Invalidation on Varnish Cache

Banning content from Varnish

Tue, 31 Aug 2021 00:00:00 +0000

Banning is a concept in Varnish that allows expression-based cache invalidation. This means that you can invalidate multiple objects from the cache without the need for individual purge calls.

A ban is created by adding a ban expression to the ban list. All objects in the cache will be evaluated against the expressions in the ban list before being served. If the object is banned Varnish will mark it as expired and fetch new content from the backend.

Purging content from Varnish

Mon, 30 Aug 2021 00:00:00 +0000

The most basic and easy-to-use cache invalidation mechanism in Varnish is purging.

The idea is that you can perform a return (purge) in vcl_recv, and Varnish will remove the object. This would free up space in the cache after an object lookup.

This means that the hash of the object is used to identify it, but return (purge) would remove it along with its variants. This isn’t out-of-the-box behavior; you won’t find it in the built-in VCL. You need to write some VCL code for it.

Cache invalidation

Wed, 20 May 2020 00:00:00 +0000

Introduction

A good caching strategy not only defines how the content should be cached, but most importantly, how it should be invalidated and evicted from cache. An object inserted in cache can be served to other clients until it expires, is evicted to make room for other objects, or is invalidated.

The TTL (Time to Live) of an object define how long an object can be cached. An object’s TTL is set when the content is generated (by the backend) or when it’s inserted (in Varnish). The TTL can be set via HTTP caching headers (i.e. Expires or Cache-Control) or via VCL. Either way, Varnish will respect the defined TTLs and evict the object when its Time to Live has expired, making room for fresher content to be inserted in cache.