Knowing how to clear Drupal's cache is an important skill for any developer. You'll likely find yourself doing it frequently in order to get Drupal to register the changes you make to your code, or other updates you make via the UI. It is also a good first step to trouble shooting problems with your Drupal site: Clear the cache before you do any other debugging to ensure it's not just a bad cache entry.

Goal

Learn three methods of clearing Drupal's cache: via the administrative UI, with Drush, and by truncating tables in the database.

Drupal comes with all of its caching features enabled by default. This improves response time, but can be frustrating for themers as it makes it harder to preview the changes you make to template files.

In this tutorial we'll look at:

• Why these features are enabled by default
• How the theme layer leverages Drupal's caches
• Why you should learn to disable them when doing development

Goal

Understand which performance-related features in Drupal are turned on by default and how this can impact theme and front-end developers.

Making Drupal fast by default implies having caching layers and CSS and JavaScript aggregation utilities enabled out-of-the-box. As a theme developer this can be annoying, because you must clear these various caches in order to preview any changes. In addition, inspecting variables with debugging tools often produces PHP errors. We'll make some recommendations for PHP settings on your local environment that can prevent these errors from happening so often.

By the end of this tutorial, you should be able to:

Many of the processes that Drupal performs when responding to a request are cached in order to increase performance. Creating the HTML for the page that a user sees or the JSON response to a REST request can require thousands of operations. Some operations are time consuming, memory heavy, CPU intense, or all 3. By performing the operation once, and then caching the result for next time, subsequent requests can be fulfilled faster. In order to make it easier to store, retrieve, and invalidate cached data, Drupal provides cache-related services you can use in your code.

The Render API is capable of detecting poorly-cacheable (highly dynamic) parts of a page and rendering them later using a process called auto-placeholdering. This works by using #lazy_builder callbacks to lazy load certain very dynamic subtrees of a render array. The place in the array where that very dynamic content would appear is first assigned a placeholder. At the very last moment it is replaced with the actual content.

The individual items that make up the content of a page impact the cacheability of that page. In order for Drupal's cache and external caches to better understand how the content varies on a page, module developers use the #cache render element property. The #cache property defines cacheability metadata for individual elements in a render array.