Reactive Programming in Java. Copyright - Syncogni Consulting Pvt Ltd. All rights reserved.

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Daniel Phelps
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3 Prerequisites: Core Java Lambda Expressions Method references Functional Programming Web - application development using JEE/Spring

4 Rules of the game You are free to interrupt me at any time with your questions I may answer it right away, if it is simple answer in the end if it takes a lot of time to question and answer it has many buts and ifs it is out of the scope of our learning plan

5 Rules of the game We may not discuss any particular project related issues here, because : It needs more time to study and understand the project specific environment The resolving may take a lot of iterations / testing / time It may not benefit everyone It may waste others time

6 Expectations from the participants?

7 Note: Some slides are a bit descriptive, because: anybody can use it even after the session without trainer it serves as PPT + study material participants read it silently/aloud during session to maximize participation

8 Teaser begins.

10 Official Reactive Manifesto Responsive Resilient Elastic Message Driven

11 Get the official ribbon!!!

16 Teaser Ends.

17 Trailer begins.

18 Reactive Programming Reactive = showing response to a stimulus Reactive / Responsive Scalable Resilient / fault tolerant Event driven Observable sequences Backpressure Represented using marble diagrams

19 Reactive Programming Async facade - design pattern Asynchronous Ex 1: Do x, y, z after p is done Ex 2: Do p, q after r is started Ex 3:

20 Reactive Programming Polyglot Implemented (Library is available) in.net Scala Clojure JavaScript Ruby Python C++, Groovy, Objective-C/Cocoa etc.

21 Reactive Programming Java Implementations: RxJava Akka-Streams Project Reactor( Spring 5 JSR 166 (headed by Doug Lee) to be implemented in Java 9 as Flow API Vert.x 3.0

22 RxJava Originally from Netflix Currently open source Works with Java 6+ Official Page:( First implementation of reactive style of programming Has very powerful APIs

23 What is RxJava? Note : RxJava = Reactive Extensions for Java A library for composing asynchronous and event based programs using observable sequences for the Java VM - Netflix

24 What is Reactive Programming? We are not blocked on some operation We will respond to an event as and when we receive the notification of that event When / if the stock price changes, we will be notified. Then only we will take appropriate action If the cricket score changes, we will hear a loud noise, then only look at the screen :)

25 What is Reactive Programming? We don t wait for the entire web page to be downloaded to display it Keep pushing the election results as and when the change happens Divide and conquer approach Keep pushing the information like blackberry Users will react to the pushed data

26 Recipe for RxJava Get inspired by: Gang of Four s Observer Pattern Iterable type s these two extra semantics: The ability to signal that there is no more data available The ability to signal that an error has occurred Mix them well with seasoning :)

27 Functional Reactive Asynchronous Values Events Push Functional Reactive Lambdas Closures Pure Composable

28 Imperative vs Reactive Iterable Observable pull push T next() onnext(t) throws Exception onerror(exception) returns; oncompleted()

29 Imperative vs Reactive // Iterable<String> contains 379 strings // Observable<String> contains 379 strings myiterable.skip( 16 ) //header.map( s -> s+ _transformed ) //blocking.foreach( e -> System.out.println(e) ); myobservable.skip(16) //header.map( s -> s+" transformed" ) //non-blocking.subscribe( e -> System.out.println(e) );

30 Scenarios: Single Multiple Sync T getdata() Iterable<T> getdata() Async Future<T> getdata() Observable<T> getdata()

31 Trailer Ends!!!

32 Movie Begins...

33 Hello World RxJava

43 Subscriber Subscriber<String> mysubscriber = new Subscriber<String>() public void onnext(string s) { System.out.println(s); public void oncompleted() { System.out.println("Done"); public void onerror(throwable e) { e.printstacktrace(); } };

44 Observable Observable<String> myobservable = Observable.create( new Observable.OnSubscribe<String>() public void call(subscriber<? super String> sub) { sub.onnext("hello, world!"); sub.oncompleted(); } });

45 Connecting code myobservable.subscribe(mysubscriber);

46 HelloWorldRxJava - Demo

47 Communication Protocol One item will be pushed to client during each onnext call at the server side For every onnext call on server, client can execute onnext and receive an item Once server executes, oncompleted or onerror, it will not execute onnext again

48 Communication Protocol Client will not initiate onnext or oncompleted or onerror, but, it is the server onerror signifies that there is some error at the observable s side The observable will call neither oncompleted nor onnext after calling onerror

49 Communication Protocol onnext* (oncompleted onerror)? * = zero or more;? = zero or one Client may call unsubscribe method at any time, to signal the same to server Server must check this using subscriber.isunsubscribed() method and must stop sending further push notifications

50 Life Cycle of an Observable lazily starts emitting items only after subscription keeps emitting items till an error is encountered or till the end of stream is reached

