Cache #

Caching refers to storing data or information in a fast, convenient location, speeding up future requests. It can store results from expensive calculations, complex queries, or web services. Different cache stores serve varying needs, but all are generally fast, with some being distributed and supporting diverse data models.

Most systems have multiple caching layers suitable for specific scenarios. For instance, Drupal websites read a lot of data from databases and APIs, both of which can be independently cached. Additionally, Drupal is built using PHP, an interpreted language, and even the code has a cache layer. While CPU caching isn’t within this handbook’s scope, it’s important to note its role in overall performance.

Drupal core includes two key caching modules:

Internal Page Cache, enabled by default, which caches web pages for anonymous users
Internal Dynamic Page Cache, recommended for all websites, which caches pages while excluding personalized content, benefiting both anonymous and authenticated users.

Cache Invalidation #

Cache invalidation in simple words is the process of making room for the new content to be cached when the content that is cached reaches its expiration time or is forcefully evicted.

Cache invalidation depends upon the following factors:

Time: Cache content is automatically invalidated when it expires. Drupal’s Cache API allows manual adjustment of expiration times.
Context: Invalidation can also occur based on conditions apart from expiration time such as user type.
Dependencies: Cache invalidation can be triggered by changes to content types, specific categories, or the curation of listing blocks, among other factors.

Cache API in Drupal #

Drupal’s Cache API manages data storage based on factors like time, context, and dependencies, known as Cacheability Metadata. Key properties include:

Cache Max-Age: Sets content caching duration.
Cache Contexts: Manages dependencies based on request context.
Cache Tags: Handles dependencies on Drupal-managed data like content types or categories.

Cache Bins #

In Drupal, cache storage is organized into bins, each storing different data types. Common bins include bootstrap, render, data, and discovery. Modules can also create custom cache bins.

Cache Systems #

Drupal’s default cache storage is in the database, which may be convenient but results in poor performance. To address this, Drupal is typically configured to store cached data in external stores like Memcache or Redis. These stores can be used to cache database data, API calls, or page rendering chunks in RAM, thereby increasing application performance.

Memcache #

Memcache is a free, open-source, high-performance memory caching system. It can be integrated into a Drupal website using the Memcache module, which also offers an administrative overview of Drupal’s interaction with Memcached and provides performance stats.

Redis #

Redis is a versatile in-memory data structure store used as a database, cache, message broker, and streaming engine. It can be configured on a Drupal website using the Redis module.

Both Memcache and Redis can enhance website performance by adding another layer of caching, with the choice between them depending on specific requirements and available features.

Check here to see how we set up Redis and Memcached in Drupal.

Web Server Caching #

Web server caching, or web proxy caching, stores copies of frequently accessed web objects to avoid hitting the application server. Varnish is a popular reverse proxy used with Drupal. Alternatively, this could also be implemented with a CDN so that the data is close to users.