Issue
How do I purge Varnish cache, and what are the recommended best practices on Acquia Cloud?
Resolution
There are major advantages to having long cache lifetimes and expires, which means that it's important to use Varnish with Drupal for performance improvements. However, when changes are made or new content is published, you don't want to wait a long time for visitors (including non-authenticated users) to see the changes. In these cases, you should periodically purge your Varnish cache.
Varnish excels at storing pages, CSS, and JavaScript files, as well as multimedia files, for as long as possible, so that visitor requests don't hit the back-end (in most scenarios, this will be Apache). This greatly helps with performance and allows a website to scale much better than it would if only Apache was serving pages. When your organization decides to implement a caching strategy, you will need to explore all available options and decide what's best for your situation. For more information, see the Using Varnish documentation page.
Select from the following methods to purge your Varnish cache:
Using Acquia Purge
If you're an Acquia Cloud-hosted site, the Acquia Purge module is your fastest and most convenient way to purge caches. It provides a non-programmatic way to purge Varnish-powered balancers through a UI, and it integrates the Rules and Cache Expiration modules for extra flexibility and proactive purging.
Read more about installing and using Acquia Purge in the Acquia Purge documentation.
Purging Programmatically
While Acquia Purge is Acquia's best recommendation when it comes to implementing a caching strategy, another good method is to use the Purge and Cache Expiration modules.
The Purge module clears URLs from reverse proxy caches like Varnish and also issues an http PURGE request to them. It works with the Cache Expiration module to act on events that are likely to expire URLs from the proxy cache and interact with Rules and Drush. This results in delivering faster content updates to end users. Acquia Purge is recommended on Acquia Cloud because it has many of the same features, and the maintainers for Purge and Acquia Purge work closely to ensure compatibility.
If you absolutely must purge a page manually from Acquia Cloud, you can learn how to manually purge a page.
Leveraging Acquia Cloud API
Acquia has developed the Acquia Cloud API for developers to be able to take advantage of even more automation as part of their daily workflow. Acquia Cloud API is a RESTful web interface that allows developers to extend, enhance, and customize Acquia Cloud. It includes developer workflow, site management, and provisioning capabilities. Of all its commands, the relevant one here is the delete the domain cache instance method:
DELETE /sites/:site/envs/:env/domains/:domain/cache
The preceding command triggers a Varnish cache purge for a specific domain. The following command works in the context of the automated Cloud API. Here's an example:
curl -s -u user:pass -X DELETE \
https://site.com/v1/sites/site/envs/prod/domains/site.com/cache.json
For more information about the Acquia Cloud API, see the Developing with the Cloud API, and check out the Cloud API reference page.
Leveraging Acquia Cloud Hooks
Cloud Hooks allow you to automate almost anything as part of your Workflow actions. A Cloud Hook is a script in your code repository that Acquia Cloud executes on your behalf when a triggering action occurs (for example, a database, some code or some files copy from one environment to the other). You can definitely implement Cloud API Varnish cache clears as part of Cloud Hooks triggers, for instance when you are deploying code from Dev to Stage.
For more information about Acquia Cloud Hooks, read our Acquia Cloud Hooks documentation page, and check out the Cloud Hooks GitHub repository .
Using other purge methods
Although Acquia Purge is the preferred and safest method of cache purging, there is another method that you can use if necessary: