acid-state-0.5.0: Add ACID guarantees to any serializable Haskell data structure.

Portabilitynon-portable (uses GHC extensions)
Maintainerlemmih@gmail.com

Data.Acid.Local

Description

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.Acid.Core).

Synopsis

Documentation

class SafeCopy st => IsAcidic st whereSource

Methods

acidEventsSource

Arguments

:: [Event st]

List of events capable of updating or querying the state.

data AcidState st Source

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 system failure (both hardware and software).

data Event st whereSource

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

type EventResult ev = MethodResult evSource

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

class Method ev => UpdateEvent ev Source

All UpdateEvents are also Methods.

class Method ev => QueryEvent ev Source

All QueryEvents are also Methods.

data Update st a Source

Context monad for Update events.

Instances

data Query st a Source

Context monad for Query events.

Instances

openAcidStateSource

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.

openAcidStateFromSource

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 ()Source

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

createCheckpoint :: SafeCopy st => AcidState st -> IO ()Source

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.

createCheckpointAndClose :: SafeCopy st => AcidState st -> IO ()Source

Save a snapshot to disk and close the AcidState as a single atomic action. This is useful when you want to make sure that no events are saved to disk after a checkpoint.

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

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.

scheduleUpdate :: UpdateEvent event => AcidState (EventState event) -> event -> IO (MVar (EventResult event))Source

Issue an Update event and return immediately. The event is not durable before the MVar has been filled but the order of events is honored. The behavior in case of exceptions is exactly the same as for update.

If EventA is scheduled before EventB, EventA will be executed before EventB:

do scheduleUpdate acid EventA
   scheduleUpdate acid EventB

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

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

update' :: (UpdateEvent event, MonadIO m) => AcidState (EventState event) -> event -> m (EventResult event)Source

Same as update but lifted into any monad capable of doing IO.

query' :: (QueryEvent event, MonadIO m) => AcidState (EventState event) -> event -> m (EventResult event)Source

Same as query but lifted into any monad capable of doing IO.

runQuery :: Query st a -> Update st aSource

Run a query in the Update Monad.