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How to force Varnish to update cache contents

Every freistilbox cluster on our managed hosting platform has its own Varnish content cache. This cache stores content according to the HTTP headers that came with it. When the cache lifetime of a specific content object expires, Varnish will remove it from its cache store and request an update from your web application.

Triggering a cache update requires connecting to the active Varnish node. Even though your freistilbox cluster might have more than one Varnish server (especially if you have a freistilbox Pro oder Enterprise cluster), the hosting platform maintains a hostname that always points to the active node. Its name consists of your Cluster ID with a “v” for “Varnish” appended, for example “c61v”.

Cache invalidation using the Drupal Purge module

In order to let your Drupal application delete stale cache content, freistilbox supports the Purge module in conjunction with the Varnish Purger module.

The Varnish Purger module allows invalidating whole sets of cache objects at once by using “Cache Tags”. This method uses the Cache-Tags HTTP header to label content with one or more tags. All content with a certain tag can then be removed from the cache using a BAN request for this tag.

Configure the Varnish Purger module using the following details:

  • Hostname: cXXv.freistilbox.net (use your own cluster ID followed by a “v” for Varnish, for example c61v.freistilbox.net)
  • Port: 80
  • Path: ‘/`
  • Request method: BAN
  • Scheme: http

Also make sure that the Varnish Purger Tags submodule is configured like this:

  • Header: Cache-Tags
  • Value: [invalidation:expression]

Manual cache invalidation

To manually remove a certain content object from the cache, you can send a PURGE request with its URL from the command line on your shell box.

As an example, let’s assume we want to refresh the cached content for the URL https://www.example.com/tralalaa.css.

The following curl command sends a HTTP request to the active Varnish instance of your cluster, using the document path we want to invalidate. However, instead of the usual GET, it uses the PURGE request method. It also includes a Host header to identify your website’s domain name.

curl -X PURGE http://c61v.freistilbox.net/tralalaa.css \
  -H "Host: www.example.com"

If your website enforces encryption by redirecting any http:// URL to its https:// equivalent, you need to add another header named X-Forwarded-Proto.

curl -X PURGE http://c61v.freistilbox.net/tralalaa.css \
  -H "Host: www.example.com" -H "X-Forwarded-Proto: https"

After you’ve issued this command, Varnish will refresh the content for the given URL.