PANTA RHEI OR REACTIVE JAVA IN PRACTICE RxJava is a library for building asynchronous and event based services using simple and powerful api. During this workshop we will create an application composed from several smaller services. These services will process requests and responses in an asynchronous manner. We will investigate advantages and drawbacks of this approach and see how RxJava helps us express such API in a way that is clear, maintainable, composable and testable. Starting from very basic RxJava usages we will venture into more demanding use cases and look how such aspects of our application as monitoring, error handling, timeouts, logging, parallelism and back pressure. APIs of internal components will also be modeled as observable sequences. This exercise is not designed to make you adopt reactive approach in every possible corner of your codebase but rather to provide fresh look at how simple and complex tasks alike can be modeled as streams and provide insight into where RxJava is and where is not a good fit.

1. Panta rhei
by Tomasz Kowalczewski
Reactive Java in practice

2. EXAMPLES REPOSITORY
https://github.com/tkowalcz/rx-java-pantha-rhei

3. REACTIVE, ASYNCHRONOUS
People has been doing it for years
(Re)invented time and again by big companies
and/or in high performance products
Now every company may need to operate at
that scale
As users demand more and faster we need
tools to do it right

4. SYNCHRONOUS DESIGN
List<String> tweets = client.getNextTweets();
List<Tweet> validTweets = Lists.newArrayList();
for (String jsonTweet : tweets) {
Tweet tweet = gson.fromJson(jsonTweet, Tweet.class);
Image image;
image = download(tweet.getUser().getProfileImage());
Tweet.setImage(image);
if (tweet.isValidTweet()) {
validTweets.add(tweet);
}
}

5. DECOMPOSING INDIVIDUAL OPERATIONS
List<String> tweets = client.getNextTweets();
List<Tweet> validTweets = Lists.newArrayList();
for (String jsonTweet : tweets) {
Tweet tweet = gson.fromJson(jsonTweet, Tweet.class);
Image image;
image = download(tweet.getUser().getProfileImage());
Tweet.setImage(image);
if (tweet.isValidTweet()) {
validTweets.add(tweet);
}
}

6. DECOMPOSING INDIVIDUAL OPERATIONS
List<String> tweets = client.getNextTweets();
List<Tweet> validTweets = Lists.newArrayList();
for (String jsonTweet : tweets) {
Tweet tweet = gson.fromJson(jsonTweet, Tweet.class);
Image image;
image = download(tweet.getUser().getProfileImage());
Tweet.setImage(image);
if (tweet.isValidTweet()) {
validTweets.add(tweet);
}
}

7. DECOMPOSING INDIVIDUAL OPERATIONS
List<String> tweets = client.getNextTweets();
List<Tweet> validTweets = Lists.newArrayList();
for (String jsonTweet : tweets) {
Tweet tweet = gson.fromJson(jsonTweet, Tweet.class);
Image image;
image = download(tweet.getUser().getProfileImage());
Tweet.setImage(image);
if (tweet.isValidTweet()) {
validTweets.add(tweet);
}
}

8. DECOMPOSING INDIVIDUAL OPERATIONS
List<String> tweets = client.getNextTweets();
List<Tweet> validTweets = Lists.newArrayList();
for (String jsonTweet : tweets) {
Tweet tweet = gson.fromJson(jsonTweet, Tweet.class);
Image image;
image = download(tweet.getUser().getProfileImage());
Tweet.setImage(image);
if (tweet.isValidTweet()) {
validTweets.add(tweet);
}
}

9. DECOMPOSING INDIVIDUAL OPERATIONS
List<String> tweets = client.getNextTweets();
List<Tweet> validTweets = Lists.newArrayList();
for (String jsonTweet : tweets) {
Tweet tweet = gson.fromJson(jsonTweet, Tweet.class);
Image image;
image = download(tweet.getUser().getProfileImage());
Tweet.setImage(image);
if (tweet.isValidTweet()) {
validTweets.add(tweet);
}
}

10. WHAT IS MISSING?
List<String> tweets = client.getNextTweets();
List<Tweet> validTweets = Lists.newArrayList();
for (String jsonTweet : tweets) {
Tweet tweet = gson.fromJson(jsonTweet, Tweet.class);
Image image;
image = download(tweet.getUser().getProfileImage());
Tweet.setImage(image);
if (tweet.isValidTweet()) {
validTweets.add(tweet);
}
}
Error
handling
Timeouts
Monitoring
Logging
Back pressure
Parallelism

11. SYNCHRONOUS DESIGN
List<String> tweets = client.getNextTweets();
List<Tweet> validTweets = Lists.newArrayList();
for (String jsonTweet : tweets) {
Tweet tweet = gson.fromJson(jsonTweet, Tweet.class);
Image image;
image = download(tweet.getUser().getProfileImage());
Tweet.setImage(image);
if (tweet.isValidTweet()) {
validTweets.add(tweet);
}
}

12. WHAT IS MISSING?
List<String> tweets = client.getNextTweets();
List<Tweet> validTweets = Lists.newArrayList();
for (String jsonTweet : tweets) {
Tweet tweet = gson.fromJson(jsonTweet, Tweet.class);
Image image;
image = download(tweet.getUser().getProfileImage());
Tweet.setImage(image);
if (tweet.isValidTweet()) {
validTweets.add(tweet);
}
}
Error
handling
Timeouts
Monitoring
Logging
Back pressure
Parallelism

13. STREAMS (ELEMENTS + TIME)
[" ", " ", " ", " ", " "]
T1 T2 T3 T4 T5

14. SUBSCRIBE, MAP, FILTER
[" ", " ", " ", " ", " "]
subscribe

15. SUBSCRIBE, MAP, FILTER
[" ", " ", " ", " ", " "]
[ , , , , ]
map
subscribe

16. SUBSCRIBE, MAP, FILTER
[" ", " ", " ", " ", " "]
[ , , , , ]
[ , , ]
map
filter
subscribe

17. Reactive flow
t
subscribe
onNext*
onCompleted | onError
unsubscribe

18. just(1, 1, 2, 3, 5, 8, 13).filter(i -> i % 2 != 0)
// 1, 1, 3, 5, 13
just(1, 1, 2, 3, 5, 8, 13).map(i -> i * 2)
// 2, 2, 4, 6, 10, 16, 26
Exercise 1a: filter, map

19. Exercise 1b: take, error handling

20. Subscriptions flow
map
filter
source
…
subscribe
subscribe
subscribe
subscribe
subscribe onNext
onNext
onNext
onNext
subscribeOn
If subscription should be
made on a different thread
observeOn
If notifications should be
made on a different thread
(e.g. UI thread)

21. Exercise 2: Observable::create

22. just([1], [1], [2, 3], [5], [], [13])
.flatMap(arr -> just(arr))
// 1, 1, 2, 3, 5, 13
Exercise 3: basic flatMap

23. Exercise 4: flatMap & async

24. Exercise 5: error handling

25. Exercise 6: cache

26. Exercise 7: testing

27. Exercise 8a: subjects
?

28. Exercise 8a: subjects

29. Exercise 8b: combineLatest

30. Backpressure

31. AVAILABLE IN A LANGUAGE OR FRAMEWORK NEAR YOU

32. FURTHER READING
Examples from this presentation
https://github.com/tkowalcz/rx-java-pantha-rhei
Reactive Java project
https://github.com/Netflix/RxJava
Other reactive projects
http://reactivex.io/
http://rxmarbles.com/
http://davesexton.com/blog/post/To-Use-Subject-Or-
Not-To-Use-Subject.aspx
https://speakerdeck.com/dlew/reactive-extensions-
beyond-the-basics