Redis object cache for WordPress, explained

Every time WordPress builds a page, it asks the database dozens or hundreds of questions: which menu items exist, what the site settings are, which products are in stock. Many of those answers are the same on every request. An object cache remembers them, so the database is asked once instead of every time.

Page cache vs object cache

A page cache saves the finished HTML of a page and hands it straight to the next visitor. It is the biggest speed win for pages that look the same for everyone, such as blog posts and landing pages.

An object cache works one level deeper. It speeds up the pages a page cache cannot help with: the WordPress admin, logged-in members, shopping carts and checkouts, search results, and anything personalised.

Why Redis

Redis keeps data in memory, which makes reading it far faster than asking the database. It is the most common choice for WordPress object caching, and it is reliable and light on resources.

When it helps most

• WooCommerce shops, where carts and checkouts are never page-cached;
• membership sites and online courses with many logged-in users;
• large sites with many plugins that each add database queries;
• a slow WordPress admin.

A small brochure site that is almost entirely page-cached will see less difference, although the admin usually still feels quicker.

Turning it on

You need a Redis server and a plugin that connects WordPress to it. On Netkeel, Redis runs on every server and you can turn the object cache on for a site with one click; each site gets its own isolated Redis database. Many hosts sell this as a paid add-on per site. With us it is included.