CDN Cache Invalidation Strategies

CDN Cache Invalidation

CDN cache invalidation is the process of removing or updating cached content before its max-age or s-maxage expires. The classic computer science quote applies: "There are only two hard things — cache invalidation and naming things."

Strategy 1: URL Versioning (Recommended)

Change the URL when content changes:

/assets/app-v2.3.1.js
/assets/app-BkQ3x9f2.js  (content hash)

Pros: Instant, reliable, no purge needed Cons: Requires build tooling and HTML updates

This is the gold standard for static assets. The Cache-Control header can be maximally aggressive because the URL itself is the version.

Strategy 2: Purge API

Most CDNs offer an API to purge specific URLs or patterns:

# Cloudflare
curl -X POST "https://api.cloudflare.com/client/v4/zones/{zone}/purge_cache" \
  -d '{"files":["https://example.com/api/products"]}'

# Fastly
curl -X PURGE "https://example.com/api/products"

Pros: Immediate invalidation, no URL changes needed Cons: API calls required, propagation delay (1–30 seconds)

Strategy 3: Surrogate Keys (Cache Tags)

Tag cached responses with logical identifiers:

Surrogate-Key: product-123 category-electronics

Then purge by tag:

curl -X POST "https://api.fastly.com/service/{id}/purge/product-123"

Pros: Purge all pages containing a specific product with one API call Cons: Not all CDNs support it (Fastly, Akamai do; Cloudflare uses Cache-Tag)

Strategy 4: Short s-maxage + SWR

Use short cache durations with stale-while-revalidate:

Cache-Control: s-maxage=60, stale-while-revalidate=86400

Pros: Self-healing, no purge infrastructure needed Cons: Content can be stale for up to 60 seconds

Choosing a Strategy

Scenario	Best Strategy
Static assets (JS/CSS)	URL versioning
Blog posts / CMS content	Purge API on publish
Product catalog	Surrogate keys
API responses	Short s-maxage + SWR