Caching

What a cache is

A cache is a small, fast store that keeps copies of data that is expensive to fetch or compute. Reading from memory takes well under a millisecond. A database query over the network often takes 5 to 50 ms.

• A hit means the data was in the cache. A miss means it was not, so you go to the slower source.
• The hit ratio is hits divided by total requests. It is the one number that tells you if a cache is worth having.
• Typical tools: Redis and Memcached (in-memory key-value servers), plus caches inside your app process, your browser and your CDN.

Why it works: real traffic is lopsided. A small share of keys gets most of the reads, so a cache that holds only the hot keys catches most requests.