Handling Backpressure and Managing Streams in Spring WebFlux

Spring WebFlux is a reactive web framework introduced by the Spring team to build scalable and non-blocking web applications. It is based on Project Reactor's Reactive Streams API that allows developers to handle backpressure and manage streams efficiently. In this article, we will explore how Spring WebFlux handles backpressure and stream management.

Understanding Backpressure

Backpressure is a mechanism used in reactive systems to deal with the scenario where the producer is generating data faster than the consumer can handle. In a traditional non-reactive world, this situation could lead to an out-of-memory error or system unresponsiveness. However, in reactive systems, backpressure comes into play to slow down the data producer and give the consumer enough time to process the data.

Publisher and Subscriber

In Spring WebFlux, a backpressure-aware communication happens between the publisher and the subscriber using the Flux and Mono classes. The Flux class represents a stream of 0 to N elements, while the Mono class represents a stream with 0 or 1 element.

When a subscriber subscribes to a Flux or Mono, it receives a subscription object that can be used to request a specific number of elements to be processed. The subscriber can use the Subscription.request(n) method to request n number of elements at a time. By default, if the subscriber doesn't explicitly request any elements, it receives a maximum of 32 elements.

Dealing With Backpressure

Spring WebFlux provides various operators to manage backpressure effectively. Let's explore a few common operators:

limitRate(int n)

This operator limits the rate of elements emitted downstream to n elements per second. It helps the subscriber to regulate the flow and avoid being overwhelmed with a large number of elements.

Flux<Integer> numbers = Flux.range(1, 1000);

In the above example, the limitRate operator limits the rate to 10 elements per second. So, the subscriber receives and processes only 10 elements per second, avoiding any potential backpressure issues.

onBackpressureBuffer(int n)

This operator buffers elements up to n elements when the consumer cannot keep up with the producer's pace. It helps to smooth out the flow and prevent data loss.

Flux<Integer> numbers = Flux.range(1, 1000);

In the above example, the onBackpressureBuffer operator buffers up to 100 elements before backpressure is applied. This allows the consumer to catch up without losing any data.

Stream Management

Spring WebFlux provides various mechanisms to manage streams efficiently. Let's dive into a few techniques:

flatMap(Function<T, Publisher<V>> mapper)

The flatMap operator is commonly used to handle streams of asynchronous results. It transforms each element into a Publisher and merges them into a single reactive stream. This allows concurrent processing and efficient resource utilization.

Flux<Integer> numbers = Flux.range(1, 1000);
numbers.flatMap(num -> Flux.just(num + 1))

In the above example, the flatMap operator takes each number emitted by the publisher and maps it to a new publisher that emits num + 1. These individual streams are then merged into a single stream during the subscription, enabling parallel processing.

concatMap(Function<T, Publisher<V>> mapper)

The concatMap operator is quite similar to flatMap, but it maintains the order of the emitted elements. It waits for the previous element to complete processing before subscribing to the next element.

Flux<Integer> numbers = Flux.range(1, 1000);
numbers.concatMap(num -> Flux.just(num + 1))

In the above example, the concatMap operator guarantees that the order of the elements remains the same. It waits for each element to complete before processing the next one, ensuring the correct order of the emitted elements.


Spring WebFlux provides powerful tools and operators to handle backpressure and manage streams efficiently. By leveraging the Reactive Streams API and the range of operators provided by Spring WebFlux, developers can build robust and scalable applications. Understanding and utilizing these concepts will enable developers to design responsive systems that can handle varying workloads effectively.

noob to master © copyleft