Cache

What is Caching?

Caching is a method to improve performance for visitors to your site by storing copies of pages and files in faster storage.

Caching is important for any content management system (like Drupal and WordPress) that generates its pages dynamically, because generating the output for a page, especially complex ones, can take a long time. Once a page is generated by the content management system, a copy of the page is saved in the site cache so it can be delivered to the next visitor without regenerating it.

How Long Are Items Cached?

If you are logged in, you can expect to see all content updates immediately. For logged out users (normal visitors), there's a small delay before they may see content updates:

Documents and images: Up to 10 minutes
Pages: Up to 2 minutes

Seeing delays?

If you notice that changes you are making to content are consistently not being reflected to logged-out visitors quickly, there may be an issue that WDS needs to resolve, and you should open a support request. Please provide as much detail as possible.

How Our Caching Works

There are several layers of caching involved on your Drupal site:

Varnish

Varnish is a caching server that stores copies of images (.png, .jpg, etc), documents (.pdf, .txt, etc), and pages. It receives all traffic for your website and checks if it has a cached version of the requested content before requesting it from your site.

Images and documents will be stored in the Varnish cache for 5 minutes before your website is checked again for the current version. This means if you replace a document with a new version and keep the same filename, visitors may still see the old version for up to 5 minutes. Likewise, if you delete a document, the document may still be visible for up to 5 minutes.

Pages are also kept in the Varnish cache. A copy of a page will remain in the cache for up to 3 hours. However, unlike for documents and images, your Drupal site will automatically clear this cache when updates are made. All visitors should see content changes you made within 1 minute after the update.

Note that when you are logged into your site, the Varnish cache is completely bypassed.

Drupal Page Cache

Just like Varnish, Drupal also saves entire pages in its own cache. Pages in this cache are saved indefinitely and are cleared automatically and immediately when the content on them has been updated. And, just like Varnish, this cache is bypassed if you are logged in. This is why the site may appear to be a bit slower when you are logged in as compared to if you are logged out, because you aren't receiving cached content.

Drupal's cache does not save copies of documents or images like Varnish does.

Browser Cache

Your browser has its own cache as well. For your Drupal site, it will store copies of images and documents for up to 5 minutes (just like Varnish does). While it also stores copies of pages (like Varnish and Drupal Page Cache), it will only store them for a maximum of 1 minute.

Many browsers allow you to perform a "hard refresh" of a page by holding down the Shift or Ctrl/Cmd key and pressing the reload button.

Clearing Caches

Uusers with the site admin role have the ability to manually clear their websites caches. Under normal circumstances, this is never necessary. Your website is designed to automatically clear appropriate cache entries as needed. However, WDS recognizes that there may be times when this is not working as expected.

If content you've updated does not appear for all visitors on you site within 5 minutes, you can try clearing the caches. If this resolves the problem, we encourage you to contact WDS and let us know the circumstances. It's possible there's an issue with the platform that we can fix!

The buttons to clear cache are found under Configuration > Development > Clear Caches.

If you believe your site is experiencing a high volume of traffic, it's best to wait for a period of relative inactivity before clearing caches, as your site will become slower for a few minutes afterwards.