Hit Rate & Metrics

A cache is valuable only if it has a high hit rate. Track:

• Hit rate: hits / (hits + misses). Aim > 90% for hot data.
• Miss rate: 1 - hit rate.
• Eviction rate: evictions / second.
• Average size: current vs capacity.

A LOW hit rate (< 50%) usually means:

• Cache is too small for working set
• Access pattern isn't stable (random keys)
• Bug: writing to wrong cache key

Modern caches expose detailed metrics:

• Caffeine: per-method hit rates, average load time
• Redis: INFO stats command (keyspace_hits, keyspace_misses)
• memcached: stats command

For hot paths, log the cache hit rate per minute and ALERT when it drops. A degraded cache can take down a service.