51 What is marble diagram?

53 Ex 2 : Client tells the server to stop //client side code Subscriber<String> mysubscriber = new Subscriber<String>() public void onnext(string s) { System.out.println(s); if (s.touppercase().contains("bahu")) { System.out.println("I got my movie!!!!"); unsubscribe(); } } };

54 Ex 2 : Client tells the server to stop //Server side Observable<String> myobservable = Observable.create(new Observable.OnSubscribe<String>() public void call(subscriber<? super String> sub) { List<String> list = Arrays.asList("Dhoom 3", "Bahubali", "Star wars", "Batman"); for (String e : list) { if (sub.isunsubscribed()) { break; } sub.onnext(e); } sub.oncompleted(); } });

55 Ex 3 : Error Handling - Server code try { for (String e : list) { if (sub.isunsubscribed()) { break; } if(e.equalsignorecase("bahubali 10")){ throw new Exception("Something is wrong!!!"); } sub.onnext(e); } sub.oncompleted(); } catch (Exception e1) { sub.onerror(e1); break; }

56 Ex 3 : Error Handling - Client public void onerror(throwable e) { e.printstacktrace(); }

57 ManualErrorHandler - Demo

58 Ex 5: Take only some values - demo //I can process only next 2 orders Observable<String> mycustomobservable = myobservable.take(2); mycustomobservable.subscribe(mysubscriber);

59 Ex 6: Take orders only for some time //I can take orders for another 3 seconds Observable<String> mycustomobservable = myobservable.take(3, TimeUnit.SECONDS); mycustomobservable.subscribe(mysubscriber);

60 Ex 7: Skip some initial values //Skip initial 3 bytes which is a useless header Observable<String> mycustomobservable = myobservable.skip(3); mycustomobservable.subscribe(mysubscriber);

61 Ex 8: Skip values for some time //Skip the values for initial 3 seconds //while they are testing microphone Observable<String> mycustomobservable = myobservable.skip(3, TimeUnit.SECONDS); mycustomobservable.subscribe(mysubscriber);

62 Ex 9: Skip and Take - Composition myobservable.skip(1).take(2).subscribe(mysubscriber);

63 Ex 10: Filter the stream myobservable.skip(1).take(2).filter(e -> e.touppercase().contains("star")).subscribe(mysubscriber);

64 SkipTakeFilterDemo

65 Ex 11: Two Subscribers - Demo myobservable.skip(1).take(2).filter(e -> e.touppercase().contains("star")).subscribe(mysubscriber1); myobservable.skip(2).take(3).filter(e -> e.touppercase().contains("gold")).subscribe(mysubscriber2);

66 Ex 12: Combinators //Combinators - Combine / Merge 2 observables Observable.merge(myObservable1, myobservable2).subscribe(mysubscriber);

67 Merge2Observables - Demo

68 Error Handling

69 Ex 13: onerrorresumenext //on error, resume with the next observable myobservable1.onerrorresumenext(myobservable2).subscribe(mysubscriber);

70 Ex 13: onerrorresumenext

71 Map vs FlatMap Both do transformation Map: maps one item to another item returns only one item synchronous FlatMap: maps one item to zero or more items returns an Observable stream asynchronous

73 FlatMap myobservable1.flatmap( s -> { String movie1 = s + " 1"; String movie2 = s + " 2"; String movie3 = s + " 3"; return Observable.just(movie1, movie2, movie3); } ).subscribe(mysubscriber);

74 Transformations

75 Group By Transformation groupby similar to group by clause in SQL groups items according to the condition given Ex : can group a sequence of numbers based on odd or even

77 Climax

78 Case studies:

79 Case study 1: DB Access

80 Case study 2 : FlatMap

81 Service Orchestration

82 Service Orchestration

83 Case study 3 : Car assembly line

84 Clients REST Controller Service DB REST Controller Service Service External API REST Controller Service External API

85 Clients REST Controller Service REST Controller Service External API Service REST Controller Service External API

86 REST Controller REST Controller Orchestrated Service Service REST Controller Orchestrated Service Service External API Service Service External API

87 Clients Controllers Service Orchestration Layer Standard Services Layer External APIs / Data Access Layer / Databases

88 Case study : Spring - DeferredResult

89 Case study 5 : Spring - SseEmitter

90 THE END

91 Now revisit the teaser & the trailer :)

92 References Google Wikipedia SO RxJava API Docs tml

93 ?

94 Thank you!!!

95 Let us stay in touch : Dinesh Bhatt DB@syncogni.com India : USA :

Reactive Programming in Java. Copyright - Syncogni Consulting Pvt Ltd. All rights reserved.

Reactive Programming in Java. Copyright - Syncogni Consulting Pvt Ltd. All rights reserved. Reactive Programming in Java Copyright - Syncogni Consulting Pvt Ltd. All rights reserved. Prerequisites: Functional Programming as in Java 8 Streams of Java 8 Lambda expressions Method references Expectations