Solving the Concurrency Problem in a Spring Boot REST Service

Learn how to effectively manage concurrency issues in your Spring Boot REST service. Explore practical solutions and best practices.
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Solving the Concurrency Problem in a Spring Boot REST Service

In the world of web services, concurrency is a common problem that can lead to inconsistencies and errors, especially in REST services built using Spring Boot. In this guide, we will tackle a practical example of a concurrency issue that arises when two APIs validate and process requests involving a numerical field, progressiveNr. This field is critical for maintaining the correct sequence of operations related to a specific voucher.

Understanding the Concurrency Issue

Let's set the scene: Imagine you have two API endpoints in your Spring Boot project. Both APIs accept the same object as input to perform different actions. They need to validate this object before processing it. The challenge lies with progressiveNr, a numeric field integral for tracking the order of requests related to vouchers.

Example Scenario

Suppose the client sends two simultaneous requests to the APIs:

API 1: {requestId: 196, voucherId: 1, progressiveNr: 2}

API 2: {requestId: 197, voucherId: 1, progressiveNr: 3}

Here lies the issue: While API 1 is still validating its request, API 2 is also triggered. If the validation for API 1 is not yet complete, the progressiveNr table still indicates that the last processed number is 1. Thus, when validating API 2, you would receive an error: "progressiveNr must be 2" as API 2 expects the number to increment correctly.

Proposed Solution

Synchronous Requests and Validation

To prevent this type of concurrency problem from happening, you should ensure that calls to your APIs are made synchronously instead of in parallel. This means users should not be able to submit multiple requests that could potentially conflict with one another without proper order.

Returning Proper Response Codes

Instead of attempting to find complex fixes, the simplest approach is to manage requests appropriately:

For requestId: 196, process it correctly and return a 200 OK response.

For requestId: 197, since it's attempting to break the sequence, immediately return a 400 Bad Request.

This ensures that your application adheres to its contract and does not allow clients to manipulate the state of the voucher that could lead to invalid data.

Why Locks Aren’t the Best Option

While you may consider using locks to solve this concurrency issue, it can add complexity and decrease the performance of your application. Locks can lead to potential deadlocks and should be used sparingly. The above solution keeps your API logic simple and robust without over-engineering.

Best Practices to Avoid Concurrency Issues

Request Management: Create a system that ensures clients can only submit requests in a controlled manner, perhaps using a queue system.

Validation Logic: Review your validation logic to ensure it effectively communicates what is required for a valid request before processing.

Client Education: Inform clients about the expected order and patterns for requests to minimize risks of errors.

Conclusion

Concurrency issues can be tricky, but with proper planning and request management, they can be effectively managed in your Spring Boot REST services. By returning appropriate response codes and ensuring synchronous processing of requests, you can retain control over the state of your resources and maintain system integrity.

Implementing these strategies will help keep your application stable and provide a better experience for your users. If you encounter other concurrency challenges, don’t hesitate to explore further or ask for help from the community.