Скачать или смотреть How to Prevent Chained Mono Execution on Error in Spring Webflux

Discover how to correctly manage `Mono` calls in `Spring Webflux` to ensure proper error handling and prevent unintended service calls.
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This video is based on the question https://stackoverflow.com/q/62559572/ asked by the user 'Andrea Damiani' ( https://stackoverflow.com/u/11971373/ ) and on the answer https://stackoverflow.com/a/62562888/ provided by the user 'Alex Shavlovsky' ( https://stackoverflow.com/u/13794675/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Managing Chained Mono Execution in Spring Webflux

In the world of reactive programming, handling errors when calling multiple services is crucial for ensuring app stability and user experience. When working with Spring Webflux, developers often face challenges related to how chained Mono executions respond to error signals. This post will delve into a common issue where a Mono continues to execute even after the preceding call returns an error. Let's break down the misunderstanding and provide effective solutions for handling these scenarios.

The Problem

Consider the following function, doSomething, which is intended to perform two service calls sequentially. The first call retrieves a Foo object based on certain identifiers, while the second call fetches a list of Baz objects associated with that Foo. However, there's an unexpected behavior: even when the first call results in an error signal (such as NOT_FOUND or UNPROCESSABLE_ENTITY), the second service call is still executed.

Here's a simplified version of the initial problematic code:

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This raises a question for many developers: Why is the second service call triggered even if the first call fails?

Understanding the Issue

The root cause of this issue lies in how flatMap and error handling are managed in reactive streams. In the provided code, the second service call is created upfront and is thus always evaluated regardless of the outcome of the first call.

Key Concepts:

flatMap: It is used to transform the result of a Mono but does not prevent the subsequent Mono from being initialized.

Error Signals: In reactive programming, an error signal is emitted on failure, which can be captured to prevent further execution.

In the original implementation, the second Mono (secondResult) is being subscribed to before the first Mono (firstResult) has a chance to signal an error.

Effective Solutions

To ensure that the second service call only executes if the first call is successful, you can modify the doSomething method in the following ways:

Solution 1: Using doOnNext

You can use the doOnNext method to execute the second call as a side-effect, only when the first call succeeds:

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Solution 2: Delaying the Execution with Mono.defer

Another approach is to use Mono.defer() to defer the execution of the second service call. This allows it to only be executed when it's actually needed:

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Solution 3: Combining Results Correctly

You can also combine the results effectively and ensure that the first Mono's result is preserved:

[[See Video to Reveal this Text or Code Snippet]]

Conclusion

Managing error signals in reactive programming with Spring Webflux requires a clear understanding of how Mono and flatMap work. By utilizing methods like doOnNext, Mono.defer, and proper flatMap usage, you can control the flow of execution and ensure that subsequent service calls are only made when appropriate.

If you encounter issues with chained Mono executions, consider the solutions presented in this post. They can help streamline your error handling and improve the reliability of your service interactions.

Remember, effective error management is key to building resilient and user-friendly applications!