Two years ago, I wrote an article about how probes work in Varnish (it's a great article - fun, informative... go read it), it covers a lot of ground, but still, it misses one important spot. More precisely, it only focused on how Varnish uses probes to know whether a backend is worth contacting, so today, we are going to look at the other side of the story: how do we tell the rest of the system that Varnish is up and ready to work?
Also, we'll see how to handle maintenance: if you need to get your Varnish node offline, it's annoying to log into all the load balancers to re-configure them; it's easier to just tell Varnish to fail incoming probes until said load balancers take the node out of their pool, and then you can wait for the active connections to end (does it ring a bell?) and then stop Varnish.
Hop on! We'll have a look at different ways of doing it - good and (mostly) bad, to try and understand how to do it and be warned of the various pitfalls to avoid.