Practical RxJava workshop v1

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Practical RxJava

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@Couchbase @InfoQFR @bbl_fr @SimonBasle

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the Plan & Goals

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RxJava 101

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migrate a Legacy application RxJava 101

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migrate a Legacy application learn Operators of interest RxJava 101

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migrate a Legacy application learn Operators of interest 9 steps + Q& A RxJava 101

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Why?

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Blocking it’s Evil, m’kay?

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Blocking it’s Evil, m’kay?

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we need asynchronous code

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we need reactive asynchronous code

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we need parallelisable asynchronous code

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we need composable asynchronous code

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we need readable asynchronous code

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but how ?

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Futures? Callbacks?

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Futures? Callbacks? too easy to block (get) complex beyond 1 level of composition no composition have you heared of Callback Hell?

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DocumentService.find("userId", new Callback>() { public void onSuccess(List result) { final List jsonList = new ArrayList(10); int taken = 0; for (Document doc : result) { if (taken >= 10) break; if (!doc.isStarred()) continue; taken++; final CountDownLatch rendezVous = new CountDownLatch(3); final JsonObject jsonBuffer = new JsonObject(); jsonBuffer.appendInt("id", doc.getId()); jsonBuffer.append("text", doc.getText()); CommentService.findForDoc(doc, new Callback>() { public void onSuccess(List comments) { final JsonObject commentArray = JsonObject.createArray(); CountDownLatch userLatch = new CountDownLatch(comments.size()); for (Comment c : comments) { JsonObject cj = new JsonObject(); cj.append("content", c.getText()); cj.append("date", c.getDate()); UserService.find(c.getUserId(), new Callback() { public void onSuccess(User user) { cj.append("author", user.getName()); cj.append("nickname", user.getLogin()); Futures? Callbacks? too easy to block (get) complex beyond 1 level of composition no composition have you heared of Callback Hell?

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RxJava 101

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RxJava 101

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Netflix OpenSource

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dual of Iterable - Iterator

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Iterable - Iterator becomes Observable - Observer

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Iterable - Iterator becomes Observable - Observer “Pull” “Push”

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compose

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compose asynchronous programs based on events

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compose asynchronous programs based on events using observable sequences

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the Reactive Manifesto reactivemanifesto.org

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reactive-streams.org standardize on the jvm

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reactive-streams.org standardize on the jvm akka - reactor - rxjava

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Show Me how it works !

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interface Observer

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interface Observer onNext(T data)

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interface Observer onNext(T data) onCompleted()

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interface Observer onNext(T data) onCompleted() onError(Throwable t)

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interface Observer onNext(T data) onCompleted() onError(Throwable t)

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Observable

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Observable compose & chain a stream

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Observable subscribe an Observer

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so much Choice, you’ll see!

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still lost… time to practice

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The Legacy app

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Wow pool API? Much Legacy!

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Doge Mining Pool Client (UI?) Rest Controller Rest Controller Rest Controller Rest Controller Service Service Service External API External API External API DB L E G A C Y

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Doge Mining Pool Client (UI?) Rest Controller Rest Controller Rest Controller Rest Controller Service Service Service External API External API External API DB

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Doge Mining Pool Client (UI?) Rest Controller Rest Controller Rest Controller Rest Controller Service Service Service External API External API External API DB

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simple Creation 1

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Migrate Services ● AdminService ● CoinService ● HashrateService ● PoolService *

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Useful Operators ● Observable.just ● Observable.from ● Observable.create ● map

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Useful Operators ● Observable.just ● Observable.from ● Observable.create ● map

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But now it doesn’t compile how to adapt the Controllers?

