Varnish ‘ban lurker’
Banning and purging content from the cache is an invalidation technique and is necessary to remove items that have changed or expired. Banning leaves the object in the cache and can allow it to be served ‘stale’ while an updated copy is being fetched. In contrast, purged items are removed from the cache immediately. When a banned object is too stale (past their grace period) it is purged.
The facility to ban content provides control of the objects in the cache. The ‘ban lurker’ is a powerful part of this as it help manage resource usage in the cache, and is generally considered a force for good in the Varnish universe. The ban lurker helps flush out cached objects that wouldn’t otherwise be purged, and it helps stop excessive amounts of ban rules in the running cache server.
The Ban Lurker is in essence a set of ban rules that are implemented versus objects in the cache rather than URLs in the requests. Because the ban is lodged versus ‘obj’ instead of ‘req’ it doesn’t need to be evaluated on every request, so it maintains high performance while achieving the same goal.
Technically, the ban lurker is a thread that walks through the cache looking for rarely used objects that can be garbage collected to make room for more valuable items. Combined with a few manipulations for the headers, that’s all it takes - create ‘ban’ calls using ‘obj’ instead of ‘req’.
How the ‘ban lurker’ works
The ban lurker actively walks the cache and matching objects against the ban list. It works solely on the ‘obj’ ojects in the cache, and has no concept of request (the ‘req’ object). In order to be ‘ban-lurker-friendly’ all you really have to do is to ban based on ‘obj’. As always, the way to get values into the ‘obj’ is to set them as values on ‘beresp’ and Varnish will add them to the cache object.
The ban lurker only really helps when long TTLs are involved. Bans based on req objects can only really be evaluated when a request for that object is made, so using bans versus ‘obj’ means that a background thread (the lurker) can find matches and remove them even though they are not being requested.
Enabling the ban lurker
The ban lurker is enabled by setting the ‘ban_lurker_sleep’ parameter of Varnish to some value other than zero. The parameter controls for how long it will sleep between each object found to be banned. In operation, it finds an expired object that matches the ban list, removes it, then it rests for the duration of the sleep parameter, then it resumes the search for candidates that match the ban list and have passed their TTL.