Introduction to Reactive Streams and Reactor 2.5

Unless otherwise indicated, these slides are © 2013-2015 Pivotal Software, Inc. and licensed under a Creative Commons Attribution-NonCommercial license: http://creativecommons.org/licenses/by-nc/3.0/
(Rx + Reactive Streams + Spring) % Java 8 =
Reactor 2.5
Stéphane Maldini
@smaldlini

Unless otherwise indicated, these slides are © 2013-2015 Pivotal Software, Inc. and licensed under a 
Creative Commons Attribution-NonCommercial license: http://creativecommons.org/licenses/by-nc/3.0/
Stéphane Maldini
• Survive in London
• Reactive Engineering @
• Project Reactor lead
• Reactive Streams &  
Reactive Streams Commons contributor
• Works on Spring Framework 5 
upcoming Reactive support
• @smaldini on Twitter
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+

Do you need to be “reactive” ?
If you ask then you probably don’t.
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How much faster is it ?
It’s slower than your usual imperative system.
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Creative Commons Attribution-NonCommercial license: http://creativecommons.org/licenses/by-nc/3.0/ 5
Is it good at anything beyond pretending ?
Yes. It scales with the request volume.

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But remember it changes the way you think code.  
It’s definitely a michelin star lunch.

Swipe Right - it’s a match
• Mobile/IoT backend API
• Server-to-Server communications (HTTP…)
• Unreliable clients
• Big Data
• Mutualized Resources (Cloud…)
• User Interfaces
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Reactive, what the sh*t?
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More details on http://fr.slideshare.net/StphaneMaldini/intro-to-reactive-programming-52821416
• Reactive is used to broadly define event-driven systems
• Reactive Manifesto defines qualities of reactive systems
• Reactive programming: moving imperative logic to async, non-
blocking, functional-style code, in particular when interacting
with “time consuming” resources

For reactive programming, we need tools :
☐ Reactive Streams
☐ Reactive APIs
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Reactive Streams
• Reactive Streams is a protocol for asynchronous stream
processing with non-blocking backpressure
• De facto standard for interop between reactive libraries
• To be included in Java 9 as java.util.concurrent.Flow
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Reactive Streams principle
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Publisher SubscriberData
Demand
• Max(InflightData) <= demand
• No data sent without demand
• Demand can be unbounded

Reactive Streams is 3+1 interfaces (+ a TCK)
1215
onSubscribe
onNext*
(onError|onComplete)?
Publisher
Subscriber
Subscription
request(n)
cancel()

Reactive Streams offers Quality of Service for the JVM
Bounded hardware use and flow prioritization
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Imperative vs Reactive Streams ?
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User rickSanchez =
userRepository.findUser(“rick”);
Blocking, not
returning until done !
Might throw
exceptions

If this blocks, and runs on a serving HTTP request ?
If the calling HTTP requests come from another “Microservice” ?
If that Microservice is also calling in a blocking way ?
If you solve scalability issues by scaling out ?
If you agreed to pay the scalability tax, how far can it help you ?
Thinking about the big picture
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Imperative vs Reactive Streams ?
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Publisher<User> rickSanchez =
rickSanchez.subscribe(new Subscriber<User>(){ … });
Might send 0, 1 or N Users ! Non Blocking - on demand data emission
Callback for start, result, error or complete

How practical is Publisher ?
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Do you want to implement Publisher yourself ?
Probably not, usually.
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For reactive programming, we need tools :
☑ Reactive Streams
☐ Reactive APIs
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• Transform, Combine, Timebox, Aggregate, Consume…
• On the JVM:
• Reactor 2.5 is 4th generation* and based on Reactive Streams
• RxJava 1.x: 2nd generation* and most used implementation
• Akka Stream 2.x: Lightbend 3rd generation* Reactive API
• Also for other languages, for example RxJS, MostJS
Reactive APIs
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* Based on http://akarnokd.blogspot.fr/2016/03/operator-fusion-part-1.html

Reactor 2.5 ecosystem
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Reactor Core 2.5
• Cross collaboration with Dávid Karnok (RxJava) and some
Spring Framework committers
• Natively based on Reactive Streams, RSC* and Java 8+
• 2 new rich Publisher types : Flux & Mono
• Strong focus on efficiency
• Ever-improving debugging, logging, testing capabilities
* ReactiveStreamsCommons is a research effort about reactive flows
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Flux (0..N elements) with ReactiveX compliant API
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Mono (0..1 element)
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Imperative vs Reactor ?
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Mono<User> rickSanchez =
rickSanchez.consume(this::userHandler,
this::errorHandler)
Single Typed Publisher !
Start the Mono and consume
its result or error.