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Naively Block on an Observable take(n) vs single() vs first() xxxOrDefault(t), toList()

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Take

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Single

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SingleOrDefault

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Transform 2

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Migrate Services ● PoolService ○ poolGigaHashrate method

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Useful Operators ● flatMap ● reduce

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flatMap

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reduce

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Filter 3

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Migrate Services ● UserService ○ findAll for now naively adapted (Observable.from) ○ compose on findAll for getUser / getUserByLogin ● SearchService

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Useful Operators ● filter ● take ● flatMap to asynchronously retrieve additional data needed for filter

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filter

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Count 4

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Migrate Services RankingService ○ use from for rankByHashrate / rankByCoins ○ complete migration for other methods

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Useful Operators ● takeUntil ● count ● take

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takeUntil

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count

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Time for a Break :)

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Side Effects 5

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doOnXXX ● doOnNext ● doOnError ● doOnCompleted ● doOnEach

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Migrate Services PoolService ○ add a line of log each time a user connects

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Combine 6

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Level: Simple Concat

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Level: Intermediate Merge

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Level: Advanced Zip

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Migrate Services StatService.getAllStats ● for each User ○ retrieve his hashrate ○ retrieve how many coins he mined ○ combine both and make a UserStat

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Live & Let Die and Retry oo 7

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The Problem StatService.lastBlockFoundBy ○ fireproof implementation (choose a random User) ○ but what if… we wanted to do it without knowing the number of users in advance?

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The Plan Generate an index randomly (within a broadly guessed limit) Retry up to 4 times if out of bounds Last resort: use a default User (eg. “Banned User”)

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Useful Operators ● flatMap ● skip ● retry ● onErrorReturn

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Retry

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OnErrorReturn

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Trap #1

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skip doesn’t error if overflow Trap #1

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skip doesn’t error if overflow combine with first/last Trap #1

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Trap #2

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just caches the value Trap #2

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just caches the value use create to regenerate at each retry Trap #2

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Chain 8

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Migrate Services ExchangeRateService ○ calls two external APIs: doge-to-dollar rate and currency-to-currency exchange rate ○ two chained REST calls ○ combination of the two gives doge-to-anyCurrency

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Useful Operators ● Observable.create ○ wrap the REST call on each subscription ○ catch RestClientException and make them into DogePoolException (especially for timeouts)

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Useful Operators ● zipWith ○ similar to zip but called from stream A instead of statically ○ combine both rates to get the final one

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Oh Hey! ! WARNING - Incoming Product Owner !

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"this free exchange rate API crashes too often, make it so we switch to an alternative non-free API when it’s the case" - the Product Owner

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"this free exchange rate API crashes too often, make it so we switch to an alternative non-free API when it’s the case" - the Product Owner

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"oh yeah and if you could go on and track costs of these calls it would be great" - the Product Owner, pouring himself a coffee Much Surprise Such Requirements wow

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The Plan ● AdminService ○ add method+variable to track cost for this month ● ExchangeRateService ○ rate retrieval method similar to current one ○ use endpoint from exchange.nonfree.api.baseUrl ● switch thanks to OnErrorResumeNext ● use side effects ○ log when we switch & track the cost

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OnErrorResumeNext

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Clean-up Controllers 9

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Asynchronous Response ● prepare a DeferredResult ○ that will be returned without blocking ● subscribe on stream ○ onNext: inject T via setResult ○ onError: create a DogePoolException and inject it via setErrorResult

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This could be Generalized ● using ReturnValueHandler and a simple adapter of Observable to DeferredResult ● each time an Observable is returned, it’ll be converted into this adapter ○ WebAsyncUtils.getAsyncManager(...). startDeferredResultProcessing(...)

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Migrate Services ● AdminController (simple) ● IndexController (trickier) ○ use zip, flatMap, single… to detect bad users ● UserProfileController (idem)

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Q& A

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TakeAway

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Nonblocking code

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without Callbacks

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with a minimum of visual cluter

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readable and understandable

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Hope you loved it!

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No content

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Merci!

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➔ RxJava Logo and Marble Diagrams - ReactiveX Documentation http://reactivex.io ➔ Cat Attack - Static416 on Flickr ➔ Stormtroopers Series - J.D. Hancock http://photos.jdhancock.com/series/stormtroopers.html ➔ Blackhole with Corona - NASA on Wikimedia ➔ Broken Keyboard - Santeri Viinamäki on Wikimedia ➔ Gobot Toys- Tom Prankerd on Wikimedia ➔ Coffee Filter - Unsplash on Pixabay ➔ Abacus - Succo on Pixabay ➔ Still from Office Space - Copyright, 20th Century Fox By By-Nc By Cc0 By By Cc0 Cc0 © Credits (1/2)

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