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. The classic example is the bank account where clients can deposit and withdraw, in which each operation needs to be atomic. For detailed discussion on the topic see this JavaOne presentation.
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 it raises the abstraction level from lock-based concurrency.
Akka's Transactors combine Actors and STM to provide the best of the Actor model (concurrency and asynchronous event-based programming) and STM (compositional transactional shared state) by providing transactional, compositional, asynchronous, event-based message flows.
When you really need composable message flows across many actors updating their internal local state but need them to do that atomically in one big transaction. Might not be often but when you do need this then you are screwed without it.
When you want to share a datastructure across actors.
You can combine Actors and STM in several ways. An Actor may use STM internally so that particular changes are guaranteed to be atomic. Actors may also share transactional datastructures as the STM provides safe shared state across threads.
It's also possible to coordinate transactions across Actors or threads so that either the transactions in a set all commit successfully or they all fail. This is the focus of Transactors and the explicit support for coordinated transactions in this section.
Akka provides an explicit mechanism for coordinating transactions across actors. Under the hood it uses a CommitBarrier, similar to a CountDownLatch.
Here is an example of coordinating two simple counter UntypedActors so that they both increment together in coordinated transactions. If one of them was to fail to increment, the other would also fail.
To include another actor in the same coordinated transaction that you've created or received, use the coordinate method on that object. This will increment the number of parties involved by one and create a new Coordinated object to be sent.
To enter the coordinated transaction use the atomic method of the coordinated object, passing in a java.lang.Runnable.
The coordinated transaction will wait for the other transactions before committing. If any of the coordinated transactions fail then they all fail.
The same actor should not be added to a coordinated transaction more than once. The transaction will not be able to complete as an actor only processes a single message at a time. When processing the first message the coordinated transaction will wait for the commit barrier, which in turn needs the second message to be received to proceed.
UntypedTransactors are untyped actors that provide a general pattern for coordinating transactions, using the explicit coordination described above.
You could send this Counter transactor a Coordinated(Increment) message. If you were to send it just an Increment message it will create its own Coordinated (but in this particular case wouldn't be coordinating transactions with any other transactors).
To coordinate with other transactors override the coordinate method. The coordinate method maps a message to a set of SendTo objects, pairs of ActorRef and a message. You can use the include and sendTo methods to easily coordinate with other transactors.
Here's an example of coordinating an increment, using an untyped transactor, similar to the explicitly coordinated example above.
To execute directly before or after the coordinated transaction, override the before and after methods. They do not execute within the transaction.
To completely bypass coordinated transactions override the normally method. Any message matched by normally will not be matched by the other methods, and will not be involved in coordinated transactions. In this method you can implement normal actor behavior, or use the normal STM atomic for local transactions.