Mono, a single-data-at-most API
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Flux, classic Rx patterns for 0..N events
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Visual (Marble) doc for Flux & Mono
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Reactor Web Console (preview)
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Concurrency Types Cheat Sheet
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No value Single value Multiple values
Blocking void T 
Future<T>
Iterable<T>
Collection<T>
Stream<T>
Non-
blocking
CompletableFuture<Void> CompletableFuture<T> CompletableFuture<List<T>>
Reactive 
Streams
Publisher<Void> Publisher<T> Publisher<T>
RxJava Observable<Void>
Completable (1.1.1)
Observable<T>
Single<T> (1.0.13)
Observable<T>
Reactor Mono<Void> Mono<T> Flux<T>

https://spring.io/blog/2016/04/19/understanding-reactive-types
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Reactive Spring
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• Spring projects are going reactive
• Reactor Core is the reactive foundation
• RxJava support provided out of the box
• You will be able to choose your web engine: 
Tomcat, Jetty, Undertow or Netty
• Most impact on Web and Data support (IO intensive)
• Spring Reactive experiment
• Spring Reactive Playground sample application

Well known Controller example
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@RestController
public class UserController {
private BlockingRepository<User> repository;
@RequestMapping(path = "/save-capitalized", method = RequestMethod.POST)
public void saveCapitalized(@RequestBody List<User> users) {
users.forEach(u -> u.setName(u.getName().toUpperCase()));
repository.save(users);
}
}
public interface BlockingRepository<T> {
void save(List<T> elements);
Iterable<T> findAll();
}

Controller with Reactive types
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@RestController
private ReactiveRepository<User> repository;
@RequestMapping(path = "/save-capitalized", method = RequestMethod.POST)
public Mono<Void> saveCapitalized(@RequestBody Flux<User> users) {
return repository.save(users.map(u -> new User(u.getName().toUpperCase()));
}
}
public interface ReactiveRepository<T> {
Mono<Void> save(Publisher<T> elements);
Flux<T> findAll();
}

Blocking vs Reactive: memory consumption
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Memoryconsumption
Time
Blocking Reactive

Blocking vs Reactive: streaming updates
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Numberofuserssaved
inthedatabase
Time
Blocking Reactive

Controller with Reactive return values
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@RestController
private ReactiveRepository<User> repository;
@RequestMapping(path = "/", method = RequestMethod.GET)
public Flux<User> findAll() {
return repository.findAll();
}
}
• Optimized serialization when using Flux instead of List
• Also perfectly suitable for Server-Sent Events

Reactive HTTP client with Mono
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import static org.springframework.web.client.reactive.HttpRequestBuilders.*;
import static org.springframework.web.client.reactive.WebResponseExtractors.*;
Mono<Person> result = webClient
.perform(get("http://localhost:8080/person")
.header("X-Test-Header", "testvalue")
.accept(MediaType.APPLICATION_JSON))
.extract(body(Person.class));

Reactive HTTP client with Flux
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import static org.springframework.web.client.reactive.HttpRequestBuilders.*;
import static org.springframework.web.client.reactive.WebResponseExtractors.*;
Flux<Person> response = webClient
.perform(get("http://localhost:8080/persons")
.accept(MediaType.APPLICATION_JSON))
.extract(bodyStream(Person.class));
Works for:
• JSON array [{"foo":"bar"},{"foo":"baz"}]
• JSON Streaming {"foo":"bar"}{"foo":"baz"}
• SSE with something like .extract(sseStream(Person.class))

Do you want to learn and play at the same time ?
Get Factorio on Steam for a few coins
40

Introduction to Reactive Streams and Reactor 2.5

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