What is caching? Browser, page and object caches

A cache is a stored copy of something that was expensive to produce, kept so the next request can be answered instantly. Websites use several layers of cache, each saving a different kind of work.

Layer	Stores	Saves
Browser cache	Images, styles and scripts on the visitor’s device	Downloading files again
CDN	Copies of files or pages near the visitor	Distance and load on your server
Page cache	Finished HTML pages on your server	Running PHP and database queries
Object cache	Results of database queries, in memory	Repeating the same query
Opcode cache	Compiled PHP code	Re-reading and compiling PHP files

Browser cache

Your server tells browsers how long to keep files with headers like Cache-Control: max-age=2592000 (30 days). Repeat visitors then load your images and styles from their own disk. Change a file’s name or add a version, such as style.css?ver=2, when you update it, so browsers fetch the new one.

Page cache

The biggest win for most sites. The first visitor to a page triggers the full work; the result is saved as a static file, and later visitors get that file. On WordPress this is done by a caching plugin or by the web server (for example Nginx’s FastCGI cache). See how much RAM a site needs for why this matters.

Object cache

A store in memory, such as Redis or Memcached, that keeps the results of database queries. It helps pages that cannot be fully cached, like shop carts and account pages.

Opcode cache

PHP’s OPcache keeps compiled code in memory. It is built into PHP and should always be on.

When caching causes problems

• You see an old version after an edit: clear the page cache or the CDN.
• One visitor sees another’s data: a page meant for logged-in users was cached. Never page-cache carts, checkouts or account pages.
• Changes do not show for everyone: different layers expire at different times. Clear from the top down: page cache, then CDN, then browser.