I am looking for a solution to strange cache behaviors...
There are tasks I don't understand. That's why I'm asking.

digest
Sends out daily digest emails for followed content.

Since it is declared to send out daily, why is it running every 5 minutes? viewupdates Which update does this task work for? If it is for a global update version - why is it activated every 5 minutes? clearcache Which cache clears this task, and how is it synchronized when e.g. cache blocks and sidebar is set to a different time? And how does this task affect when using eg nginx server for cache server? Appreciate any clarification...Thanks.

Those tasks do their own things in batches. Doing everything at once would just lock the task with a timeout error otherwise. Especially on big boards.

 

viewupdates is for storing the temporary views into their proper column in the content's table (like topics views). Updating constantly the table would lock it all the time, the code instead saves each view as a new row in a temporary table and then the task counts the rows and adds only once the views in the proper table.

 

clearcache drops the caches from the core_cache table and the guest output cache. The code simply drops rows that are already expired. Other code in different files checks if a block/sidebar/widget cache is expired and refreshes them.