What purging is

Purging is the process of removing cached objects from a CDN. A purge tells the edge nodes that a file should no longer be served from cache. The CDN will then fetch a fresh copy from the origin the next time a client requests it.

Purging differs from TTL expiry. TTLs control how long content is considered valid by default. Purges are active interventions to remove content before it would have expired.

Why purging matters

There are several reasons to purge cached content. Website updates may require users to see new HTML or scripts immediately. Errors such as broken images or incorrect responses need to be corrected quickly. Some industries face legal obligations to remove content at once. Purging allows these changes to propagate through the CDN without waiting for TTLs to expire.

Purge methods

CDNs offer multiple ways to purge. The most common are:

  • Single-file purge: remove one object, usually identified by its URL.
  • Wildcard purge: remove many objects with matching prefixes, for example all images in a folder.
  • Cache tags: remove all objects that were tagged at upload or delivery time.
  • Full purge: remove every object in the cache. This is disruptive and should be rare.

How purges propagate

When a purge request is made, the CDN pushes an invalidation message to its edge nodes. Each node then marks the object as invalid and removes it from local cache. Propagation speed depends on the provider. Some CDNs complete a global purge in seconds, others may take minutes.

In practice, there can be a short window where different edges serve different versions. Monitoring is important if precise consistency is required.

APIs and automation

Most CDNs provide purge APIs. These allow integration into deployment pipelines and content management systems. A purge can be triggered by a script after a code release or by an editor updating a page. Many providers also expose dashboards, but APIs are more reliable for automation.

In a CI/CD setup, HTML pages might be purged on every deployment while static assets remain versioned. This avoids manual steps and keeps content fresh.

Performance considerations

Purging reduces the cache hit ratio. When an object is removed, the next requests will miss and go to the origin. This increases latency and origin load until the cache is repopulated. Frequent or large purges can therefore harm performance.

Scheduling large purges during off-peak hours and monitoring hit ratio after a purge helps minimize impact.

Best practices

Version assets such as JavaScript and CSS. Instead of purging them on each release, publish them with unique filenames and update references in HTML. This avoids purges while ensuring users always fetch the correct version.

Use purges for objects that cannot be versioned, such as HTML or API responses. Automate them through APIs rather than dashboards. Monitor logs and metrics after each purge to confirm success.

Edge cases and failures

Purges are not always instantaneous. A failed node, network delay, or provider limit can cause inconsistent states. Some CDNs batch purge requests, which slows propagation. Testing and monitoring are essential to confirm the purge worked as intended.

In a multi-CDN setup, purges must be sent to every provider. Differences in propagation speed and API semantics can complicate coordination.

Other caches in the path

Purging a CDN does not guarantee that clients will see the latest version. Several other caches may still serve old content.

  • Browser cache: Browsers cache assets locally based on response headers. A CDN purge does not clear these; only cache directives or cache-busting URLs can.
  • Intermediate proxies: Enterprises, schools, and ISPs sometimes run transparent or inline proxies that keep their own caches. These obey HTTP headers but may lag behind CDN purges.
  • Application gateways: Some organizations use additional reverse proxies in front of origins, which may also cache responses.

Understanding these layers helps explain why a change may not appear immediately even after a CDN purge.

Cache busting as a tool

A common debugging step is to append a random query string to a URL, forcing a cache miss. This ensures the request bypasses caches and fetches fresh content. In development and staging, cache busting is useful to verify that changes are visible.

In production, indiscriminate cache busting is risky. It can defeat CDN caching entirely and drive traffic back to the origin. This lowers the cache hit ratio, raises latency, and can overload the origin. Worse, automated systems or user-shared links with cache-busting parameters may propagate widely, amplifying the problem.

Protecting against cache busters

There are several ways to limit the damage from unwanted cache busting:

  • Ignore irrelevant query strings: Configure the CDN to exclude known tracking or random parameters from the cache key.
  • Normalize requests: Strip or rewrite parameters that are not meant to alter content.
  • Educate teams: Reserve cache busting for staging and make sure production deployments use controlled purge or versioning instead.

These safeguards allow debugging workflows to remain flexible in lower environments while keeping production caches efficient.

Patterns in real-world use

A common pattern is to purge only HTML on deploy. The HTML points to versioned static assets that never require purges. Another is to use cache tags for grouped objects, for example removing all images in a product catalog when an item is deleted.

Full purges are rare but may be used in emergencies such as legal takedowns or major configuration errors. These should be tested carefully to avoid large traffic spikes to the origin.

See also Cache-Control & TTLs for how freshness settings interact with purge strategies.