What is cache warming?

Cache warming is requesting your pages through the CDN right after a purge or deploy, so the CDN fetches them from the origin and stores them before visitors arrive. The first real visitor then gets a cache hit instead of waiting for the origin. It is also called cache preloading or cache priming.

deploy -> purge -> warm: GET each URL in the sitemap through the CDN
                       -> read the cache status of each response
                       -> repeat from each region that has visitors

Why a purge leaves the cache cold

A purge removes, or marks stale, the cached copies of your pages in every PoP. Until something requests a page again, the cache is cold for it. CDNs fill caches on demand, one PoP or tier at a time, so busy pages recover quickly in busy regions while everything else stays cold longer.

What warming changes

• Origin load: the misses happen at a pace you choose, instead of as a burst from the first visitors and crawlers. Request collapsing only softens that burst per location.
• TTFB: a warmed page skips the origin round trip, which also helps LCP, because the browser cannot render before the HTML arrives.
• Hit ratio: the purged pages are cached again within minutes instead of whenever traffic reaches them.
• Crawling: crawlers that arrive after a deploy meet cached, fast responses, which Google counts towards crawl capacity.

How to warm a cache well

• Start from the list of pages that matter, usually your XML sitemap, most important pages first.
• Request through the CDN, from each region where you have visitors. A request fills the PoP, and the upper tier, that served it, not the others.
• Send the headers your visitors send, such as Accept-Encoding, so you warm the same cache key and `Vary` variant they will ask for.
• Rate-limit the requests so warming does not become the traffic spike it was meant to prevent.
• Verify each response: read the CDN's cache status and check that a second request is a hit. A 200 OK alone does not tell you the page was cached.

When warming does not help

• Pages the CDN does not cache, such as personalized pages, or responses with `no-store` or private. Cloudflare, for example, does not cache HTML by default.
• Pages with so much traffic everywhere that the first visitors refill the cache within seconds.
• Fingerprinted static assets with a long `immutable` lifetime, which deploys do not purge.

How warming works on a given CDN depends on its cache-status header, purge options and tiers. The Top 10 CDNs comparison scores ten CDNs on those criteria, with a guide for each one, from Cloudflare and Fastly to Google Cloud CDN and Azure Front Door.