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Configuring external cache settings¶

Note for Acquia-hosted websites

Cloud Platform customers use Varnish cache. For more information about Varnish and how to configure Varnish cache for your Cloud Platform website, see Using Varnish.

Cloud Platform subscriptions also support Memcached which does not consider cron to be a cache-clearing event. If using Memcached, your pages will not regenerate after cron. For more information, see Using Memcached.

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

To find the settings, in the Admin menu, go to Configuration > Performance for Drupal 7 and 8 (see the site path.)

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).

Note for Cloud Platform customers

Use the Cloud > Cron page for scheduled tasks, rather than the default Drupal 7 Cron or any of the contributed Cron modules, such as Elysia Cron or Ultimate Cron. Relying on Cloud Platform ensures Cron tasks run on the schedule you specify, and not when visiting the website. For more information, see Using scheduled jobs to support your application. If you must use Elysia cron, see Elysia cron on Cloud Platform.

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

ESI¶

The ESI, or Edge Side Includes, module is not recommended. The module is useful for small pieces of permanently cached content. In most cases, users want to have the small section of content uncached, which bootstraps Drupal, defeats the purpose of Varnish, and other caching mechanisms. In various use cases, ESI has a negative impact on system performance.