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Spring Boot 4 Microservices & REST APIs · Lesson

Backpressure and Operators

Transform and control reactive streams.

Backpressure and Operators is a free Spring Boot 4 Microservices & REST APIs lesson on CoddyKit — lesson 4 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Spring Boot 4 Microservices & REST APIs learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Operators Build Pipelines

Reactive operators chain together to form a processing pipeline. Each operator returns a new publisher describing the next step.

Common ones: map, flatMap, filter, take, buffer.

map vs flatMap

map transforms each item synchronously (1-to-1). flatMap transforms each item into another publisher and flattens the results — use it for async work.

Using map

map for pure, synchronous transformations.

Flux<Integer> lengths = Flux.just("a", "bb", "ccc")
    .map(String::length);
lengths.subscribe(System.out::println); // 1, 2, 3

Using flatMap

flatMap when each item triggers an async call returning a Mono or Flux.

Flux<User> users = Flux.just("1", "2", "3")
    .flatMap(id -> userService.findById(id));
users.subscribe(System.out::println);

flatMap Ordering

flatMap interleaves results as inner publishers complete, so order is not guaranteed. Use concatMap to preserve order.

Flux<User> ordered = Flux.just("1", "2", "3")
    .concatMap(id -> userService.findById(id));

What is Backpressure

Backpressure is the mechanism by which a slow consumer tells a fast producer to slow down.

Subscribers request a number of items; the producer only emits up to that demand. This prevents memory overflow.

Limiting with take

take requests only the first N items, then cancels the upstream.

Flux<Long> first3 = Flux.interval(Duration.ofMillis(100))
    .take(3);
first3.subscribe(System.out::println); // 0, 1, 2

Buffering Items

buffer groups items into lists, useful for batching.

Flux<List<Integer>> batches = Flux.range(1, 10)
    .buffer(3);
batches.subscribe(System.out::println);
// [1,2,3], [4,5,6], [7,8,9], [10]

onBackpressureBuffer

When a producer is faster than the consumer, strategies control overflow: buffer, drop, or latest.

Flux.interval(Duration.ofMillis(1))
    .onBackpressureBuffer(100)
    .subscribe(System.out::println);

Throttling with limitRate

limitRate caps how many items are requested upstream at a time, applying controlled backpressure.

Flux.range(1, 1000)
    .limitRate(10)
    .subscribe(System.out::println);

Combining Operators

Real pipelines chain several operators to filter, transform, and limit a stream.

Flux.range(1, 100)
    .filter(n -> n % 2 == 0)
    .map(n -> n * n)
    .take(5)
    .subscribe(System.out::println);

Quick Check

Test your understanding of operators and backpressure.

Recap

You learned about operators and backpressure:

  • map = sync transform; flatMap = async, unordered; concatMap = async, ordered
  • Backpressure lets slow consumers control fast producers
  • take, buffer, limitRate shape the stream

Frequently asked questions

Is the “Backpressure and Operators” lesson free?

Yes — the full text of “Backpressure and Operators” is free to read here on the web, and the Spring Boot 4 Microservices & REST APIs course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Spring Boot 4 Microservices & REST APIs course, upgrade to CoddyKit PRO.

What will I learn in “Backpressure and Operators”?

Transform and control reactive streams. You practise Spring Boot 4 Microservices & REST APIs with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Spring Boot 4 Microservices & REST APIs?

No prior experience is required. Spring Boot 4 Microservices & REST APIs on CoddyKit is structured for beginners through advanced learners; this is — lesson 4 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Backpressure and Operators” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this Spring Boot 4 Microservices & REST APIs lesson?

Yes. Every Spring Boot 4 Microservices & REST APIs lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

All lessons in this course

  1. Mono and Flux Basics
  2. Reactive Controllers
  3. WebClient for Reactive Calls
  4. Backpressure and Operators
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