REST (Representational State Transfer) has become the standard architectural style for building API-driven applications. Its popularity is largely due to its simplicity, flexibility, and scalability.

When building a new RESTful API, Spring Boot is one of the most popular Java frameworks used today thanks to its powerful auto-configuration and robust feature set for building RESTful web services. However, there are some key aspects of RESTful API design that beginners can often overlook when starting out.

Here are some common mistakes to avoid when designing RESTful APIs with Spring Boot:

1. Not Using HTTP Methods Properly

REST APIs must properly utilize the different HTTP methods available like GET, POST, PUT, DELETE and PATCH. Each method has a specific meaning and purpose:

• GET is used for retrieving resources.
• POST is used for creating new resources.
• PUT/PATCH is used for updating existing resources.
• DELETE is used for deleting resources. Misusing the HTTP methods can lead to confusion for API consumers and cause architectural issues.

2. Not Using Proper HTTP Status Codes

Along with HTTP methods, REST APIs should return appropriate HTTP status codes that provide extra meaning and context to the API consumer. For example:

• 200 OK should be returned on a successful GET request.
• 201 Created should be returned on a successful resource creation using POST.
• 404 Not Found should be returned when a resource is not found.
• 400 Bad Request indicates a malformed request.
• 500 Internal Server Error indicates server-side errors. Proper use of status codes is important for handling errors, troubleshooting and enabling consumers to understand the responses.

3. Improper Resource and URL Design

REST URLs should be designed based on the resources, not the operations performed on them. URLs should consist of nouns and not verbs. For example:

GET /api/users 
POST /api/users

This is preferable over something like:

GET /api/getUsers
POST /api/createUser

The URLs should also be hierarchical and nested as needed to enable logical grouping and organization of APIs.

4. Not Implementing HATEOAS

Hypermedia as the Engine of Application State (HATEOAS) is a key principle of REST that enables navigation to related resources and operations through hypermedia links returned in the responses. This allows the API to evolve independently by decoupling the client and server.

However, HATEOAS is commonly not implemented properly in most REST APIs today. At a minimum, APIs should provide links to related resources in responses to implement HATEOAS.

5. Lack of Versioning

REST APIs will evolve over time and maintaining backwards compatibility is essential. Hence, proper versioning strategy (e.g. v1, v2) should be implemented, preferably via the URL:

https://api.example.com/v1/users

Versioning should not rely on custom request headers or parameters that can be error-prone.

6. Improper Pagination, Filtering and Searching

For REST APIs that return lists of resources, proper pagination, filtering, sorting and searching capabilities should be implemented. Pagination using limit and offset parameters helps to control the number of resources returned in requests to prevent performance issues. Filtering, sorting and searching based on fields enables consumers to query the API efficiently.

7. Not Securing APIs Properly

Security is a key concern for REST APIs. Authentication using OAuth2 or JSON Web Tokens should be implemented to identity consumers. Appropriate authorization should be performed to restrict access to resources. Rate limiting could be applied to prevent abuse.

Read more: How Improve JWT Algorithm And Boosting Microservice Performance?

8. Not Handling Errors Properly

REST APIs should return consistent and well-defined error schemas and messaging to help consumers handle errors and exceptions seamlessly. The errors should provide enough context to enable debugging issues.

9. Lack of Proper Documentation

REST APIs should be well documented with an OpenAPI spec or Swagger definitions. This enables excellent discoverability and guides proper API consumption. Documentation helps prevent breaking changes down the road.

10. Inconsistent API Design

Guidelines and conventions should be created for REST API design upfront. This includes consistent naming, URL structure, request/response formats, versioning strategy, etc. This helps avoid haphazard design and prevents confusion when APIs evolve.

11. Overloading APIs and Resources

APIs and resources should have focused functionality. For example, a /users API should focus on core user management capabilities. Overloading it with unrelated functionality leads to bloated APIs that are hard to use and maintain.

By keeping these common REST API design pitfalls in mind, beginners can develop well-architected and scalable RESTful APIs with Spring Boot. What matters is sticking to core REST principles and conventions for building clean, intuitive and sustainable APIs.