Learning Rx by example (2)

Given at Oredev 2016: https://vimeo.com/190922794

(For a beginner's version of this talk check here: https://www.youtube.com/watch?v=k3D0cWyNno4)

Rx (Reactive extensions) is a powerful API for Asynchronous programming.

It has a steep learning curve. Surprisingly though, the easiest way to grasp the concepts is by examples. So in this talk, we'll look at just 3 examples.

These are marginally complex requirements that most developers would run into these days. We dissect the problem using Rx and try to come up with elegant and simple solutions to an otherwise complicated problem.

1. Learning Rx
 by example
2. 2 mt intro to Rx 3 (intermediate) examples 1
 2
3. 2 mt Intro to Rx
4. API for Asynchronous programming
5. Observer pattern done right Best ideas from: Observer pattern Iterator pattern Functional programming
6. Observer pattern done right Observable.just(1) .subscribeOn(Schedulers.io()) .observeOn(AndroidSchedulers.mainThread()) .subscribe(getSubscriber()); .useSomeFunkyOperators()
7. Example 1 Loading from Disk Cache + Network Call
8. Cached data
 Fast Example 1 Database Fresh data
 Slow Server Requirement:
9. .concatEager( .merge( Example 1 Code: Observable .subscribeOn(Schedulers.io())
 .observeOn(AndroidSchedulers.mainThrd())
 .subscribe(); .concatEager getDiskResults(), getNetworkResults()) .merge
10. .concatEager( .merge( Example 1 Code: Observable .subscribeOn(Schedulers.io())
 .observeOn(AndroidSchedulers.mainThrd())
 .subscribe(); .concatEager getDiskResults(), getNetworkResults()) .merge Both of these methods return Observable<Result>
11. Example 1 Database Network request Problem: .merge
12. Example 1 Database Network request Problem: .merge
13. Example 1 Code: .merge Observable .subscribeOn(Schedulers.io())
 .observeOn(AndroidSchedulers.mainThrd())
 .subscribe(); .merge( getDiskResults(), getNetworkResults()) .subscribe(new Subscriber<Result>() {
 @Override
 public void onCompleted() { //... }
 
 @Override
 public void onError(Throwable e) { //... }
 
 @Override
 public void onNext(Result result) {
 if ( list.contains(result) && isExistingResultFromNetwork(result))
 return;
 // usual "add result to list" logic
 list.add(result); list.refresh();
 }
 });
14. Example 1 Code: .merge Observable .subscribeOn(Schedulers.io())
 .observeOn(AndroidSchedulers.mainThrd())
 .subscribe(); .merge( getDiskResults(), getNetworkResults()) .subscribe(new Subscriber<Result>() {
 @Override
 public void onCompleted() { //... }
 
 @Override
 public void onError(Throwable e) { //... }
 
 @Override
 public void onNext(Result result) {
 if ( list.contains(result) && isResultFromNetwork(result))
 return;
 // usual "add result to list" logic
 list.add(result); list.refresh();
 }
 }); NO!
 The Rx is not strong with this code https://twitter.com/JakeWharton/status/786363146990649345
15. Example 1 Code: getNetworkData() .publish( network -> ) getDiskData() .takeUntil(network) Observable .merge( network, )
16. Example 1 Code: Get database results but stop after network results getDiskResults() .takeUntil(getNetworkResults()) getDiskButStopAfterNetwork() =
17. Example 1 Code: Get disk results that occur before Network starts + Network results getDiskResults() .takeUntil(getNetworkResults()) getDiskButStopAfterNetwork() = getDiskButStopAfterNetwork() ) getNetworkResults() Observable .merge(
18. Example 1 Code: Get disk results that occur before Network starts + Network results ) .publish( getNetworkResults() getDiskResults() .takeUntil(getNetworkResults()) Observable .merge(
19. Example 1 Code: Get disk results that occur before Network starts + Network results Observable .merge( , ) .publish( getNetworkResults() network -> getNetworkResults() getDiskResults() .takeUntil(getNetworkResults())
20. Example 1 Code: Get disk results that occur before Network starts + Network results Observable .merge( , .publish( getNetworkResults() network -> network, ) getDiskResults() .takeUntil(getNetworkResults())
21. Example 1 Code: Get disk results that occur before Network starts + Network results Observable .merge( , .publish( getNetworkResults() network -> ) getDiskResults() .takeUntil(getNetworkResults()) network network,
22. Example 1 Code: Observable .subscribeOn(Schedulers.io())
 .observeOn(AndroidSchedulers.mainThrd())
 .subscribe(); .concat( getSlowCachedDiskData(), getFreshNetworkData()) .publish getNetworkData()
 .publish( network ->
 Observable .merge( network, getDiskData() .takeUntil(network) ) )
23. Example 2 Pagination
 (using Subjects)
24. Example 2b Setup starting seq. once
 (network call for results) Observable Observable
 .just(1)
 .flatMap(pageNo -> getNetworkResults(pageNo))
25. compose Observable<R> Observable<T> map R T flatmap R Observable<T> Holy trinity of RxConversion
26. Example 2b Setup starting seq. once
 (network call for results) Subscribe here Observable Observable
 .just(1)
 .flatMap(pageNo -> getNetworkResults(pageNo)) .subscribe(new Subscriber<List<Item>>() {
 @Override
 public void onCompleted() {
 // all items downloaded
 }
 
