Actor v.s. STM

Actors are excellent for solving problems where you have many independent processes that can work in isolation and only interact with other Actors through message passing. This model fits many problems. But the Actor model is unfortunately a terrible model for implementing truly shared state. E.g. when you need to have consensus and a stable view of state across many components.

Software Transactional Memory (STM) on the other hand is excellent for problems where you need consensus and a stable view of the state by providing compositional transactional shared state. Some of the really nice traits of STM are that transactions compose and that it raises the abstraction level from lock-based concurrency.

Akka allows you to combine Actors and STM into what we call Transactors (short for Transactional Actors), these allow you to optionally combine Actors and STM provides IMHO the best of the Actor model (simple concurrency and asynchronous event-based programming) andSTM (compositional transactional shared state) by providing transactional, compositional, asynchronous, event-based message flows. You don’t need Transactors all the time but when you do need them then you really need them.

Reference: http://jonasboner.com/2010/01/04/introducing-akka.html