Upgrade to Pro — share decks privately, control downloads, hide ads and more …

Practical RxJava workshop

Simon Baslé
February 08, 2016

Practical RxJava workshop

v2 of the Practical RxJava workshop, first presented at JFokus 2016

Simon Baslé

February 08, 2016
Tweet

More Decks by Simon Baslé

Other Decks in Programming

Transcript

  1. Futures<T>? Callbacks? too easy to block (get) complex beyond 1

    level of composition no composition have you heared of Callback Hell?
  2. DocumentService.find("userId", new Callback<List<Document>>() { public void onSuccess(List<Document> result) { final

    List<String> jsonList = new ArrayList<String>(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<List<Comment>>() { public void onSuccess(List<Comment> 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<User>() { public void onSuccess(User user) { cj.append("author", user.getName()); cj.append("nickname", user.getLogin()); Futures<T>? Callbacks? too easy to block (get) complex beyond 1 level of composition no composition have you heared of Callback Hell?
  3. 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
  4. Doge Mining Pool Client (UI?) Rest Controller Rest Controller Rest

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

    Controller Rest Controller Service Service Service External API External API External API DB
  6. Migrate Services • UserService ◦ findAll for now naively adapted

    (Observable.from) ◦ compose on findAll for getUser / getUserByLogin • SearchService
  7. Useful Operators • filter • take • flatMap to asynchronously

    retrieve additional data needed for filter
  8. Migrate Services StatService.getAllStats • for each User ◦ retrieve his

    hashrate ◦ retrieve how many coins he mined ◦ combine both and make a UserStat
  9. The Problem StatService.lastBlockFoundBy ◦ Has intermittent bug that causes it

    to crash with an IndexOutOfBoundsException ◦ We’d like to retry the call when this happens, to prevent the error.
  10. The Plan Make the method Observable (generate in a defer,

    log in a doOnNext, pick the user via flatMap and elementAt…) Migrate code as before, it still randomly fails Make it automatically retry
  11. The Plan (how to test) Edit the PoolController.lastBlock() method so

    it doesn’t catch and recover Execute PoolControllerTest.testLastBlock() and verify it succeeds Sometimes it should print several “ELECTED” messages, this is the retry in action
  12. 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
  13. Useful Operators • Observable.create ◦ wrap the REST call on

    each subscription ◦ catch RestClientException and make them into DogePoolException (especially for timeouts)
  14. Useful Operators • zipWith ◦ similar to zip but called

    from stream A instead of statically ◦ combine both rates to get the final one
  15. "this free exchange rate API crashes too often, make it

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

    so we switch to an alternative paid APi when it’s the case" - the Product Owner !
  17. "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
  18. 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
  19. Asynchronous Response • prepare a DeferredResult<T> ◦ that will be

    returned without blocking • subscribe on stream ◦ onNext: inject T via setResult ◦ onError: create a DogePoolException and inject it via setErrorResult
  20. 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(...)
  21. Testing • using mockMvc’s andReturn() to get a MvcResult ◦

    assert status().isOk() and request().asyncStarted() • trigger the async processing ◦ mockMvc.perform(asyncDispatch(mvcResult)) • from this point assert as before
  22. Migrate Services • AdminController (simple) • IndexController (trickier) ◦ use

    zip, flatMap, single… to detect bad users • UserProfileController (idem)
  23. ➔ 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)
  24. ➔ Hand Reaching - Kinseykick on Pixabay ➔ Sidecar Motocross

    - Jean-Daniel Echenard on Flickr ➔ Twin Falls - Blese on Flickr ➔ Tumbeast Gnawing on Servers - Matthew Inman on Wikimedia ➔ Trap - Anticiv on Flickr ➔ Kane Cleaning Supplies - Collinanderson on Flickr ➔ Chain - Stomchak on Wikimedia ➔ Takeaway - Edimburgh Blog on Flickr ➔ The End Sands - Elektro-Plan on Pixabay Cc0 By-Nd By-Nc By By-Sa By By By Cc0 Credits (2/2)