Dear hAkkers,

we—the Akka committers—proudly present the first development milestone for Akka 2.4. Since the release of Akka 2.3.0 (already 14.5 months ago) much has happened, in particular around Streams & HTTP. It may therefore surprise you that these additions to the Akka toolkit are not yet part of the 2.4 development branch, but if you read on for a little while the pieces of the puzzle will fall into place. Besides a plethora of small improvements the main changes relative to the 2.3.x series are:

we dropped support for Java 6 & 7 as announced in the last roadmap update, requiring now Java 8 or later

we dropped support for Scala 2.10, kept 2.11 and added 2.12 (which is at milestone 1 currently as well); this is in keeping with our policy to support the Scala version that is “current” when the first milestone comes out plus any later version that is published during this series’ lifetime

we added the experimental Akka Typed module previously codenamed Project Gålbma, a new way of formulating Actor interactions with full type-system support

we promoted the ClusterSingleton, ClusterClient, DistributedPubSub and ClusterSharding patterns to fully supported Cluster tools

ClusterSharding also learnt a few new tricks (asynchronous shard allocation, reviving entries after migration [thanks to Dominic Black], and more flexible use of roles [thanks to Richard Marscher])

Akka Persistence can now use multiple different Journals within one ActorSystem (thanks to Andrei Pozolotin, who also split out ClusterMetrics into their own module)

we also removed the default Journal plugin and removed the hard dependency on LevelDB (since that was not a recommended production configuration anyway)

we added support for binding to a different host & port than what the ActorSystem advertises to its peers, a.k.a. “allow Akka to run in Docker containers”

the SLF4J logging adapter can now apply the log filtering rules prior to sending to the EventStream, see the migration guide

but the biggest feature is probably that Akka 2.4 will be binary backwards compatible with Akka 2.3, see the detailed description below.

Being binary compatible means that applications and libraries built on top of Akka 2.3.x continue to work with Akka 2.4.x without recompilation (subject to the conditions below), which implies that Akka Streams & HTTP as well as the upcoming Play Framework 2.4 can be combined with Akka 2.4.

Akka 2.4 Roadmap

What remains to be done before we can release 2.4.0-RC1 is to

complete the Java API and the documentation for Akka Typed

finalize Akka Persistence on the Write Side (i.e. PersistentActor will be non-experimental; the work on the Read Side will be deferred until Akka 2.5)

incorporate Streams & HTTP once ready (to become an experimental module, as usual)

incorporate Patrik’s akka-data-replication under the new module name akka-distributed-data

We will perform this work within the next weeks, releasing further milestones when appropriate. Please do what you usually do so well: try out our latest and report back when things break, not work as advertised, feel strange, or even when you are happy :-) Especially concerning binary compatibility we will need help from the community (you!) since we cannot run all possible programs ourselves; we base our BC efforts on the MiMa plugin but that is no perfect guarantee that everything will work out of the box.

Binary Compatibility

Akka 2.4.x is backwards binary compatible with previous 2.3.x versions (exceptions listed below). This means that the new JARs are a drop-in replacement for the old one (but not the other way around) as long as your build does not enable the inliner (Scala-only restriction). It should be noted that Scala 2.11.x is is not binary compatible with Scala 2.10.x, which means that Akka’s binary compatibility property only holds between versions that were built for a given Scala version— akka-actor_2.11-2.4-M1.jar is compatible with akka-actor_2.11-2.3.11.jar but not with akka-actor_2.10-2.3.11.jar .

Binary compatibility is not maintained for the following:

akka-testkit and akka-multi-node-testkit

experimental modules, such as akka-persistence and akka-contrib

features, classes, methods that were deprecated in 2.3.x and removed in 2.4.x

The dependency to Netty has been updated from version 3.8.0.Final to 3.10.3.Final. The changes in those versions might not be fully binary compatible, but we believe that it will not be a problem in practice. No changes were needed to the Akka source code for this update. Users of libraries that depend on 3.8.0.Final that break with 3.10.3.Final should be able to manually downgrade the dependency to 3.8.0.Final and Akka will still work with that version.

The dependency to Typesafe Config has been updated from 1.2.1 to 1.3.0 which should be binary compatible for the vast majority users, except for obscure edge cases as its changelog points out. This change was made in order in order to use new JDK8 specific features in the library as well as to align Akka with Play which is now also depending on 1.3.0.

When migrating a code base to 2.4 please refer to the migration guide in order to profit from some of the improvements.

General Remarks

We are extraordinarily proud of the long list of contributors to this release, looking at the commit history we find more than 100 names! We are particularly thankful to hepin1989 (a.k.a. kerr) who contributed already 19 pull requests to Akka. Other noteworthy mentions besides Andrei Pozolotin’s, Dominic Black’s and Richard Marscher’s (which are already listed above) are:

Improve remote watching mechanism by Thibaut Robert

Improve AtLeastOnceDelivery by Adam Warski

Get current topics in DistributedPubSub by Ben Poserow

The complete list of closed tickets can be found in the 2.4-M1 github issues milestone.

For the full stats see the announcement on the website.

Happy hakking!