Glossary

Caching

Caching is storing the result of an expensive operation (SQL query, HTTP call, rendering) for reuse without recomputing it. Proper caching is one of the most effective ways to speed up an application without changing algorithms.

Cache layers

HTTP cache — browsers and proxies cache responses based on Cache-Control headers
CDN — edge nodes cache static assets and even dynamic responses
Application cache — Redis or Memcached store query results and computed data
OPcache — PHP caches compiled bytecode
DB query cache — some databases cache query results (removed in MySQL 8+)

Invalidation

The hardest part of caching is deciding when data is stale. Two strategies: TTL (time to live) — simpler, but data may be outdated; event-based invalidation — more precise, but harder to implement.