The document discusses reactive programming and the Reactive Streams specification. It introduces reactive programming as a programming paradigm for concurrent and asynchronous processing using a stream-based approach. It then describes the Reactive Streams specification, which defines interfaces and protocols for building asynchronous streams with non-blocking back pressure. The rest of the document discusses an implementation of Reactive Streams called Project Reactor and how it can be used with Spring frameworks to build reactive applications.

1. From Streams to
Reactive Streams
Oleg Tsal-Tsalko
JavaDay Kyiv 2017

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OLEG TSAL-TSALKO

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Agenda
Reactive Manifesto
Reactive Programming
Reactive Streams spec
Project Reactor

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Nowadays challenges

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Non-blocking architecture
• Can’t write imperative code anymore
• Can’t assume single thread of control
• Must deal with async results (listeners/callbacks)
• Everything becomes stream of events

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What is Reactive Programming?

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Concurrent
Programming
Reactive
Programming

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Reactive Use Cases
External Non-blocking Service Calls
Highly Concurrent Message Consumers
Spreadsheets
Abstraction Over (A)synchronous Processing
…

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Existing implementations

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One spec to rule them all

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Reactive Streams spec
Asynchronous stream processing with non-blocking back pressure
The Problem
Handling streams of “live” data
whose volume is not predetermined so that
fast data source does not overwhelm the stream destination
Scope
Minimal set of interfaces, methods and protocols to build
asynchronous streams of data with non-blocking back pressure

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What is back-pressure?

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Back-pressure

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Reactive Streams API

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Publish-Subscribe Flow

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Difference to CompletableFuture
If you have a reference to the Future,
it means the task processing an asynchronous result is already
executing!
Deferred - nothing happens before the call to subscribe()
Pull - Subscriber pull the next chunk of data when needed
Push - Producer push new data to Consumer according to demand
CompletableFuture is push only model
ReactiveStreams is deferred pull-push model

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Spring Reactor

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Spring Reactor evolution

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Reactor in Spring ecosystem

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Three things you can do with Flux/Mono:
1. operate on it
2. subscribe to it
3. configure it
Simple and clear, isn’t it?

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Fluent API

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Programming model is very similar
Both heavily use operators chaining and lambdas
Java8 Streams
Reactive
Streams
However purpose is different!!!
Reactive Streams operate on
reactive data
and represent data over time
Java8 streams operate on
collections
and have all or nothing semantics

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Reactive streams lifecycle
• Assembly-time: When you compose your stream
just().subscribeOn().map()
• Subscription-time: When Subscriber subscribes to a stream
and it triggers a "storm" of subscriptions under the hood
• Runtime: When items are generated followed by zero or one
terminal event of error/completion

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Links
Exercises to practice: https://github.com/reactor/lite-rx-api-
hands-on
Examples shown:
https://github.com/olegts/ReactiveStreams/tree/master/src/
test/java/org/reactivestreams/reactor/javaday
Reactive Streams: https://github.com/reactive-
streams/reactive-streams-jvm
Project Reactor: https://projectreactor.io/docs

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