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Review Observable/Design Pattern

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Presentation on theme: "Review Observable/Design Pattern"— Presentation transcript:

1 Review Observable/Design Pattern
Observables Reactive Libs Review Observable/Design Pattern Observables Streams JavaRx Demo ScalaRxShell Demo

2 RxLibs Microsoft open source Open source implementations of ReactiveX: Open source implementations of ReactiveX: Netflix open source: api.html Additions on ReactiveX, competition to akka

3 Observable/Observer not reactive
Java Design pattern. Not useful for async or distributed systems. Pre 2000. package com.example; import java.util.Observable; import java.util.Observer; //models customer going to coffeeshop. can change name. leave observer public class MyObserver implements Observer{ private String name=null; private boolean atCoffeeShop=false; MyObserver(String n){; } @Override public void update(Observable o, Object arg) { // TODO Auto-generated method stub System.out.println("calling update callback for:"+name);

4 Observable package com.example; import java.util.Observable;
public class CoffeeShop extends Observable{ public void enterCoffeeShop(MyObserver obs){ addObserver(obs); } public void notifyCustomers(){ setChanged(); notifyObservers();

5 package com.example; public class Main{
public static void main(String []args){ CoffeeShop cs = new CoffeeShop(); cs.addObserver(new MyObserver("cust1")); cs.addObserver(new MyObserver("cust2")); cs.addObserver(new MyObserver("cust3")); cs.notifyCustomers(); } Output: calling update callback for:cust3 calling update callback for:cust2 calling update callback for:cust1

6 Run instr: Download zip:
Unarchive, cd TestCode- master/TestObservable, mvn compile $ mvn exec:java - Dexec.mainClass=com.example.Main [WARNING] Warning: killAfter is now deprecated. Do you need it ? Please comment on MEXEC-6. calling update callback for:cust3 calling update callback for:cust2 calling update callback for:cust1

7 RxJava NOT ASYNC the Coffeeshop/Observable notifies all the observers via callback function. JavaRx, JavaRxScala add to this design pattern for event streams and to make async programming possible. EventStreams=Subscriptions Async: Try/onNext,onComplete No Locks, Threads, etc...abstracted away Note: code style cleaner than Observer/Observable. Everything in subscription class. No separate driver program needed

8 RxJava Create an observerable stream using .just(1,2,3)
Add a subscriber(lazy eval), similar to observer but add the duality of iterables. Iterable/Observer stream programming. Iterables block/bad perf. JavaRx adds threads + async onNext <-> next() onComplete ↔ hasNext() onError ↔ throws exception

9 RxJava Create stream, print
package com.example; import rx.Observable; public class TestObservable{ public static void main(String []args){ System.out.println("asdf"); Observable.just(5,6,7,8).subscribe(new MySubscriber<Integer>()); //endless stream //Observable.just(5,6,7,8).repeat().subscribe(new MySubscriber<Integer>()); }

10 Subscriber import rx.Subscriber;
class MySubscriber<Integer> extends Subscriber<Integer>{ @Override public void onCompleted() { // TODO Auto-generated method stub System.out.println("onCompleted"); } public void onError(Throwable throwable) { System.err.println(Error: " + throwable.getMessage()); public void onNext(Object arg0) { System.out.println("onNext: " + arg0);

11 Run instr: cd TestObservable/TestJavaRx mvn clean;mvn compile
$ mvn exec:java - Dexec.mainClass=com.example.TestObservabl e [WARNING] Warning: killAfter is now deprecated. Do you need it ? Please comment on MEXEC-6. onNext: 5 onNext: 6 onNext: 7 onNext: 8 onComplete

12 RxJava Create static <T> Observable<T> create(Observable.OnSubscribe<T> f) Returns an Observable that will execute the specified function when a Subscriber subscribes to it.

13 Create a separate class
package com.example; import rx.*; //implement logic when to call onNext(), onError(), onComplete() by using create() //demo only public class CreateObservable<Integer> implements Observable.OnSubscribe<Integer>{ @Override public void call(Subscriber<? super Integer> arg0) { // TODO Auto-generated method stub Subscriber subscriber = (Subscriber) arg0; try{ if(!subscriber.isUnsubscribed()){ for(int i=0;i<5; i++){ subscriber.onNext(i); } subscriber.onCompleted(); }catch(Exception e){ subscriber.onError(e);

14 Add subscribe. No drivers!!!
package com.example; import rx.*; import rx.functions.Action1; public class ObservableCreate { public static void main(String args[]){ Observable.create(new CreateObservable<Integer>()).subscribe(new Action1<Integer>() { @Override public void call(Integer integer) { System.out.println("onNext: " + integer); } });

15 Run Instr: Run from eclipse

16 Run from eclipse Run function Action1 everytime subscriber subscribes. Lazy eval, have to call subscribe to start data processing

17 RxJava API Example Some things are easier in RxJava, create a counter once per second. No locks, no threads. Errors and cancellation(unsubscribe) built into the API. Less custom code and less testing.

