vcl_error restart

[Le Roux Bodenstein]

I'm trying to setup varnish so that it will try one backend and then
if there's an error (for example - the backend isn't up at all - not
even throwing 500 errors), it tries a fallback backend. I'm trying to
get varnish to immediately try another backend if I restart my app. So
the first backend is my django powered web app and the second one is a
smaller, dumb one that just tries to read from a cache that I manage.
I only want to use this cache as a fallback - it isn't a max-age based
thing.

I think backend polling isn't perfect here - I don't want requests to
die for seconds before health polling figures it out - the app will be
restarted within seconds already anyway. I also don't want to use the
grace functionality here - this is only for non-cacheable (not by
varnish, anyway) web app urls that need to be re-evaluated all the
time. (I'm thinking of sending static, cacheable urls to a third
backend - a traditional webserver)

So basically, I want to do something like this:

backend b1 {
    .host = "localhost";
    .port = "8000";
}

backend b2 {
    .host = "localhost";
    .port = "8001";
}

sub vcl_recv {
    if (req.restarts == 0) {
        set req.backend = b1;
    } else {
        set req.backend = b2;
    }
}

sub vcl_error {
    if (req.restarts == 0) {
        set req.backend = b2;
        restart;
    }
}


But my second backend never gets a request. Is this possible? Has
anyone done something like this before?


Le Roux