Caching Explained: The Strategies Every Web Developer Should Know

Cache invalidation is determining when cached data is stale and removing it so the next request gets fresh data. It's considered one of the two hard problems in computer science (along with naming things). The difficulty: you need to know when upstream data changes, track which cached entries depend on that data, and invalidate exactly those entries without invalidating everything (which defeats the purpose of caching). In practice: time-based expiration (TTL) is simple but causes lag between data changes and cache updates. Event-driven invalidation is more accurate but requires infrastructure to propagate change events. Most systems use a combination.

Cache-Control is the primary directive. 'Cache-Control: max-age=31536000, immutable' means cache for 1 year and the content won't change (used for versioned assets). 'Cache-Control: no-cache' means the browser can store it but must revalidate with the server on each use. 'Cache-Control: no-store' means don't cache at all. 'Cache-Control: public' allows CDNs to cache; 'private' means only browser cache, not shared caches. ETag provides a fingerprint of the content — the browser sends back 'If-None-Match: [etag]' on subsequent requests; if the ETag matches, the server returns 304 Not Modified instead of the full content.

Both are in-memory data stores used for caching. Memcached is simpler — key-value pairs, fast, good for simple caching scenarios, multi-threaded. Redis is more capable — it supports data structures (lists, sets, sorted sets, hashes), persistence to disk, pub/sub messaging, Lua scripting, and cluster mode. For pure caching with no other needs: either works, Memcached has slightly lower overhead. For caching plus other features (sessions, job queues, pub/sub, leaderboards): Redis is almost always the choice. Redis has largely replaced Memcached in new deployments because the additional capabilities come with minimal added complexity.