Can there be in whatever way to dynamically load and unload webservers from the reverse proxy?

For instance let us say I've got a cluster of 100 apache servers and I wish to patch the servers. I'd like the possess a setup where I've got a pool of up-to-date servers along with a pool of "being up-to-date" servers. I'd like overturn proxy be familiar with what's being patched and never read from all of these webservers.

I'd rather not use only a config file to configure overturn proxy.

You should use varnish's backend polling. You place simple check script on each server in after sales and let varnish to poll it. Each poll means asking some "master server" if "slave" has right version, enough disk space etc...