Upcoming Changes to Varnish over SSL on Acquia Cloud Site Factory

On Wednesday, 29 January 2020, Acquia will be enabling Varnish caching over SSL on the Acquia Cloud Site Factory platform.

What does this mean for my application?

Currently, the Acquia Cloud platform allows customer applications to bypass the Varnish caching layer when traffic is using TLS to connect to your application. When your application uses this configuration option, all traffic served via TLS hits your instance backend directly. This results in higher load on your application infrastructure and negates some of the performance and stability benefits Varnish caching provides.

We are implementing a change to the way traffic is served that will force all applications to utilize Varnish caching over SSL. This does not mean that all traffic must be cacheable. Bypassing Varnish caching for certain content or features of your application remains possible at the application level.

This change is part of our ongoing efforts to improve Acquia Cloud and enforce best practices on the platform, while ensuring that Acquia products and services are secure, reliable, and scalable.

How do I prepare for this change?

As of 29 January 2020, Varnish over SSL will be enabled for all customer applications on the Acquia Cloud Site Factory platform. Applications already using this setting will not be affected.

In most cases, enabling this feature will have no negative impacts for your application. Drupal and most modules already set the appropriate Cache-Control header. As a result, most customer applications will see a  performance improvement as a result of this change Because implementing Varnish caching over SSL results in more traffic being cached, there will be less load to your database and file system infrastructures.

However, certain custom or in-house developed modules may not always set the appropriate “Cache-Control” header. This may result in unexpected content being cached once this feature is enabled. In these cases, applications may require additional code changes to ensure they continue to function as expected. Should you encounter any issues after this feature is enabled, please file a Support request via the Acquia Support Help Center and one of our support engineers will be able to assist you further.

Bypassing Varnish at the Drupal application level

Drupal core and most contributed modules already set the appropriate Cache-Control header. However, Varnish caching can still be bypassed by configuring one of the following Drupal modules:

• https://www.drupal.org/project/cacheexclude

• https://www.drupal.org/project/http_cache_control

For further details, see Varnish: Bypassing the cache.

Additional resources providing further details on how Varnish caching works on the Acquia Cloud platform are available as follows:

• Introduction to Varnish

• Using Varnish

• Varnish: Bypassing the cache