Stratégies d’invalidation et de purge du cache

There is a famous saying that one of the two hard things in computing is cache invalidation. The challenge is removing stale content everywhere it lives without harming performance.

• Content is replicated across many edge locations
• Removing too aggressively kills your hit ratio
• Removing too little serves stale data

The simplest purge targets one exact URL. Fast and precise, but tedious when many related objects change at once.

Purging by prefix clears everything under a path, like all files in /assets/v3/. Useful after deploying a directory of changed assets.

Tag-based purging attaches labels (surrogate keys) to responses, then purges everything sharing a tag in one call. Perfect for invalidating all pages that reference a changed product.

A full purge clears the entire cache. It is a blunt instrument: it guarantees freshness but causes a temporary surge of origin traffic as everything re-fills. Reserve it for emergencies.

Hard purge deletes content immediately. Soft purge marks it stale so the edge can still serve it while revalidating, smoothing the origin load.

• Hard: instant removal, possible miss storm
• Soft: graceful refresh, slight staleness window

Often the best invalidation is to not invalidate at all. Embed a version or hash in the asset URL so new content gets a new key and old caches simply age out.

Purges are not always instant. They propagate across PoPs over seconds. Plan deploys so users do not see a mix of old and new assets during the window.

In a Redis plus CDN stack, a write must invalidate the right layers. Deleting a Redis key alone will not clear a CDN edge copy; coordinate purges across both.

Wire purges into your deploy pipeline so the right tags or paths are cleared automatically after each release, avoiding manual, error-prone purging.

Match the tool to the change:

• One file changed: purge by URL
• A directory deployed: purge by prefix
• Related content across pages: tag-based purge
• Frequent asset changes: versioned URLs (no purge)
• Emergency only: full purge

Test your invalidation knowledge.

You learned cache invalidation strategies: purge by URL, prefix, or surrogate-key tag; soft versus hard purge; full purge as a last resort; and avoiding purges entirely with versioned URLs. You also saw how to coordinate invalidation across cache layers and automate it in CI/CD.