Cats STM

Build Status Latest version Discord Scala Steward badge

Cats STM is a library for writing composable in-memory transactions which will handling correct locking, optimistic concurrency and automatic retries for you. It is intended as a safe, composable alternative to locks and semaphores that also allows a higher level of concurrency.

Transactional and Safe

Write Txn expressions in terms of mutable TVars (analogous to Ref from Cats Effect). Run Txn[A] expressions transactionally to obtain an IO[A] (or F[A] for the Async[F] of your choice). The STM runtime will determine what locks need to be acquired and in what order to avoid deadlock.


Trivially compose transactional expressions into larger ones without explicit locking or requiring any knowledge of the internals of the subexpressions and what locks they require.

Automatic retries

Specify pre-conditions and the committing of a transaction will be automatically retried until the pre-conditions are satisifed.

Fine-grained, optimistic concurrency

There are no global locks and per-TVar locks are acquired only when a transaction has succeeded and should be committed.