Manual template cache management

[brandonkelly]

The current template caching system has a high dosage of automagic cache busting:

• {% cache %} tags record:
  • each of the element IDs whose properties were accessed (stored in templatecacheelements)
  • any element queries that were executed (stored in templatecachecriteria)
• Whenever an element is saved/deleted, template caches will be deleted if:
  • they are associated with the element’s ID (via templatecacheelements)
  • any of the element queries associated with the template cache are now pulling in the element’s ID (via templatecachecriteria)

The element query-based cache busting works in general, but there are a few edge cases (e.g. #1667) where it doesn’t, and it is also generally a huge performance suck, and the cause of many stalled tasks. It’s so expensive that some sites resort to disabling it altogether, via the cacheElementQueries config setting.

We should probably just completely remove element query-based cache busting, and replace it with manual cache management, plus the ability to define automated cache busting strategies.

[brandonkelly]

Element query caching has been completely rewritten for Craft 3.5, in a way that no longer suffers from the performance issues / long-runnning background jobs that have plagued {% cache %} tags to date. Details here: #1507 (comment)