Building multiplayer game using Streams v2

In this talk you will learn how stream-based programming can be used to implement web frontend and multiplayer backend of the classic game: Snake.

Building dynamic applications using imperative approach tends to create lots of unmaintainable code. Stream-based programming tries to solve this problem by introducing fully declarative way of defining application logic. While using streams, you will focus on WHAT needs to be done, not HOW and WHEN.

The talk is divided into 3 parts. In the first part you will learn how to create a frontend of the Snake web game using streams as building blocks. Then, we will move to the server side and use Scala and Akka Streams library to create backend service that will allow the game to be played by multiple players. In the third part, we will discuss reactive streams and how they make asynchronous communication safe.

Transcript

1. michalplachta.com @miciek Building multiplayer game using Streams Michał Płachta
2. michalplachta.com @miciek Streams Streams? way of defining application logic level of abstraction Threads Futures Actors
3. michalplachta.com @miciek Streams? focused on data transformations Element Our Code New Element
4. michalplachta.com @miciek Streams? explicit time dependencies time
5. michalplachta.com @miciek Streams? declarative programming Functional developer’s sanity Imperative Declarative
6. michalplachta.com @miciek What’s inside? • how frontend web app written in streams looks like • how backend app written using streams looks like • how do Reactive Streams keep async communication safe?
7. michalplachta.com @miciek Meet Snake & Fruit Snake Fruit
8. michalplachta.com @miciek Our First Stream: “Ticks” stream
9. michalplachta.com @miciek “Directions” stream
10. michalplachta.com @miciek “Snake’s Head Positions” stream
11. michalplachta.com @miciek Stream operators: map & scan stream.map(f) • transforms values from stream using function f • returns a new stream with transformed values stream.scan(init, f) • accumulates values from stream using function f ◦ f(acc,val)=>new_acc • returns a new stream that outputs accumulator value
12. michalplachta.com @miciek Once again: the code
13. michalplachta.com @miciek Displaying values from the stream
14. michalplachta.com @miciek Streams are reusable
15. michalplachta.com @miciek Streams & Operators are lazy No elements or No terminal function
16. michalplachta.com @miciek “You call this a game?”
17. michalplachta.com @miciek “Key presses” stream
18. michalplachta.com @miciek “Left Key Presses” stream
19. michalplachta.com @miciek “Right Key Presses” stream
20. michalplachta.com @miciek Stream operators: filter stream.filter(p) • checks each value from stream against the predicate passed as p • returns a new stream that outputs only values that satisfy condition p Already used: • map(f) • scan(init, f(acc, val))
21. michalplachta.com @miciek “Left Rotations” stream
22. michalplachta.com @miciek “Right Rotations” stream
23. michalplachta.com @miciek “Actions” stream
24. michalplachta.com @miciek Stream operators: merge stream.merge(stream2) • stream is merged with stream2 • returns a new stream that outputs values from both stream and stream2 as they appear Already used: • map(f) • scan(init, f(acc, val)) • filter(p)
25. michalplachta.com @miciek New “Directions” stream based on user input
26. michalplachta.com @miciek Recap: Old “Directions” stream
27. michalplachta.com @miciek “Directions” stream: Better, but no timing! vs
28. michalplachta.com @miciek Stream operators: sampledBy stream.sampledBy(stream2) • last value from stream is taken when any value appears in stream2 • returns a new stream that outputs values from stream sampled by values from stream2 Already used: • map(f) • scan(init, f(acc, val)) • filter(p) • merge(stream2)
29. michalplachta.com @miciek “Directions” stream: user input & timing
30. michalplachta.com @miciek “Snake Head Positions”: no change needed
31. michalplachta.com @miciek User moves the snake: the code
32. michalplachta.com @miciek Current implementation • “ticks” • “key presses” • “left key presses” • “right key presses” • “left rotations” • “right rotations” • “actions” • “dir changes” • “directions” • “snake head positions” + map, scan, filter, merge & sampledBy 10 streams
33. michalplachta.com @miciek Stream operators: slidingWindow stream.slidingWindow(n) • aggregate last n values • returns a new stream that outputs arrays of last n values from the stream • can be implemented using scan Already used: • map(f) • scan(init, f(acc, val)) • filter(p) • merge(stream2) • sampledBy(stream2)
