HTTP ETag and Conditional Requests — If-None-Match and 304 Not Modified

Master HTTP conditional requests using ETag, If-None-Match, If-Match, and 304 Not Modified responses for efficient caching and optimistic concurrency control. This guide covers everything you need to know about this topic, including common causes, step-by-step solutions, and answers to frequently asked questions.

Here are the key things to understand: ETag (Entity Tag) is a unique identifier for a specific version of a resource. If-None-Match: sends the cached ETag — server returns 304 if unchanged, 200 with new data if changed. If-Match: used for write operations — server rejects the update with 412 if the ETag changed (optimistic locking). Strong ETags ("abc123") indicate byte-for-byte identical content; weak ETags (W/"abc123") indicate semantically equivalent. ETags reduce bandwidth by avoiding redundant full-content responses. Understanding these fundamentals will help you diagnose and resolve this issue more effectively.

The most common reasons this occurs include: Server not generating ETags, forcing full content download on every request. ETag mismatch during updates causing 412 Precondition Failed (concurrent edit detected). Load balancer routing requests to different servers with different ETag generation. CDN caching stale ETags after content updates on the origin server. Identifying the root cause is the first step toward finding the right solution.

To resolve this, follow these recommended steps: Enable ETag generation on your server: Nginx and Apache generate ETags by default for static files. For APIs: generate ETags from a hash of the response body or the resource's last-modified timestamp. Include If-None-Match in GET requests to enable 304 responses and reduce bandwidth. Use If-Match in PUT/PATCH requests to prevent lost updates from concurrent editing. Ensure all servers behind a load balancer generate identical ETags for the same content. After a 412 Precondition Failed: re-fetch the current version, merge changes, and retry the update. If these steps do not resolve the issue, consider consulting additional resources or a qualified professional.

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