Reactive Programming

3 days Online / Zoom English Certificate 15 people max Victor Rentea €11200 ~~14000~~

This training is for you if your project uses Spring 5 WebFlux, you need to optimize a high-load API or batch workflow, or you've encountered bugs and challenges when working with Reactor. It’s also for you if you simply want to master one of the most complex programming paradigms in the world.

Introduction

The first challenge when approaching reactive programming is understanding why and when not to use it. Starting with ‘classic’ blocking web endpoints, we’ll benchmark the bottlenecks that occur under heavy load and then explore the various historical alternatives to Reactor. While we’ll examine dozens of operators, the focus will remain on understanding the signals that drive all reactive flows—this is key to unlocking the mysteries of backpressure, hot publishers, and avoiding common pitfalls when beginning to work with Reactor. We will then use this knowledge to approach several typical reactive flows, drawing conclusions about the best ways to write code that is maintainable and safe. After a comprehensive tour of Spring WebFlux features and quirks, we’ll move on to two tough tasks: testing reactive flows and progressively migrating blocking code to reactive flows. If time permits, we’ll also discuss performance tuning.

Agenda

Introduction

[optional, basics] Recap: functional pipelines with Java8 Streams
Blocking REST Endpoints: trace a thread pool starvation issue
Alternatives to Reactive: CompletableFuture, coroutines(kt), Virtual Threads (Project Loom)
Reactive Streams Spec
Mono, Flux
Understanding Marble Diagrams
Signals: next, complete, error, request, subscribe, cancel

Core Operators

Immediate: just, empty, error, never, range, fromIterable
Delayed: delay, interval, delayElements
Immediate Operators: distinct, filter, map, first, take, contains, all

Enriching Data

flatMap vs concatMap vs flatMapSequential
Batching requests: buffer, bufferWindow
Handling Empty: defaultIfEmpty, switchIfEmpty
Async filtering with.filterWhen
Parallelizing work: Tuples, zip, zipWith, zipWhen
Use-case Context design pattern

Performing Side Effects

Fire-and-forget using doOnNext
Parallelization, cancellation

[Hands-on] Migrating a blocking codebase to Reactive Programming

Schedulers: CPU vs IO, best practices, ParallelFlux
Calling blocking code in a safe way on boundedElastic scheduler
CompletableFuture integration
Bridging to a callback-based model: Sinks
Reactive REST: Endpoints, and WebClient
Reactive NoSQL: Mongo, Cassandra, Redis
Reactive SQL: R2DBC
Reactive Messages: send/receive
Kafka Streams [optional]: KStream, KTable, Spring Cloud Stream Functions

Infinite Fluxes

Preventing cancellation with onErrorContinue
GroupedFlux
Time series operators: scan, reduce, window, buffer, sample, merge, concat, combineLatest
Hot vs Cold Publisher
In-memory broadcast using Sinks
Caching Hot Publishers: replay, cache, connect, autoConnect

Resilience

Error handling patterns
Timeout and Retry
Back-pressure
Throttling: rate-limiting, resilience4j integration
Challenges of distributed systems under heavy load - brainstorming

Testing

StepVerifier vs .block()
TestPublisher
PublisherProbe + custom extension
Detecting blocking code with BlockHound
Controlling Virtual Time [optional]

Debugging, Monitoring and Tuning

Propagating metadata via Reactor Context
Checkpoints and Hooks.onOperatorDebug
Monitoring using .metrics, .elapsed, .timed

Common Pitfalls and Workarounds

Not subscribing / Resubscribing to Cold Publisher
Loosing Data Signals (empty)
.subscribe()
Blocking in non-blocking threads
Unexpected Cancelation

Review of Code

Sample from the audience

Victor is a Java Champion, Java Enterprise Architect, and Independent Trainer, delivering intense masterclasses for companies worldwide since 2014. He is passionate about software architecture, clean code, refactoring, and unit testing. He is a regular speaker at top international conferences. He's also the founder of the Bucharest Software Craftsmanship Community with 4000 members.

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