STM in Haskell
- 1. Software Transactional Memory Steve Severance – Alpha Heavy Industries
- 2. Motivation • Innovations in concurrency has not kept pace • Still using locks • Immutability has helped in Haskell
- 3. STM Basics • Transactional Concurrency • Atomicity – Results are visible all at once • Isolation – Action unaffected by other threads • Everything you need is in the stm-2.4.2 package
- 4. Haskell and STM • Exploits Three Haskell Features: – Purity (Transactions must be pure) – Monads – Immutability (Values are never modified) • Utilizes the Type System – Anything involved in the transaction is marked (TVar, TChan, STM, etc…)
- 5. Basic Transaction Pattern • atomically $ do … • atomically evaluates the transaction main :: IO () main = do var <- newTVarIO False atomically $ do val <- readTVar var if val == False then writeTVar var True else return ()
- 6. Primitives • TVar (Always has a Value) • TMVar (Analogous to Regular MVar) • TChan (Multicast Capabilities) • TQueue (FIFO Queue) • TBQueue (Bounded Queue) • TSem (Semaphore)
- 7. Retrying • Use the retry function to retry the transaction • The transaction will only be run when one of the inputs has changed withdrawFunds :: TVar Account -> Money -> STM Money withdrawFunds accountVar amount = do account <- readTVar accountVar if (aBalance account) > amount then writeTVar accountVar account{aBalance = (aBalance account) `subtract` amount} >> return amount else retry
- 8. Choices • orElse runs a second option if the first retrys withdrawEvil :: TVar Account -> TChan Account -> Money -> STM Money withdrawEvil accountVar nsfChan amount = do withdrawFunds accountVar amount `orElse` withdrawNonSufficientFunds accountVar nsfChan amount
- 9. Using Alternative • You always wanted to use those cool operators • Use <|> instead of orElse
- 10. Dealing with Exceptions • Revisiting the Atomic Guarantee – No need for rollback unlike MVar and Chan • STM can throw and catch
- 11. Bounded Queues • TBQueue limits its size • Writers will block until space becomes available • Useful for performance and memory management
- 12. Being Strict • Use NFData (deepseq) • We internally use special write functions to keep work from leaking across thread boundaries • Transaction TimewriteTVar’ :: NFData a => TVar a -> a -> STM () {-# INLINE writeTVar' #-} writeTVar' var val = rnf val `seq` writeTVar var val
- 13. Other Languages • Clojure • Scala • C++ • C#/.Net
- 14. References • Composable Memory Transactions • Beautiful Concurrency • STM Retrospective for .Net • Gists: – Sample 1