 @Override
 public void onError(Throwable e) {
 // handle error
 }
 
 @Override
 public void onNext(List<Item> items) {
 
 }
 }); private Observable<List<Item>> getNetworkResults(int pageNo) {
 // make network request // get List of Items for page number
 } addToList(items);
27. Example 2b Setup starting seq. once
 (network call for results) Subscribe here Observable Observable
 .just(1)
 .flatMap(pageNo -> getNetworkResults(pageNo)) .subscribe(new Subscriber<List<Item>>() {
 @Override
 public void onCompleted() {
 // all items downloaded
 }
 
 @Override
 public void onError(Throwable e) {
 // handle error
 }
 
 @Override
 public void onNext(List<Item> items) {
 
 }
 }); .concatMap(pageNo -> getNetworkResults(pageNo)) addToList(items);
28. Example 2b Setup starting seq. once
 (network call for results) Subscribe here like page numbers! Observable What if I could keep feeding inputs to get the next set of results?
29. Network call 
 to get results for page n
 Subject Code: PublishSubject<Integer> paginator = PublishSubject.create(); Example 2b Observable
 .just(1)
 .concatMap(pgNo -> getNetworkResults(pgNo)) paginator .observeOn(AndroidSchedulers.mainThread())
 .subscribe(new Subscriber<List<Item>>() {
 @Override
 public void onCompleted() {
 // all items downloaded
 }
 
 @Override
 public void onError(Throwable e) {
 // handle error
 }
 
 @Override
 public void onNext(List<Item> items) {
 
 }
 }); addToList(items);
30. Network call 
 to get results for page n
 Subscribe here Example 2a Code: Subject void onReachedEndOfList() {
 //...
 
 } paginator.onNext(nextPage); .concatMap(pgNo -> getNetworkResults(pgNo)) paginator .observeOn(AndroidSchedulers.mainThread())
 .subscribe(new Subscriber<List<Item>>() {
 @Override
 public void onCompleted() {
 // all items downloaded
 }
 
 @Override
 public void onError(Throwable e) {
 // handle error
 }
 
 @Override
 public void onNext(List<Item> items) {
 
 }
 }); addToList(items);
31. Example 3 TrueTime: Implementing NTP with Rx github.com/instacart/truetime-android
32. Example 3 TrueTime NTP
 
 Pool DNS
 Resolve SNTP Rq 2 SNTP Request 1 SNTP Rq 3 SNTP Rq 4 SNTP Rq 5 Least
 roundtrip delay 5 IP1 IP2 IP3 IP4 Sort by clock offset + Pick Median
33. Example 3 TrueTime NTP
 
 Pool DNS
 Resolve SNTP Rq 2 SNTP Request 1 SNTP Rq 3 SNTP Rq 4 SNTP Rq 5 Least
 roundtrip delay 5 IP1 IP2 IP3 IP4 TrueTime Sort by clock offset + Pick Median
34. Observable
 .just(ntpPool)
 .compose(resolveNtpPool())
 NTP DNS
 Resolve time.apple.com Code: compose Observable<Type1> Observable<Type 2> IP1 IP2 IP3 IP4
35. Observable
 .just(ntpPool)
 .compose(resolveNtpPool())
 NTP DNS
 Resolve time.apple.com Code: private Transformer<String, String> resolveNtpPool(){
 return ntpPool -> {
 try {
 return Observable.from( InetAddress.getAllByName(ntpPool)); } catch (UnknownHostException e) {
 return Observable.error(e);
 }
 })
 }

