Akka and the Java Memory Model
A major benefit of using the Typesafe Stack, including Scala and Akka, is that it simplifies the process of writing concurrent software. This article discusses how the Typesafe Stack, and Akka in particular, approaches shared memory in concurrent applications.
The Java Memory Model
Prior to Java 5, the Java Memory Model (JMM) was ill defined. It was possible to get all kinds of strange results when shared memory was accessed by multiple threads, such as:
- a thread not seeing values written by other threads: a visibility problem
- a thread observing 'impossible' behavior of other threads, caused by instructions not being executed in the order expected: an instruction reordering problem.
With the implementation of JSR 133 in Java 5, a lot of these issues have been resolved. The JMM is a set of rules based on the "happens-before" relation, which constrain when one memory access must happen before another, and conversely, when they are allowed to happen out of order. Two examples of these rules are:
- The monitor lock rule: a release of a lock happens before every subsequent acquire of the same lock.
- The volatile variable rule: a write of a volatile variable happens before every subsequent read of the same volatile variable
Although the JMM can seem complicated, the specification tries to find a balance between ease of use and the ability to write performant and scalable concurrent data structures.
Actors and the Java Memory Model
With the Actors implementation in Akka, there are two ways multiple threads can execute actions on shared memory:
- if a message is sent to an actor (e.g. by another actor). In most cases messages are immutable, but if that message is not a properly constructed immutable object, without a "happens before" rule, it would be possible for the receiver to see partially initialized data structures and possibly even values out of thin air (longs/doubles).
- if an actor makes changes to its internal state while processing a message, and accesses that state while processing another message moments later. It is important to realize that with the actor model you don't get any guarantee that the same thread will be executing the same actor for different messages.
To prevent visibility and reordering problems on actors, Akka guarantees the following two "happens before" rules:
- The actor send rule: the send of the message to an actor happens before the receive of that message by the same actor.
- The actor subsequent processing rule: processing of one message happens before processing of the next message by the same actor.
In layman's terms this means that changes to internal fields of the actor are visible when the next message is processed by that actor. So fields in your actor need not be volatile or equivalent.
Both rules only apply for the same actor instance and are not valid if different actors are used.
Futures and the Java Memory Model
The completion of a Future "happens before" the invocation of any callbacks registered to it are executed.
We recommend not to close over non-final fields (final in Java and val in Scala), and if you do choose to close over non-final fields, they must be marked volatile in order for the current value of the field to be visible to the callback.
If you close over a reference, you must also ensure that the instance that is referred to is thread safe. We highly recommend staying away from objects that use locking, since it can introduce performance problems and in the worst case, deadlocks. Such are the perils of synchronized.
STM and the Java Memory Model
Akka's Software Transactional Memory (STM) also provides a "happens before" rule:
- The transactional reference rule: a successful write during commit, on an transactional reference, happens before every subsequent read of the same transactional reference.
This rule looks a lot like the 'volatile variable' rule from the JMM. Currently the Akka STM only supports deferred writes, so the actual writing to shared memory is deferred until the transaction commits. Writes during the transaction are placed in a local buffer (the writeset of the transaction) and are not visible to other transactions. That is why dirty reads are not possible.
How these rules are realized in Akka is an implementation detail and can change over time, and the exact details could even depend on the used configuration. But they will build on the other JMM rules like the monitor lock rule or the volatile variable rule. This means that you, the Akka user, do not need to worry about adding synchronization to provide such a "happens before" relation, because it is the responsibility of Akka. So you have your hands free to deal with your business logic, and the Akka framework makes sure that those rules are guaranteed on your behalf.