Glossary
Caching in TYPO3
The intermediate storage of rendered content, so TYPO3 does not recalculate everything on every request.
Why cache at all
A TYPO3 page is assembled from many parts: the page record, content elements, menus, file references, TypoScript configuration. Repeating that work on every request would be waste, since the result is almost always the same.
So TYPO3 stores the finished output and delivers it directly on the next request. That is the single most important reason a TYPO3 site can be fast.
Several layers
Inside TYPO3 there are several stores: the page cache for rendered output, caches for configuration and class mappings, and caches belonging to individual extensions. They are cleared separately because they differ in how expensive they are to rebuild.
Outside, further layers join in: a server-side cache in front, a CDN, the browser. If you cannot see a change, work outwards from the inside.
What cannot be cached
Anything that differs per visitor: a shopping basket, a logged-in area, a form with an error message. Parts like that are handled separately so the rest of the page can still come from the cache.
Overlook it and you get the most unpleasant fault in this category: one visitor seeing another’s cached view. So with personalised content we always check specifically what is delivered in which state.