What is Cache Hit Ratio?
Cache hit ratio is the share of cacheable work served from a cache instead of an origin or recomputation step. It can be weighted by request count, transferred bytes, or another workload metric, and each weighting answers a different question.
How Cache Hit Ratio works
This ratio describes cache effectiveness for a selected observation window and population of requests. A monitoring system first sets aside bypassed and otherwise uncacheable traffic, classifies the remaining eligible lookups as hits, misses, validations, or errors, then applies a declared weighting method. Operators interpret it alongside object popularity, eviction, response size, and origin latency, because the same ratio can represent very different delivery cost for thumbnails, large video segments, and personalized responses.
Key facts
- 1A request-weighted ratio is hits divided by cacheable lookups, while a byte-weighted ratio compares served bytes; neither metric can be substituted for the other without changing the question.
- 2Cold deployments, rare-object traffic, and a long tail of unique transformation URLs can lower the ratio even when caching rules for popular assets are correct.
- 3A high aggregate value can conceal expensive misses if many tiny objects hit while a few large media objects repeatedly reach the origin, so origin bytes and latency remain necessary companion metrics.
When Cache Hit Ratio matters
Track hit ratio to assess delivery latency, origin load, and the effectiveness of cache policies. A low ratio may reveal fragmented keys or short lifetimes, but it is expected when traffic is dominated by uncacheable, dynamic responses.
Common use cases for delivery
These examples cover delivery broadly, not specifically Cache Hit Ratio.
- Serving image, audio, video, and document derivatives to a geographically distributed audience.
- Protecting private assets worldwide with expiring or signed requests.
- Reducing repeated processing and origin traffic by caching deterministic results.
Working with delivery
This guidance covers delivery broadly, not just Cache Hit Ratio.
A client requests an asset using a URL or playback manifest. A delivery layer evaluates authorization and cache state, serves a cached response when possible, or retrieves the asset from its origin before forwarding and optionally caching it.
Delivery choices determine more than download speed. Cache keys, origin behavior, authorization, geographic routing, invalidation, and egress cost decide whether an asset is fast, current, and available to the right audience.
What you gain
- Edge caching places frequently requested assets closer to viewers.
- Explicit cache and authorization rules reduce avoidable origin work.
- Multiple delivery variants let clients request an asset suited to their context.
What it costs
- Long cache lifetimes improve hit ratio but make replacement and invalidation more difficult.
- Signed access protects private media but adds key management, clock, and cache-partitioning concerns.
- More variants improve client fit while increasing storage, cache fragmentation, and operational complexity.
Before production
- 1Define cache keys, cache lifetime, invalidation, and authorization behavior explicitly.
- 2Measure time to first byte, cache-hit ratio, egress, and behavior after an origin failure.
- 3Test signed and unsigned requests at the CDN edge, not only against the origin.