A news article, for instance, might show up on a number of URLs.
Reverse proxies usually provide a channel to receive such notifications, typically through special HTTP requests.
Caution While cache invalidation is powerful, avoid it when possible.
A ban is a feature specific to Varnish and one that is frequently misunderstood.
It enables you to ban Varnish from serving certain content in memory, forcing Varnish to fetch new versions of these pages.
The best way would be to let Varnish Cache keep the object in memory forever (mostly) and then tell the object when to refresh. HTTP Purging is the most straightforward of these methods.
Instead of sending a HTTP purging falls short when a piece of content has a complex relationship to the URLs it appears on.
How often you get the latter will depend on the efficiency of the cache — this is known as the “hit rate.” A cache miss depends on two factors: the volume of traffic and the average time to live (TTL), which is a number indicating how long the cache is allowed to keep an object.
As system administrators and developers, we can’t do much about the traffic, but we can influence the TTL.
However, to have a high TTL, we need to be able to invalidate objects from the cache so that we avoid serving stale content.