acid-state-0.6.6: src/Data/Acid/Local.hs
{-# LANGUAGE DeriveDataTypeable, BangPatterns #-}
-----------------------------------------------------------------------------
-- |
-- Module : Data.Acid.Local
-- Copyright : PublicDomain
--
-- Maintainer : lemmih@gmail.com
-- Portability : non-portable (uses GHC extensions)
--
-- 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").
--
module Data.Acid.Local
( openLocalState
, openLocalStateFrom
, createArchive
, createCheckpointAndClose
) where
import Data.Acid.Log as Log
import Data.Acid.Core
import Data.Acid.Common
import Data.Acid.Abstract
import Control.Concurrent ( newEmptyMVar, putMVar, takeMVar, MVar )
--import Control.Exception ( evaluate )
import Control.Monad.State ( runState )
import Control.Applicative ( (<$>), (<*>) )
import Data.ByteString.Lazy ( ByteString )
--import qualified Data.ByteString.Lazy as Lazy ( length )
import Data.Serialize ( runPutLazy, runGetLazy )
import Data.SafeCopy ( SafeCopy(..), safeGet, safePut
, primitive, contain )
import Data.Typeable ( Typeable, typeOf )
import Data.IORef
import System.FilePath ( (</>) )
{-| 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 LocalState st
= LocalState { localCore :: Core st
, localCopy :: IORef st
, localEvents :: FileLog (Tagged ByteString)
, localCheckpoints :: FileLog Checkpoint
} deriving (Typeable)
-- | 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
-- @
scheduleLocalUpdate :: UpdateEvent event => LocalState (EventState event) -> event -> IO (MVar (EventResult event))
scheduleLocalUpdate acidState event
= do mvar <- newEmptyMVar
let encoded = runPutLazy (safePut event)
--evaluate (Lazy.length encoded) -- It would be best to encode the event before we lock the core
-- but it hurts performance /-:
modifyCoreState_ (localCore acidState) $ \st ->
do let !(result, !st') = runState hotMethod st
-- Schedule the log entry. Very important that it happens when 'localCore' is locked
-- to ensure that events are logged in the same order that they are executed.
pushEntry (localEvents acidState) (methodTag event, encoded) $ do writeIORef (localCopy acidState) st'
putMVar mvar result
return st'
return mvar
where hotMethod = lookupHotMethod (coreMethods (localCore acidState)) event
scheduleLocalColdUpdate :: LocalState st -> Tagged ByteString -> IO (MVar ByteString)
scheduleLocalColdUpdate acidState event
= do mvar <- newEmptyMVar
modifyCoreState_ (localCore acidState) $ \st ->
do let !(result, !st') = runState coldMethod st
-- Schedule the log entry. Very important that it happens when 'localCore' is locked
-- to ensure that events are logged in the same order that they are executed.
pushEntry (localEvents acidState) event $ do writeIORef (localCopy acidState) st'
putMVar mvar result
return st'
return mvar
where coldMethod = lookupColdMethod (localCore acidState) event
-- | Issue a Query event and wait for its result. Events may be issued in parallel.
localQuery :: QueryEvent event => LocalState (EventState event) -> event -> IO (EventResult event)
localQuery acidState event
= do st <- readIORef (localCopy acidState)
let (result, _st) = runState hotMethod st
return result
where hotMethod = lookupHotMethod (coreMethods (localCore acidState)) event
-- Whoa, a buttload of refactoring is needed here. 2011-11-02
localQueryCold :: LocalState st -> Tagged ByteString -> IO ByteString
localQueryCold acidState event
= do st <- readIORef (localCopy acidState)
let (result, _st) = runState coldMethod st
return result
where coldMethod = lookupColdMethod (localCore acidState) event
-- | 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.
createLocalCheckpoint :: SafeCopy st => LocalState st -> IO ()
createLocalCheckpoint acidState
= do mvar <- newEmptyMVar
withCoreState (localCore acidState) $ \st ->
do eventId <- askCurrentEntryId (localEvents acidState)
pushAction (localEvents acidState) $
do let encoded = runPutLazy (safePut st)
pushEntry (localCheckpoints acidState) (Checkpoint eventId encoded) (putMVar mvar ())
takeMVar mvar
-- | 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.
createCheckpointAndClose :: SafeCopy st => AcidState st -> IO ()
createCheckpointAndClose abstract_state
= do mvar <- newEmptyMVar
closeCore' (localCore acidState) $ \st ->
do eventId <- askCurrentEntryId (localEvents acidState)
pushAction (localEvents acidState) $
pushEntry (localCheckpoints acidState) (Checkpoint eventId (runPutLazy (safePut st))) (putMVar mvar ())
takeMVar mvar
closeFileLog (localEvents acidState)
closeFileLog (localCheckpoints acidState)
where acidState = downcast abstract_state
data Checkpoint = Checkpoint EntryId ByteString
instance SafeCopy Checkpoint where
kind = primitive
putCopy (Checkpoint eventEntryId content)
= contain $
do safePut eventEntryId
safePut content
getCopy = contain $ Checkpoint <$> safeGet <*> safeGet
-- | Create an AcidState given an initial value.
