Getting into the flow building applications with reactive streams as presented at J-Fall 2018.

1. Tim van Eijndhoven @TimvEijndhoven
Getting into the Flow
Building applications with reactive streams
https://pxhere.com/en/photo/1429185

2. Let’s meet
@TimvEijndhovenJ-Fall 2018
Tim van Eijndhoven

3. Reactive Streams
@TimvEijndhovenJ-Fall 2018
“Reactive Streams is an initiative to provide a standard for
asynchronous stream processing with non-blocking back
pressure. This encompasses efforts aimed at runtime
environments (JVM and JavaScript) as well as network
protocols.”

4. Reactive Streams
@TimvEijndhovenJ-Fall 2018
“The scope of Reactive Streams is to find a minimal set of
interfaces, methods and protocols that will describe the
necessary operations and entities to achieve the goal—
asynchronous streams of data with non-blocking back
pressure.”

5. Reactive Streams
• Java API
• Specification
• Technology Compatibility Kit (TCK)
• Example implementations
@TimvEijndhovenJ-Fall 2018

6. Reactive Streams
@TimvEijndhovenJ-Fall 2018
Publisher SubscriberProcessor
Subscription
Control
Data

7. @TimvEijndhovenJ-Fall 2018
Publisher
• void subscribe(Subscriber<T> s) Publisher<T>

8. @TimvEijndhovenJ-Fall 2018
Subscriber<T>
Subscriber
• void onComplete ()
• void onError (Throwable t)
• void onNext (T t)
• void onSubscribe (Subscription s)

9. @TimvEijndhovenJ-Fall 2018
Processor<T,R>
Subscriber<T>
Processor
• void subscribe(Subscriber<T> s)
• void onComplete ()
• void onError (Throwable t)
• void onNext (T t)
• void onSubscribe (Subscription s)
Publisher<R>

10. @TimvEijndhovenJ-Fall 2018
Subscription
Subscription
• void cancel ()
• void request (long n)

11. Easy, right…?
@TimvEijndhovenJ-Fall 2018
Publisher SubscriberProcessor
Subscription
Control
Data
https://github.com/reactive-streams/reactive-streams-jvm#Specification

12. Sequence
@TimvEijndhovenJ-Fall 2018
Publisher Subscriber
Subscription
create(…)
onSubscribe(…)
request(…)
onNext(…) / onError(…)
cancel(…)
onComplete(…)
loop

13. Implementations
@TimvEijndhovenJ-Fall 2018
RxJava Akka Streams Project Reactor

14. @TimvEijndhovenJ-Fall 2018
RxJava
• Java implementation of Reactive eXtensions
• Targets Java 6
• Used by Vert.X
• Currently on version 2
• RX has implementations in various programming languages

15. @TimvEijndhovenJ-Fall 2018
RxJava
Flowable FlowableSubscriberFlowableProcessor
<various>
Control
Data

16. @TimvEijndhovenJ-Fall 2018
Project Reactor
• Project from Pivotal
• Targets Java 8
• Used by Spring 5
• Currently on version 3
• Has a feature to pass context

17. @TimvEijndhovenJ-Fall 2018
Project Reactor
Flux CoreSubscriberFluxProcessor
<various>
Control
Data

18. @TimvEijndhovenJ-Fall 2018
Akka Streams
• Project from Lightbend
• Part of Akka
• Scala library with Java DSL
• Many integrations available through Alpakka library

19. @TimvEijndhovenJ-Fall 2018
Akka Streams
Source SinkFlow
?
Control
Data

20. Java 9 Flow Support
• Added reactive stream interfaces to java 9 (JEP 266):
• Flow.Publisher / Flow.Subscriber / Flow.Subscription /
Flow.Processor
• Same API as reactive streams
• Reactive stream implementations offer adapters to/from
Flow
@TimvEijndhovenJ-Fall 2018

21. Back Pressure
@TimvEijndhovenJ-Fall 2018
Publisher Subscriber
Give me n items
∞ Data
Subscriber
n items Data
😢😁

22. Back pressure in real life
@TimvEijndhovenJ-Fall 2018
https://beeldbank.rws.nl/MediaObject/Details/441293

23. @TimvEijndhovenJ-Fall 2018
Scheduling
• The reactive streams
implementations are not async
by default: Subscription by
default happens on the calling
thread
• The implementations often offer
separate schedulers for IO
operations (these are
expensive, blocking threads
while waiting)
https://www.flickr.com/photos/chefranden/310767408

24. @TimvEijndhovenJ-Fall 2018
Hot or Cold?
• Cold publisher: starts
producing items when
subscription happens
• All items are sent to all
subscribers
• Hot publisher:
• produces items before
subscription happens
• Items produced before
subscription are lost
https://commons.wikimedia.org/wiki/File:Emoji_hot_and_cold.svg

25. Working with resources
• try-with-resources does not work on async streams
• Use the implementation specific functions:
• RxJava: Flowable.using(…)
• Project Reactor: Flux.using(…)
• Akka Streams: FileIO.fromPath(…)
@TimvEijndhovenJ-Fall 2018

26. Testing
• Testing async stuff with jUnit is hard
• Use the implementation specific test tooling:
• RxJava: TestSubscriber
• Reactor: StepVerifier
• Akka Streams: TestSource TestSink (akka-stream-testkit)
@TimvEijndhovenJ-Fall 2018

27. Testing
• Testing reactive streams often comes down to:
• Validating the flow of events (emitted items, errors,
completion)
• Manipulate time and/or performance characteristics
@TimvEijndhovenJ-Fall 2018

28. Trickery and Witchcraft
• Operator fusion
• Macro fusion: Replacing subsequent operators with a single operator
• Micro fusion: Share resources or internal structure between operators
@TimvEijndhovenJ-Fall 2018
• Benefits performance:
• Optimizes thread/actor usage
• Eliminates queues and boundaries
https://pxhere.com/en/photo/1193558

29. Summary
• Reactive Streams provide asynchronous stream processing
• Several implementations
• Interoperability through reactive streams specification and
Java Flow
• Easy interfaces, hard to get right
@TimvEijndhovenJ-Fall 2018

30. @TimvEijndhovenJ-Fall 2018
Questions?

31. @TimvEijndhovenJ-Fall 2018
Thanks for your time.
Got feedback? Tweet it!
All pictures belong
to their respective
authors
https://github.com/timve/reactivestreams-demo