On behalf of the Reactor team, it is my pleasure to announce that reactor-core 3.1.0.RC1 has been released 🎉. This is a big last step towards GA release of 3.1 at the end of the month, the long term support version that will back Spring 5!

It is also complemented by various releases, all tied together in the Bismuth-M4 Release Train and BOM:

• reactor-test, reactor-extra, reactor-adapter and reactor-logback all made the cut to 3.1.0.RC1
• reactor-netty has seen significant updates and bug fixes in the new 0.7.0.M2 milestone
• reactor-kafka has been released in its 1.0.0.M4 milestone

In order to get this release, the best way is to use the BOM, as described in the reference guide here. Make sure to read the part about milestones (transposing it to Bismuth-M4).

Important Changes in Reactor-Core

Since we did not make many announcements since MILESTONE 1, let’s have a slightly deeper look at the main changes in reactor-core 3.1.0, which has the most changes.

We reserved the option of making API changes until 3.1.0.RELEASE, and this allowed us to collect great feedback from our community and from the Spring Framework team. Thus we were able to polish the API and come up with something we feel is 100% suited for the long term ride along Spring Framework 5.0.

These changes have been progressively made since the first MILESTONE of Reactor 3.1.0, and you can head to the release notes on Github for a complete changelog, especially if you haven’t upgraded to any of these since the 3.0.7.RELEASE.

Here is a non-exhaustive overview of these changes:

Better Alignment of Flux and Mono APIs

• Mono and Flux error handling operators have been adjusted. (M1)
• Mono#flatMap has become flatMapMany and Mono.then (Mono<V> then(Function<T, Mono<V> other)) is now flatMap. (M1)
• Tuple-returning Mono.when static methods and Mono#and are now zip and zipWith, consistent with their Flux counterparts. Note the when and and methods still exist in 3.1.0.RC1 but they are specific to the case where one is only interested in the completion of several Publisher objects (represented as a Mono<Void> return type). (RC1)
• The Flux#firstEmitting static method is now called first and the Flux#firstEmittingWith instance method is now called or. (RC1)

Better Operator Consistency

• A *When suffix is used for operator variants that take a companion Publisher or that perform the same task as their unsuffixed counterparts asynchronously. (M1)
• All delayError parameter variants have been removed in favor of adding an alias suffixed with DelayError (for example flatMapDelayError). (M1)
• All *Millis variants (taking a long duration) have been removed in favor of Duration based variants. (M1)

These changes usually have the added benefit of removing any ambiguity about which override of
the method to apply when using lambdas, especially from languages like Kotlin.

Mono Specific APIs

Mono has several APIs that deal with completion of tasks. They basically ignore the elements (if any) and propagate only the onComplete signals.

Before RC1, these APIs would also have overrides that wouldn’t be focused on this task-completion use case, introducing some confusion. As of RC1 these variants have all been renamed (eg. the Mono<Tuple2> and(Mono<V> other) has been renamed zipWith).

The task-completion oriented variants, which have been kept, can be spotted by the fact that they return a Mono<Void>. They now work with any Publisher, not just Monos:

• when static methods fire several provided tasks at once and wait for all of them to complete
• and does the same when called from a Mono, with a single provided Publisher
• then is about forgetting the element(s) of the source to only signal its completion

Other Changes

• reactor-test sources have been moved to the same repo as reactor-core and the artifact is now under the io.projectreactor groupId in Maven. (M2)
• Hooks are cumulative (M3) and can be partially set/reset using a key (RC1)
• WorkQueueProcessor and TopicProcessor have many construction options, now represented as a Builder (M2)
• Processor implementations in Reactor have been heavily reworked and a facade Sink interface has been created to ease direct interactions with processors. Use Processor#sink() to obtain and keep a reference to this facade. (M3)
• A Flux/Mono can be tagged with multiple tag annotations or a name annotation. (M3)
• It is now possible to tie a Context to a reactive sequence upon subscribing to it, using subscriberContext(Context). This context is propagated upstream along with the subscription signal. (RC1)

The Road from Here

The next two weeks will be all about preparing the GA release, for which we do not expect any new change other than documentation and a few new operators.

But it is also the last opportunity for you to try the changes out and give feedback! So please give it a roll as soon as you can, and come back to us with any feedback you might have (on GitHub or via Gitter chat)

Happy reactive coding!