Slides from my madlab presentation on Akka Streams & Reactive Kafka (October 2015), full slides and source here: https://github.com/markglh/AkkaStreams-Madlab-Slides

1. AKKA STREAMS
FROM ZERO TO KAFKA
Createdby /MarkHarrison @markglh

2. HOW IT ALL BEGAN
“Reactive Streams is an initiative to provide a
standard for asynchronous stream processing
with non-blocking back pressure. This
encompasses efforts aimed at runtime
environments (JVM and JavaScript) as well as
network protocols.”

3. WHY
Ef ciently processing large indeterminate streams is hard
Avoiding blocking is essential to maximise performance
Every stage in the stream needs to be able to push and pull
We don't want to overload (or starve!) downstream
consumers...

5. HOW
Treat data as a stream of elements
Asynchronous non-blocking data and demand ows
Demand ows upstream, causing data to ow downstream
Data ow is therefore restricted by demand
Back Pressure!!
Demand happens on a separate ow!

6. WHAT
The Reactive Streams speci cation is just that
A collection of interfaces methods and protocols
Provides example implementations and a TCK for
veri cation
Aimed at providing a way to build common
implementations

7. INTRODUCING AKKA STREAMS!!
AKKA'S IMPLEMENTATION OF REACTIVE STREAMS

8. DESIGN PRINCIPLES
Explicitness over magic (I'm looking at you Shapeless!)
Fully composable
Each component, or set of componenents can be combined
Each building block is immutable
Fully compatible with other Reactive Stream implementations

9. BUILDING BLOCKS

10. BUILDING BLOCKS CONT...
Source
Traditionally known as a producer
Supplies messages that will ow downstream
Exactly one output stream
Sink
Traditionally known as a consumer
End point of the stream, this is where messages end up

11. BUILDING BLOCKS CONT...
Flow
A processing stage in the Stream
Used to compose Streams
Exactly one input and one output stream
See also BidirectionalFlow (two in -> two out)

12. BUILDING BLOCKS CONT...
RunnableGraphs
A pre-assembled set of Stream components, packaged into
a Graph.
All exposed ports are connected (between a Source and
Sink)
This can then be Materialized

13. BUILDING BLOCKS CONT...
Composite Flows
It is possible to wrap several components into more
complex ones
This composition can then be treated as one block
Partial Flow Graphs
An incomplete Flow (Graph)
Can be used to construct more complex Graphs easily

14. BUILDING BLOCKS CONT...
Materializer
Once complete, the ow is Materialized in order to start
stream processing
Supports fully distributed stream processing
Each step must be either serializable immutable values
or ActorRefs
Fails immediately at runtime if the Graph isn't complete

15. ERRORS VS FAILURES
Errors handlied within the stream as normal data elements
Passed using the onNext function
Failure means that the stream itself has failed and is collapsing
Raises the onError signal... (???)
Each block in the ow can choose to absorb or propagate the
errors
Possibly resulting the the complete collapse of the ow

17. FIRST THINGS FIRST
We need to create an ActorSystem and Materializer
implicit val system = ActorSystem("actors")
implicit val materializer = ActorMaterializer()

18. SIMPLE STREAM
We need to create an ActorSystem and Materializer
Source(1 to 5)
.filter(_ < 3) // 1, 2
.map(_ * 2) // 2, 4
.to(Sink.foreach(println))
.run()
//prints 2 4

19. COMPOSING ELEMENTS TOGETHER
We can combine multiple components together
Composing elements together
val nestedSource = Source(1 to 5)
.map(_ * 2)
val nestedFlow = Flow[Int]
.filter(_ <=
.map(_ + 2)
val sink = Sink.foreach(println)
//link up the Flow to a Sink
val nestedSink = nestedFlow.to(Sink.foreach(println))
// Create a RunnableGraph - and run it! Prints 4 6
nestedSource.to(nestedSink).run()

20. COMPOSING ELEMENTS TOGETHER CONT...
Alternatively we could do this, linking them in one step
nestedSource
.via(nestedFlow)
.to(Sink.foreach(println(_)))

21. COMPOSING ELEMENTS TOGETHER CONT...

22. GRAPH PROCESSING STAGES
Fan Out
Broadcast[T] – (1 input, N outputs)
Balance[T] – (1 input, N outputs)
...
Fan In
Merge[In] – (N inputs , 1 output)
...
Timer Driven
groupedWithin(Int, Duration)
Groups elements when either the number or duration is
reached (whichever is rst). Very useful for batching
messages.
See the Akka Stream docs for more!

23. GRAPH PROCESSING STAGES CONT...

24. THE GRAPH DSL
Whenever you want to perform multiple operations to
control the Flow of a Graph, manually constructing them as
above can become very clumbersome and tedius, not to
mentioned hard to maintain.
For this reason the Akka team have written a DSL to help
write complex Graphs.

25. THE GRAPH DSL
val g = FlowGraph.closed() {
implicit builder: FlowGraph.Builder[Unit] =>
//This provides the DSL
import FlowGraph.Implicits._
val in = Source(1 to 3)
val out = Sink.foreach(println)
//2 outputs, 2 inputs
val bcast = builder.add(Broadcast[Int](2))
val merge = builder.add(Merge[Int](2))
val f1, f2, f3, f4 = Flow[Int].map(_ + 10)
in ~> f1 ~> bcast ~> f2 ~> merge ~> f3 ~> out
bcast ~> f4 ~> merge
}
g.run() //Prints 31 31 32 32 33 33

26. THE GRAPH DSL CONT...

27. EXAMPLE - REACTIVE KAFKA
The guys at SoftwareMill have implemented a wrapper for
Apache Kafka
Tried and tested by yours truly
https://github.com/softwaremill/reactive-kafka

28. EXAMPLE - REACTIVE KAFKA CONT...
Source is a Kafka Consumer
Sink is a Kafka Publisher
val kafka = new ReactiveKafka()
val publisher: Publisher[StringKafkaMessage] =
kafka.consume(
ConsumerProperties(...)
)
val subscriber: Subscriber[String] =
kafka.publish(
ProducerProperties(...)
)
Source(publisher).map(_.message().toUpperCase)
.to(Sink(subscriber)).run()

29. A REAL WORLD EXAMPLE

30. A REAL WORLD EXAMPLE CONT...
FlowGraph.closed() {
implicit builder: FlowGraph.Builder[Unit] =>
import FlowGraph.Implicits._
val in = Source(kafkaConsumer)
val out = Sink.foreach(println)
val bcast = builder
.add(Broadcast[StringKafkaMessage](2))
val merge = builder
.add(Merge[StringKafkaMessage](2))
val parser1, parser2 = Flow[StringKafkaMessage]
.map(...)
val group = Flow[StringKafkaMessage].grouped(4)
in ~> bcast ~> parser1 ~> merge ~> group ~> out
bcast ~> parser2 ~> merge
}.run()

31. IT'S BEEN EMOTIONAL...
Slides at
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Slides
@markglh