Your site is down, and you're receiving an error message with the following large, bold text:
"This server is experiencing technical problems. Please try again in a few moments. Thanks for your continued patience, and we're sorry for any inconvenience this may cause."
This error message indicates that the error preventing users from accessing your site is being encountered by Varnish, the page caching technology that runs on your Acquia Cloud load balancers. You don't have direct access to logs and configuration files for Varnish, and we'll be more than happy to help you look into any Varnish problems your site may be experiencing. However, there are a few Drupal application configuration issues that are known to cause Varnish problems on Acquia Cloud. Checking your application for the following problematic configuration issues may help you to work around the problem more quickly.
- Is page compression enabled in Drupal? Not only is Drupal's gzip compression mechanism not needed on our platform (Varnish automatically saves compressed and uncompressed versions of any pages it caches), but it can also cause problems when Varnish attempts to unzip certain pre-compressed pages before delivery. You can disable Drupal's page compression mechanism on your site by navigating to the "/admin/config/development/performance" page, and under "BANDWIDTH OPTIMIZATION", uncheck the box next to "Compress cached pages.", and click the "Save configuration" button.
- Are you delivering a large number of response headers? Varnish on our platform is configured to serve a maximum of 60 individual response headers (or a combined 23kb of header data) before responding with an error like the one mentioned above. Drupal applications that set a large number of response headers may cause your server to exceed this limit during the course of delivering a response. Drupal 8 sites experiencing Varnish errors for authenticated users only are likely experiencing a known issue with the "Link" response headers set by Drupal. Luckily, D8 site developers should be able to work around this issue with a simple code fix, as explained in our documentation here.
While the application issues above cause a good portion of the Varnish problems we see on our platform, you may be experiencing a Varnish issue that isn't mentioned above, and whose fix may lie beyond your control. As mentioned before, we're happy to help you look into these issues, and have some additional diagnostic tools that can be run on your load balancers to trace the cause of these Varnish errors. If you'd like our help with one of these issues, simply file a support ticket with the following information:
- The URL of the page you were attempting to access when you received the error.
- Whether you're receiving the error as an anonymous user, or only when logged-in to Drupal.
- A screenshot of the the full error message you're receiving (including all the smaller text below the large header).
This information should help us to more quickly narrow in on the type of error that Varnish is experiencing, and reproduce the problem on our end if needed.