Some Trickery or Resilience With Varnish
As of now, Varnish has no means to detect whether a backend is available or at good health before sending a request (periodic checking is scheduled for ver 2.0 and will presumably work with the cluster mode as well). So if you’ve got two or more backends, and under some condition can’t or won’t serve a request immediately or want to send it elsewhere depending on some circumstance, you can do this using HTTP return code or header with the not-so-well-documented feature ‘restart' (then again, what feature is well documented in Varnish?).
‘restart’ will effectively increase a counter by 1 and re-run vcl_recv(). You can set how many times a restart should take place before giving up entirely - should you not use the counter in a condition prior to it reaching the limit - by starting
-p max_restarts=n or
param.set max_restarts 1 on the CLI. This variable defaults to 4, and you can of course set conditions depending on the number of restarts.
Here’s a sample VCL to do this:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
In this simple VCL, a request destined for this instance of Varnish which doesn’t return 200 or 302 from the backend, is effectively sent to 10.0.0.2 which may have something else in store for the visitor!
If I for instance use the above VCL and set be1 to return a 301 for / and send a request to Varnish, this is what shows up in varnishlog:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35
You can of course use this for very basic resilience as well, but that’s definitely a job for your load balancer. Also be aware about the overhead in this, since the request after all is sent to the backend and processed before passed on to the other node.
Maybe it’s not the most useful feature in the world, but I thought it was nifty!