Drupal Caching: 3 Levels

In our continuing mission (well, not a mission; it's actually a blog series) to help you improve your Drupal website, let's look at the power of caching.

In our previous post, we debunked some too common Drupal performance advice. This time we're going positive, with a simple, rock-solid strategy to get you started: caching is the single best way to improve Drupal performance without having to fiddle with code.

At the basic level, it is easy enough for a non-technical user to implement. Advanced caching techniques might require some coding experience, but for most users, basic caching alone will bring about drastic performance improvements.

Caching in Drupal happens at three separate levels: application, component, and page. Let's review each level in detail.

Application-level caching

This is the caching capability baked right into Drupal. You won't see it in action unless you dig deep into Drupal's internal code. It is enabled by default and won't ever show older, cached pages.

With application-level caching, Drupal essentially stores cached pages separately from the site content (which goes into the database). You can't really configure this, except for telling Drupal where to save cached pages explicitly. You might see improved performance if you use Memcached on cached pages, but the effect is not big enough to warrant the effort.

Drupal stores many of its internal data and structures in efficient ways to improve frequent access when application-level caching. This isn't information that a site visitor will see per se, but it is critical for constructing any page. Basically, the only enhancements that can be made at this level are improving where this cached information is stored, like using Memcached instead of the database.

You just need to install Drupal and let the software take care of caching at the application-level.

Component-level caching

This works on user-facing components such as blocks, panels, and views. For example, you might have a website with constantly changing content but a single block remains the same. In fact, you may have the same block spread across dozens of pages. Caching it can result in big performance improvements.

Component-level caching is usually disabled by default, though you can turn it on with some simple configuration changes. For the best results, identify blocks, panels, and views that remain the same across your site, and then cache them aggressively. You will see strong speedups for authenticated users.

Page-level caching

This is exactly what it sounds like: The entire page is cached, stored and delivered to a user. This is the most efficient type of caching. Instead of generating pages dynamically with Drupal bootstrap, your server can show static HTML pages to users instead. Site performance will improve almost immeasurably.

Page-level caching gives you a lot of room to customize. You can use any number of caching servers, including Varnish, which we use at Acquia Cloud. You can also use CDNs like Akamai, Fastly, or CloudFlare to deliver cached pages from servers close to the user's location. With CDNs, you are literally bringing your site closer to your users.

Keep in mind that forced, page-level caching works only for anonymous users by default. Fortunately, this forms the bulk of traffic to any website.

It bears repeating: Caching should be your top priority for boosting Drupal performance. By identifying and caching commonly repeated components and using a CDN at page-level, you'll see site speed improvements that you can write home about.

Next time: How to Evaluate Drupal Modules for Performance Optimization.