No code here yet, just notes for now
This project will implement cache chaining for D7.
I have wanted this for ages. Subsequently found out about #365088: Implement a hierarchical (primary / secondary) engine which has some code but is stalled. Apparently http://drupal.org/project/cache has something like this, but I've not reviewed code yet. Have a very specific idea how I want this to work, so will likely work on that here, then review cache project to see how it compares. Neither cacherouter nor cache have 7.x branches, this could end up being the 'port', or maybe it should be a new project.
Requirements
The main use case for this is a site with at least two web servers. Drupal 7 has the cache_bootstrap bin, which is used for a relatively small number of cache items, that shouldn't be updated too frequently, and are often quite large (variables, system_list etc.).
The plan is to have this bin cached in APC (or file caching on shm) on each local webserver. Some sites do this already, but this leaves you with cache coherency issues, and means that each web server needs to build it's own cache item.
Instead of this, we'd keep the local cache storage, but have a second layer underneath it - most likely memcache.
There are two primary things this needs to do: