Cache is a buffer memory in which the finished form of a page or its parts is stored so that it does not have to be regenerated on every visit. It is applied at several levels – in the visitor's browser, on the server, in the content management system, and in the content delivery network. The benefit is a significant speed increase and lower server load. The risk is displaying outdated content, which causes real problems with prices, product availability and opening hours. The solution is to set a different validity period for immutable files and for content that changes, and to ensure automatic invalidation on update. When fine-tuning a website, it is important to remember that a change may not appear immediately, which is often a source of needlessly hunting for non-existent bugs.
See also: Server response time, CDN and content distribution, Core Web Vitals.