36. Observable
 .just(ntpPool)
 .compose(resolveNtpPool())
 NTP DNS
 Resolve time.apple.com Code: IP1
37. Code: Observable
 .just(ntpPool)
 .compose(resolveNtpPool())
 .flatMap(bestResponseAgainstSingleIp())
 IP1 T (IP1) Observable<R> (SNTP response) Request 1 SNTP Request 2 SNTP Request 3 SNTP Request 4 SNTP Request 5 SNTP
38. compose Observable<R> Observable<T> map R T flatmap T Observable<T> Holy trinity of RxConversion
39. Code: IP1 Observable
 .just(ntpPool)
 .compose(resolveNtpPool())
 .flatMap(bestResponseAgainstSingleIp())
 bestResponseAgainstSingleIp() Func1 <String, Observable<Response>> return Observable .just(singleIp)
 .repeat(5) { Request 1 SNTP Request 2 SNTP Request 3 SNTP Request 4 SNTP Request 5 SNTP
40. Code: .flatMap(ipAddress -> sntpNtwrkReq(ipAddress)) Observable
 .just(ntpPool)
 .compose(resolveNtpPool())
 .flatMap(bestResponseAgainstSingleIp())
 bestResponseAgainstSingleIp() return Observable .just(singleIp)
 .repeat(5) { IP1 Request 1 SNTP Request 2 SNTP Request 3 SNTP Request 4 SNTP Request 5 SNTP Func1 <String, Observable<Response>>
41. Code: IP1 Observable
 .just(ntpPool)
 .compose(resolveNtpPool())
 .flatMap(bestResponseAgainstSingleIp())
 bestResponseAgainstSingleIp() return Observable .just(singleIp)
 .repeat(5) { 5 .flatMap(ipAddress -> sntpNtwrkReq(ipAddress)) Request 1 SNTP Request 2 SNTP Request 3 SNTP Request 4 SNTP Request 5 SNTP 
 .doOnError(throwable -> {
 // this request alone failed // retry this req alone
 })
 .retry(5)) } Func1 <String, Observable<Response>>
42. Code: IP1 Observable
 .just(ntpPool)
 .compose(resolveNtpPool())
 .flatMap(bestResponseAgainstSingleIp())
 bestResponseAgainstSingleIp() return Observable .just(singleIp)
 .repeat(5) { 5 .flatMap(ipAddress -> sntpNtwrkReq(ipAddress)) Request 1 SNTP Request 2 SNTP Request 3 SNTP Request 4 SNTP Request 5 SNTP } Func1 <String, Observable<Response>> 
 .doOnError(throwable -> {
 // this request alone failed // retry this req alone
 })
 .retry(5)) ) 
 .toList()
43. 
 .map(responseList -> {
 Collections.sort(responseList, comparator); 
 return responseList.get(0); }) Least
 roundtrip delay Code: 
 .toList() Observable
 .just(ntpPool)
 .compose(resolveNtpPool())
 .flatMap(bestResponseAgainstSingleIp())
 bestResponseAgainstSingleIp() Func1 <String, Observable<Response>> { } 
 .doOnError(throwable -> {
 // this request alone failed // retry this req alone
 })
 .retry(5)) )
44. Example 3 TrueTime NTP
 
 Pool DNS
 Resolve SNTP Rq 2 SNTP Request 1 SNTP Rq 3 SNTP Rq 4 SNTP Rq 5 Least
 roundtrip delay 5 IP1 IP2 IP3 IP4 Sort by clock offset + Pick Median
45. Example 3 TrueTime NTP
 
 Pool DNS
 Resolve SNTP Rq 2 SNTP Request 1 SNTP Rq 3 SNTP Rq 4 SNTP Rq 5 5 IP1 IP2 IP3 IP4 Sort by clock offset + Pick Median
46. .map(filterMedianResponse()) Code: Observable
 .just(ntpPool)
 .compose(resolveNtpPool())
 .flatMap(bestResponseAgainstSingleIp())
 .toList() filterMedianResponse() Func1 <List<Response>, Response> return responseList -> { } Collections.sort(responses, comparator);
 Sort by clock offset return bestResponses .get(bestResponses.size() / 2); Pick Median Sort by clock offset + Pick Median
47. Observable
 .just(ntpPool)
 .compose(resolveNtpPool())
 .flatMap(bestResponseAgainstSingleIp())
 .doOnNext(response -> convertToTime(response)); .map(filterMedianResponse()) .toList() Code: TrueTime Sort by clock offset + Pick Median
48. Example 3 TrueTime NTP
 
 Pool DNS
 Resolve SNTP Rq 2 SNTP Request 1 SNTP Rq 3 SNTP Rq 4 SNTP Rq 5 Least
 roundtrip delay 5 IP1 IP2 IP3 IP4 TrueTime Sort by clock offset + Pick Median
49. ✌ fragmentedpodcast.com tech.instacart.com @kaushikgopal kaush.co My thanks to @cyrilmotier who graciously allowed me to rip-off his slide deck theme