### begriffs

Steve Severance has used Software Transactional Memory (STM) pretty intensively in trading and web crawling applications. He shares things he’s learned, some gotchas, and a comparison of the way STM works in Haskell vs other languages.

### Summary

• Motivation for STM
• CPUs are getting more cores yet our tools remain fairly primitive
• Locks and lock convoys
• Immutability has helped Haskell some with this
• What is STM? It is in-memory transactional concurrency that gives us two properties
• Atomicity: all results of your transaction will be visible at once
• Isolation of underlying primitives
• Haskell is the best language in the world in which to implement an STM
• It was originally 100 lines of code, and even now it’s only about 600 lines of C!
• It exploits three properties of the language: purity, immutability and laziness
• Learning about STM was when Steve first started to appreciate laziness; up until that point laziness had only hurt, never helped
• Haskell retries an STM transaction as needed. Immutability allows it to do this fearlessly, and laziness avoids fully evaluating the transaction
• STM is harder in other languages. For instance in Clojure you have to manually implement rollback functionality.
• There are some helpful auxiliary packages such as stm-chans
• How about performing unsafe IO in the STM monad? Never! If you’re wanting to do this you’re “doing it wrong.”
• Discussion of the primitives TVar, TMVar, TChan, TQueue, TBQueue, TSem
• How the retry function works by putting our thread to sleep
• Using orElse and the Alternative typeclass
• How often does a transaction block get re-run? Does it get inefficient? Use the stm-stats package to see how well your transactions are performing.
• The asyc package internally uses STM and has great integration with it
• Other languages have STM of varying convenience. Clojure, Scala, C++, C#
• The C# team spent about three years trying to write an STM. They gave up and wrote a retrospective on why it was so hard.
• Side effects make STM hellish in languages other than Haskell.
• Q&A