When configuring external caching for your Drupal site, the following important settings may cause trouble for users, if not properly understood:

• Minimum cache lifetime
• Expiration of cached pages

To find the settings, in the Admin menu, go to Configuration > Performance (see the site path.)

Minimum cache lifetime

The minimum cache lifetime prevents Drupal from clearing webpages and block caches for a period of time after making changes which means Drupal will not regenerate webpages until at least the period of time has passed, and a cache-clearing event has taken place. Webpages are not necessarily regenerated after the minimum cache lifetime has passed.

The default value for the minimum cache lifetime is none, which means the default lifetime is 0 seconds. Unless you are familiar with configuring cache, and you have different requirements, best practices would have you keep the setting as none.

If something triggers the cache to clear, and the minimum cache lifetime is set to a value greater than none, the combination can cause a cache stampede. The problem occurs when a cached webpage is no longer cached, and several website visitors request the webpage within a short time frame. When servers try to render the webpage simultaneously, none of them will rebuild or re-cache the webpage, because every attempt to do so will time out. The server timeouts could result in downtime for your website.

Cloud Platform customers should always set minimum cache lifetime to 0, unless specific circumstances dictate otherwise. Setting the minimum cache lifetime to a non-zero value can cause issues with Varnish.

When a Drupal website’s cron interval is set to its default, the cron interval will clear the cache routinely, which could result in downtime. The default Drupal 7 cron (Poor man’s cron) is enabled by default, and you should disable it, setting the run time to Never. To disable the default Drupal 7 cron on your website, in the Admin menu, go to Configuration > System > Cron ([site_url]/admin/config/system/cron).

Expiration of cached pages

Expiration of cached pages refers to the maximum time a webpage can remain in the cache. If you set the expiration of cached pages to 0 seconds which is the maximum-age header Drupal sends to the proxy server, the pages will not be cached. A reasonable setting is 6-12 hours. Often misinterpreted, expiration of cached pages does not mean the webpage will regenerate after the amount of time you select—the term means, after the period of time, the cache checks with Drupal to see if a new version of the webpage exists. The webpage will regenerate after a cache-clearing event happens.

Depending on the content they contain, you may want to set the cache maximum age for specific webpages. For example, if you have a routinely updated news section, you may want to set a brief cache lifetime as opposed to the longer lifetime for static content. For more information, see Set max-age headers for specific webpages, files, or paths.

Edge Side Includes (ESI)

Cloud Platform does not support Edge Side Includes (ESI).

Configuring external cache settings