*Dear hakkers,*

we—the Akka committers—are proud to announce the FIRST RELEASE CANDIDATE 
for the upcoming release of Akka 2.4.2. The main change in this release is 
that it includes Streams & HTTP, with the akka-stream, akka-stream-testkit, 
akka-parsing, and akka-http-core modules no longer being marked 
“experimental”. In comparison to Streams & HTTP 2.0 the main changes are:

   - significant performance improvement for HTTP handling, now reaching 
   roughly 75% of Spray’s performance—this is not the end of the performance 
   work, we have only just begun
   - replacement of all uses of the Unit type (represented as BoxedUnit in 
   Java) with the more descriptive types akka.Done (for signaling successful 
   completion) and akka.NotUsed (for materialization results of stages that do 
   not produce a value)
   - usage of Java 8 types in the Java DSLs: java.util.Optional instead of 
   scala.Option and java.util.concurrent.CompletionStage instead of 
   scala.concurrent.Future

Especially the second and third point mean that porting code from Streams & 
HTTP 2.0 to Akka 2.4.2 will require some mechanical source code changes, 
please refer to the migration guide 
<http://doc.akka.io/docs/akka/2.4.2-RC1/scala/stream/migration-guide-2.0-2.4-scala.html>
 for 
the details. While the core team was concentration on these sweeping 
changes some community members; Alexander Golubev, Stefano Galarraga, Gilad 
Hoch and @2beaucoup to name a few, got busy contributing new functionality 
and improving the existing one:

   - Automatic and manual reset of the BackoffSupervisor
   - CircuitBreaker implemented as an Actor pattern
   - Noisy logging of cluster heartbeats now silenced but available through 
   a config setting
   - A rare bug where a node joining a cluster crashed before join 
   completed and then not being able to connect after restart was fixed
   - Option to eagerly initialize persistence plugins
   - Improved OSGi manifests for HTTP and streams
   - New stream combinators added: batch, batchWeighted, reduce and 
   watchTermination. 
   - HTTP - server side request timeouts added
   - Connection pools no longer fail if they are limited to a single 
   connection
   - New directives to turn a posted form to maps and sequences

We intend to release version 2.4.2 as soon as we are confident that it is 
reasonably bug free and the documentation is top notch as well: we are 
aware that the getting started experience for Streams & HTTP is not perfect 
in places and there are some features that are entirely missing—stay tuned 
for updates and please let us know of anything you find to be suboptimal.

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_2.11-2.4.2-RC1.jar is compatible with 
akka-actor_2.11-2.3.14.jar but not with akka-actor_2.10-2.3.14.jar.

Binary compatibility is *not* maintained for the following:

   - akka-testkit, akka-multi-node-testkit and akka-persistence-tck
   - experimental modules:
      - akka-persistence-query-experimental
      - akka-distributed-data-experimental
      - akka-typed-experimental
      - akka-http-experimental
      - akka-http-testkit-experimental
      - akka-http-spray-json-experimental
      - akka-http-xml-experimental
      - akka-http-jackson-experimental
   - features, classes, methods that were deprecated in 2.3.0 or earlier 
   and removed in 2.4.x

Credits

Thanks to the Community! For this release we had the help of 32 committers. 
For the full list of stats see the announcement on the web site: 
http://akka.io/news/2016/01/26/akka-2.4.2-RC1-released.html

Special thanks to Alexander Golubev for his involvement in the development 
of Akka Stream.

Happy hakking!

– The Akka Team

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