--
-- This will create or resume a log found in the \"state\/[typeOf state]\/\" directory.
openLocalState :: (Typeable st, IsAcidic st)
=> st -- ^ Initial state value. This value is only used if no checkpoint is
-- found.
-> IO (AcidState st)
openLocalState initialState
= openLocalStateFrom ("state" </> show (typeOf initialState)) initialState
-- | 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.
openLocalStateFrom :: (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)
openLocalStateFrom directory initialState
= do core <- mkCore (eventsToMethods acidEvents) initialState
let eventsLogKey = LogKey { logDirectory = directory
, logPrefix = "events" }
checkpointsLogKey = LogKey { logDirectory = directory
, logPrefix = "checkpoints" }
mbLastCheckpoint <- Log.newestEntry checkpointsLogKey
n <- case mbLastCheckpoint of
Nothing
-> return 0
Just (Checkpoint eventCutOff content)
-> do modifyCoreState_ core (\_oldState -> case runGetLazy safeGet content of
Left msg -> checkpointRestoreError msg
Right val -> return val)
return eventCutOff
eventsLog <- openFileLog eventsLogKey
events <- readEntriesFrom eventsLog n
mapM_ (runColdMethod core) events
ensureLeastEntryId eventsLog n
checkpointsLog <- openFileLog checkpointsLogKey
stateCopy <- newIORef undefined
withCoreState core (writeIORef stateCopy)
return $ toAcidState LocalState { localCore = core
, localCopy = stateCopy
, localEvents = eventsLog
, localCheckpoints = checkpointsLog
}
checkpointRestoreError msg
= error $ "Could not parse saved checkpoint due to the following error: " ++ msg
-- | Close an AcidState and associated logs.
-- Any subsequent usage of the AcidState will throw an exception.
closeLocalState :: LocalState st -> IO ()
closeLocalState acidState
= do closeCore (localCore acidState)
closeFileLog (localEvents acidState)
closeFileLog (localCheckpoints acidState)
-- | Move all log files that are no longer necessary for state restoration into the 'Archive'
-- folder in the state directory. This folder can then be backed up or thrown out as you see fit.
-- Reverting to a state before the last checkpoint will not be possible if the 'Archive' folder
-- has been thrown out.
--
-- This method is idempotent and does not block the normal operation of the AcidState.
createArchive :: AcidState st -> IO ()
createArchive abstract_state
= do -- We need to look at the last checkpoint saved to disk. Since checkpoints can be written
-- in parallel with this call, we can't guarantee that the checkpoint we get really is the
-- last one but that's alright.
currentCheckpointId <- cutFileLog (localCheckpoints state)
-- 'currentCheckpointId' is the ID of the next checkpoint that will be written to disk.
-- 'currentCheckpointId-1' must then be the ID of a checkpoint on disk (or -1, of course).
let durableCheckpointId = currentCheckpointId-1
checkpoints <- readEntriesFrom (localCheckpoints state) durableCheckpointId
case checkpoints of
[] -> return ()
(Checkpoint entryId _content : _)
-> do -- 'entryId' is the lowest entryId that didn't contribute to the checkpoint.
-- 'archiveFileLog' moves all files that are lower than this entryId to the archive.
archiveFileLog (localEvents state) entryId
-- In the same style as above, we archive all log files that came before the log file
-- which contains our checkpoint.
archiveFileLog (localCheckpoints state) durableCheckpointId
where state = downcast abstract_state
toAcidState :: IsAcidic st => LocalState st -> AcidState st
toAcidState local
= AcidState { _scheduleUpdate = scheduleLocalUpdate local
, scheduleColdUpdate = scheduleLocalColdUpdate local
, _query = localQuery local
, queryCold = localQueryCold local
, createCheckpoint = createLocalCheckpoint local
, closeAcidState = closeLocalState local
, acidSubState = mkAnyState local
}