34. michalplachta.com @miciek “Snakes” stream
35. michalplachta.com @miciek Quiz: Eating a fruit? using 6 operators • map(f) • scan(init, f(acc, val)) • filter(p) • merge(stream2) • sampledBy(stream2) • slidingWindow(n) & 11 streams • “ticks” • “key presses” • “left key presses” • “right key presses” • “left rotations” • “right rotations” • “actions” • “direction changes” • “directions” • “snake head positions” • “snakes”
36. michalplachta.com @miciek “Fruit eaten events” stream
37. michalplachta.com @miciek Frontend in action!
38. michalplachta.com @miciek Architecture: 11 streams, 2 ins & 2 outs
39. michalplachta.com @miciek Snake’s going multiplayer FINALLY SOME SCALA CODE
40. michalplachta.com @miciek Snake Server App Requirements • clients send snake positions to the server • server generates fruits • server keeps scores • server broadcasts: ◦ positions of all snakes ◦ positions of fruits ◦ current scoreboard
41. michalplachta.com @miciek Let’s design Server App Flow! one player flow Game Event
42. michalplachta.com @miciek WebSocket connected to the Flow
43. michalplachta.com @miciek WebSocket as a stream: Server App
44. michalplachta.com @miciek Snake App Flow (one player)
45. michalplachta.com @miciek Moved to server: “Fruit eaten events” stream
46. michalplachta.com @miciek “Fruits” stream
47. michalplachta.com @miciek “Fruits” stream using Akka Stream
48. michalplachta.com @miciek Snake App Flow (one player)
49. michalplachta.com @miciek “Score updates” stream
50. michalplachta.com @miciek “Scores” stream
51. michalplachta.com @miciek “Scores” stream using Akka Stream
52. michalplachta.com @miciek Snake App Flow (one player)
53. michalplachta.com @miciek Game Logic Server Flow
54. michalplachta.com @miciek Game Logic Flow using Akka Stream
55. michalplachta.com @miciek Snake App Flow (one player)
56. michalplachta.com @miciek Merging Player Flows outputs
57. michalplachta.com @miciek Server app flow using Akka Stream
58. michalplachta.com @miciek Backend in action!
59. michalplachta.com @miciek What if there are thousands of players?
60. michalplachta.com @miciek Crossing Async Boundary Push Model • not safe when Subscriber slower • perfect when Subscriber faster • too slow when Subscriber faster • perfect when Subscriber slower Pull Model
61. michalplachta.com @miciek Reactive Streams Streams with supply & demand • “as fast as possible, but not faster” • consumer wants to pull elements (= demand) • only then publisher can push ◦ it supplies elements until demand is satisfied
62. michalplachta.com @miciek Reactive Streams Lack of demand = Backpressure
63. michalplachta.com @miciek Example of Reactive Streams
64. michalplachta.com @miciek Reactive Streams
65. michalplachta.com @miciek Reactive Streams API public interface Publisher<T> { public void subscribe(Subscriber<? super T> s); } public interface Subscriber<T> { public void onSubscribe(Subscription s); public void onNext(T t); public void onError(Throwable t); public void onComplete(); } public interface Subscription { public void request(long n); public void cancel(); } public interface Processor<T, R> extends Subscriber<T>, Publisher<R> {}
66. michalplachta.com @miciek Simple API, hard implementation... https://github.com/reactive-streams/reactive-streams-jvm/blob/v1.0.0/README.md#specification
67. michalplachta.com @miciek Reactive Streams API implementations • Project Reactor • Ratpack • Vert.x • Slick • Akka Stream • JDK 9 • Integrations: ◦ Kafka, ◦ Mongo, ◦ Cassandra
68. michalplachta.com @miciek Akka Stream model Publisher => Source Processor => Flow Subscriber => Sink Our Snake implementation uses Reactive Streams!
69. michalplachta.com @miciek • NO ◦ frontend is pushing every 100ms • Possible solutions: ◦ Reactive Streams web frameworks? ◦ conflate/expand? ◦ buffer between frontend & backend? Is Snake Multiplayer safe?
70. michalplachta.com @miciek Links Build YOUR OWN Snake using Streams - blog post (step-by-step tutorial) michalplachta.com/2016/05/11/reactive-ui-by-example/ Client Side Code github.com/miciek/web-snake-react-bacon Server Side Code github.com/miciek/snake-multiplayer-akka-streams Play with streams and operators! rxmarbles.com/ Akka Streams doc.akka.io/docs/akka/2.4.9/scala/stream/ BaconJS - streams in JavaScript baconjs.github.io/
71. michalplachta.com @miciek Michał Płachta THANK YOU www.michalplachta.com