The uOttawa Web CMS uses two different layers of caching to maximize performance. The result of this is lightning-fast page load times for most users and a very light load on our servers. However, a small drawback is that changes can take some time before a front-end user can see them.
The first caching layer is Drupal’s built-in caching system. Drupal only caches pages when:
The visitor is not logged in,
The visitor does not have some specific cookies that remain set even after logging out,
The visitor is not trying to access a page with an error, warning or status message on it,
The content of the page does not identify itself as being “un-cacheable”
In order to bypass this cache, you should login to your site. If you would like to make sure you are seeing the cached version of the page, you should use your browser’s anonymous browsing feature (called “Incognito” in Google Chrome and “Private Browsing” in Firefox). The Drupal cache is cleared on every site every night around midnight. We also have several special tools that identify when you make changes to content and clear the Drupal cache for related items as well.
Next is our Varnish caching. Varnish is a piece of software that sits between our proxy server and the web server. It examines each incoming request and, if possible, returns a cached result. It is faster than Drupal. Varnish will cache all requests that are not made over HTTPS for a period of twelve hours and will continue to serve pages to the visitor even if Drupal itself malfunctions. If you would like to bypass the Varnish cache, make sure you are using HTTPS.
The rest of this section will focus on how you can manage and clear these caches. If you are not seeing a change you performed on your site, clearing the caches is one way to resolve this issue. If you are going to clear both the Varnish and the Drupal cache, clear the Drupal one first and then the Varnish cache.