Mastering Spring Webflux: Switching Between Reactive Streams in Java

Learn how to effectively manage data in `Spring Webflux` and solve common issues with reactive stream operations. Dive into an example solution for creating records efficiently.
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Mastering Spring Webflux: Switching Between Reactive Streams in Java

In the world of reactive programming with Java, particularly when using Spring Webflux, developers often face challenges when trying to manage data flow efficiently. One common problem arises when creating new records in a database, especially when the new record depends on fetching existing data from another record. This guide will explore a typical scenario and provide a clear solution to the problem at hand.

The Problem

The task at hand is to create a new record in the database while fetching details from an existing record. The code is utilizing a Mono<Subject> to represent the new record but encounters an issue: the object is not inserted into the database, and no errors are thrown. Let's take a look at the relevant code:

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

The expectation is that before inserting the new record, we would populate certain fields based on the fetched parent record.

Understanding the Mistake

The key issue here is the improper use of the doOnNext operator. While doOnNext allows for side effects—like updating the child object—it does not execute unless there is a subscriber that requests the data. In essence, the following happens:

The operation inside doOnNext is not executed because the subject is not getting subscribed.

As a result, fields in the Subject class remain unpopulated, leading to the record not being inserted.

The Solution

To resolve the issue, we need to avoid using doOnNext for this scenario, since it is meant for side effects rather than for core business logic or I/O operations. Instead, we will perform the necessary updates directly within the flatMap operations. Below is the corrected version of the code:

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

Breakdown of the Solution

First flatMap:

We fetch the parent record using the findById method, then we map the parent data to the new subject.

Updating Fields:

After the parent record is retrieved, we update the fields of the subject directly within the map method.

Inserting into Database:

Finally, we use another flatMap to insert the updated subject back into the database seamlessly.

Key Takeaways

Always ensure that any side-effect code involving data updates is executed within a context that is guaranteed to run, like map or another flatMap.

Do not rely on doOnNext for business logic; it should only be used for logging or monitoring purposes.

Handlers like flatMap provide the context needed to manage asynchronous operations effectively.

By following this structured approach, you can effectively switch between reactive streams and manipulate your data with confidence in Spring Webflux. Feel free to adjust the solution to your specific use cases as needed!