18 package com.example; import rx.*;
import rx.functions.Action1; import java.util.concurrent.*; public class Timer { static CountDownLatch latch = new CountDownLatch(5); public static void main(String args[]) throws InterruptedException{ Observable.interval(1,TimeUnit.SECONDS).subscribe(new Action1<Long>(){ public void call(Long counter){ latch.countDown(); System.out.println("Timer Secs:"+counter); } }); latch.await();

19 Run Inst; Run in eclipse

20 RxJava Map Java7 Uses functions to replace the x=>f(x) notation
Modify the timer example to use a map to print out the thread it is operating on Use the testing code. The countdown latch for interval sequences when another thread creates the sequence. Interval is threaded, see scheduler operator table: Scheduler#using-schedulers

21 Timer threads The observable does the work in one thread and the results via the subscriber are displayed in another thread. class RxThread[T](o:Observable[T]) { def execAsync[T] = { o.subscribeOn(Schedulers.newThread) .observeOn(AndroidSchedulers.mainThread()) .materialize } /** * Convert implicitly a Java Observable in a Scala Observable. */ object RxThread { implicit def Observable2Notification[T](o: Observable[T]) = new RxThread(o)

22 Add func public static void debugTimer() throws InterruptedException{
Observable.interval(1,TimeUnit.SECONDS).map(new Func1<Long,Long>(){ @Override public Long call(Long i){ System.out.println(Thread.currentThread().getName()); return i; } }).subscribe(new Action1<Long>(){ public void call(Long counter){ latch.countDown(); System.out.println("Timer Secs:"+counter); }); latch.await();

23 How about a sequence? public static void debugSeq() {
Observable.just(1,2,3,4,5).map(new Func1<Integer,Integer>(){ @Override public Integer call(Integer i){ System.out.println(Thread.currentThread().getName()); return i; } }).subscribe(new Action1<Integer>(){ public void call(Integer i){ System.out.println("onNext: " + i); });

24 Run inst; Comment out either debugTimer() or debugSeq() , run in Eclipse or maven

25 RxJava Twitter Ex. Building APIs
rxjavas Turning Twitter4J into Observable. Creates API for stream processing/multiuser nonblocking. Goal: API for user, rolling count of tweets

26 Making RxJava Async and Parallel
Trick from lecture: if the return type is <T> this is blocking. If Observable<T> or Future<T> then this is async. Look at BlockingObservable return types. Marble diagram=>parallel

27 Marble=>parallelism

28 JavaRx parallel JavaRx controls thread pools using schedulers as describe in lecture when EM compared ExecutionContext from Promises/Futures to Observables. Just using flatMap doesn't guarantee multiple threads. A flatMap returns Observable<T> which is the first criteria for parallelism

29 Schedulers add parallelism
Debug:Thread.getCurrentThread().getName(), getCount() Debug threads in JavaRx/Java adding to map() using Func1 Debug in Rx/Scala adding .debug() to package.scala threading-examples/

30 Multiple ways to add threads
Docs not clear, look at the JavaRx test code Test code does this: Creating your own threads In the observable call? OnNext()? Blog does this: Using the thread pool from the scheduler Add spark executors or create your own executor framework using zookeeper(TBD)

31 Rx/Scala Scala wrapper around a RxJava Scala REPL for Observables Scala REPL for Observables Not avail from websearch Change $TOOL_PATH under $SCALA_HOME/bin or copy the jars from suggestions/lib_managed/ and rxjava-scala into $SCALA_HOME/lib dir.

32 JavaRx The APIs are different between JavaRx/JavaRxScala/JavaRxNET
From lecture: Create a stream, notify subscribers on change to represent processing, schedulers manage thread pools vs. ExecutionContext in Futures, apply ops to streams which return Observables

33 Repl lib path

34 Scala REPL bug scala> import rx.lang.scala._
scala> val ob = Observable(1,2,3,4) java.lang.NoSuchMethodError: scala.collection.JavaConverters$.asJavaIterableC onverter(Lscala/collection/Iterable;)Lscala/collectio n/convert/Decorators$AsJava; at rx.lang.scala.Observable$.apply(Observable.scala :1924)

35 Upgrade to latest scala version; 2.11.2; 2 I/Fs, RxScala
scala> import rx.lang.scala._ import rx.lang.scala._ scala> val ob = Observable(1,2,3,4) ob: rx.lang.scala.Observable[Int] = scala>

36 Importing the wrong lib, RxJava
scala> import rx._ import rx._ scala> val foo = Observable.just(1,2,3,4) foo: rx.Observable[(Int, Int, Int, Int)] = scala>

37 Upgrade to latest scala version; 2.11.2
scala> import rx.lang.scala._ import rx.lang.scala._ scala> val ob = Observable(1,2,3,4) ob: rx.lang.scala.Observable[Int] = scala>

38 Import the netflix jars(no scheduler)
Download from maven central, use the versions not the versions in the hw rxjava-async-util jar rxjava-computation-expressions jar rxjava-core jar rxjava-core jar rxjava-scala jar rxjava-scala jar

39 Subscribe to the Observable, much easier than Java7
scala> foo.subscribe(x=>println(x)) 1 2 3 4 res0: rx.lang.scala.Subscription =

40 Observables Repeat Rx in Scala
Observables require subscribers to send notifications to when data is ready in the Observable. An observable is the dual of an iterator but can return 0 or many events or an Error. Key is the 0 event. Doesn't have to be async An observable interface contains: onNext onError onCompleted

41 RxScala Homework apply() (for maps, flatMap)
Add what on top of adding subscribers to observables?

42 Subjects For async programming, a subject is like a promise. A promise contains a future just as a future has to wait for a promise to write the data A subject contains an Observable<T> or Observer<T>?

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