typeable st, Binary st)

=> [Event st]

List of events capable of updating or querying the state.

-> st

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

-> IO (AcidState st)

Create an AcidState given a list of events (aka. transactions) and an initial value.

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

closeAcidState :: AcidState st -> IO ()

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

createCheckpoint :: Binary st => 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 st -> 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 st -> event -> IO (EventResult event)

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