HTTP Caching Headers — Cache-Control, ETag, and Expires Explained

Complete guide to HTTP caching headers including Cache-Control directives, ETag validation, Expires headers, and troubleshooting stale cache issues. 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: Cache-Control is the primary header for controlling caching behavior in HTTP/1.1+. Common directives: max-age, no-cache, no-store, public, private, must-revalidate. ETag provides content-based cache validation — the server returns 304 Not Modified if content unchanged. Last-Modified/If-Modified-Since provides time-based cache validation. Expires header is the older HTTP/1.0 approach — Cache-Control takes precedence when both are present. Understanding these fundamentals will help you diagnose and resolve this issue more effectively.

The most common reasons this occurs include: Stale content served from cache due to overly long max-age values. no-cache misunderstood — it means 'revalidate before using cache', not 'do not cache'. CDN caching old content after server-side updates. Browser serving cached responses for dynamic content that should be fresh. Identifying the root cause is the first step toward finding the right solution.

To resolve this, follow these recommended steps: Set Cache-Control: no-store for sensitive data (banking, medical) to prevent any caching. Use Cache-Control: public, max-age=31536000 for static assets with content hashes in filenames. Implement ETag headers for dynamic content that changes infrequently. Use Cache-Control: no-cache for content that should always be revalidated before display. Add Vary header when response differs by Accept-Encoding, Accept-Language, or other request headers. Purge CDN cache after deployments: use your CDN's purge API or deploy with new asset URLs. If these steps do not resolve the issue, consider consulting additional resources or a qualified professional.

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