How do you use if-match header to implement optimistic concurrency control in RESTful APIs?

1 What is the if-match header?

The if-match header is an HTTP request header that allows a client to conditionally update a resource on a server based on its current version. The if-match header contains one or more entity tags (ETags), which are identifiers that represent the state of a resource. The server compares the ETags in the if-match header with the current ETag of the resource and only performs the update if they match. If they do not match, the server returns a 412 Precondition Failed response, indicating that the resource has been modified by another client.

2 How to use the if-match header in RESTful APIs?

To use the if-match header in RESTful APIs, you need to generate and return an ETag for each resource in the response headers of GET, HEAD, or OPTIONS requests. This can be done by using any algorithm such as hashing, timestamps, or version numbers, as long as it is unique and consistent for each resource state. The ETag should be stored on the client side in memory, local storage, or a cache. When you want to update the resource, send a PUT, PATCH, or DELETE request with the if-match header containing the stored ETag. The server will then check if the ETag in the header matches the current ETag of the resource. If they match, the server will perform the update and return a 200 OK or 204 No Content response with the new ETag in the response headers. If they do not match, a 412 Precondition Failed response with the current ETag will be returned. The client will then handle this response accordingly; if successful it will update its stored ETag and other information about the resource; if unsuccessful it will either retry with a new ETag, discard changes, or resolve the conflict with user input or another strategy.

4 What are the challenges of using the if-match header?

Using the if-match header to implement OCC in RESTful APIs also has some challenges. Clients and servers must support and follow the HTTP specification and semantics for conditional requests and ETags, though not all may implement or adhere to these standards correctly or consistently. Furthermore, this increases the complexity and responsibility of the client, as it must store and manage the ETags and other information about the resources, as well as handle conflicts and errors. Additionally, a caching or synchronization mechanism may be required to keep the ETags up to date and consistent across different devices or instances. Finally, this approach may not work for some scenarios or use cases where conflicts are frequent or unavoidable, such as collaborative editing, real-time updates, or offline access; in these cases, other techniques or protocols may be more suitable or complementary.

5 How to test and debug the if-match header?

Testing and debugging the if-match header in RESTful APIs can be done with various tools and methods. For example, you can use a REST client application or a browser extension such as Postman, Insomnia, or RESTed to send HTTP requests with custom headers and inspect the responses. Additionally, you can use a proxy or middleware like Charles, Fiddler, or WireShark to intercept and log the HTTP requests and responses between the client and the server. Testing frameworks or libraries like Mocha, Jest, or Chai can also be used to simulate and verify the HTTP requests and responses with different scenarios and expectations. Finally, debugging tools or consoles such as Chrome DevTools, VS Code, or Node.js debugger allow you to set breakpoints, watch variables, and execute commands in the client or server code.

6 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?