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Learn how to make sequential REST API calls using Spring Boot WebClient with `concatMap` for handling lists in order.
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How to Sequentially Call REST APIs with Spring Boot WebClient using concatMap

In today's fast-paced coding environment, handling multiple asynchronous calls can pose challenges, especially when the order of operations matters. One common scenario you're likely to encounter is the need to call multiple REST APIs in sequential order. In this guide, we will explore an effective way to accomplish this using Spring Boot's WebClient, focusing on maintaining the order of execution while working with a list of values.

The Problem

We often face situations where we have a list of items, and we need to make an API call for each item based on its specific criteria. Take the example of a list containing elements that begin with letters "A" and "B". The requirement is to call apiA for elements starting with "A" and apiB for those starting with "B". The challenge is ensuring these API calls happen in a sequential manner.

Here's a simplified version of the initial code for clarity:

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

This approach, however, executes the calls in a non-blocking manner, which doesn't guarantee sequential processing. So, what's the solution?

The Solution: Using concatMap

To achieve sequential API calls while maintaining order, we can leverage the concatMap method available in the Project Reactor's Flux class. This allows us to transform the elements emitted by Flux asynchronously into Publishers while preserving their order through concatenation.

Step-by-Step Implementation

Convert List to Flux: First, we need to convert our list of values into a Flux stream.

Use concatMap: Utilize concatMap to process each element one at a time. Inside concatMap, we’ll check the condition for each element and call the appropriate API.

Here's how you can write this using Spring's WebClient:

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

Explanation of the Code

Flux.fromIterable(values): This converts our list of values into a reactive stream.

concatMap: This method takes each value from the stream one at a time. If it starts with "A", it calls callApiA; otherwise, it calls callApiB. This operation ensures that the APIs are called sequentially, waiting for each call to complete before moving to the next one.

subscribe(): This line is essential as it triggers the stream; without subscribing, no calls will be executed.

Advantages of Using concatMap

Sequential Processing: Unlike traditional loops, concatMap ensures that each API call completes before the next one starts.

Non-Blocking: Even though the calls are sequential, you get the benefits of working with a reactive approach, allowing for efficient handling of resources.

Conclusion

When working with lists in Spring Boot's WebClient, using concatMap is an effective strategy for making sequential API calls. This not only automates the process but also keeps your code clean and more manageable.

Incorporate this approach into your projects to handle similar challenges gracefully and enhance the responsiveness of your applications. Happy coding!