---
title: "What is a cache key? How CDNs decide what counts as \"the same\" request — warmup.rocks Glossary"
description: "A cache key is the identifier a CDN builds from URL, host, query string and selected headers to store a response. Why query-string noise and unnecessary key components destroy hit ratios."
canonical: https://warmup.rocks/glossary/cache-key
---

# Cache key

The **cache key** is the identifier a cache builds to store and look up a response — typically host + path + query string, optionally extended by headers, cookies or geo attributes. Two requests share a cache entry only if their keys match _exactly_. Most "why is my hit ratio terrible?" stories end at the cache key.

## What goes into the key

```
default key   = host + path + query string
custom key    = default ± query params ± headers ± cookies ± device/geo
```

CDNs let you customize this per route: ignore marketing parameters, include a language header, key on a device-type bucket. Everything you add multiplies the number of entries; everything you ignore merges requests into one entry.

## The classic failure: query-string noise

These four URLs are one page but four cache keys:

```
/product/42
/product/42?utm_source=newsletter
/product/42?utm_source=x&utm_campaign=july
/product/42?fbclid=IwAR3…
```

Every ad click and tracking link becomes a cache miss — each 3.5× slower than a hit at the median, [per our production data](https://warmup.rocks/blog/cdn-cache-miss-data). The fix is to strip known tracking parameters from the cache key (most CDNs support ignore-lists) or normalize/sort query strings, so all variants collapse into one hot entry.

## Key design rules of thumb

-   **Only include what changes the response.** Any component that doesn't affect the bytes just splits your cache.
-   **Prefer enumerable buckets over raw values** — a `mobile|desktop` flag, not the User-Agent string (see [Vary header](https://warmup.rocks/glossary/vary-header) for how raw headers explode).
-   **Remember: purge and warm operate on keys, not URLs.** A [purge](https://warmup.rocks/glossary/cache-purge) of `/product/42` may leave variant keys alive, and [warming](https://warmup.rocks/blog/what-is-cache-warming) must request each variant you actually serve.

To see how a specific URL behaves at the edge — including whether query variants hit the same entry — test it with the [cache checker](https://warmup.rocks/cache-checker).