Serving a maintenance page with varnish

Marc Weistroff 2012-01-18

Yesterday during the SOPA Blackout, this website was on strike. The
billions of readers of this blog had a static page explaining why this
website was offline instead of the normal content. As a good nerd, I
turned this operation into a technological one and instead of serving a
static html with nginx, I decided to use
varnish. Yes, varnish. I never wrote
anything about the absolutely amazing architecture of this blog but
basically, static files are generated from
RestructuredText, served by
nginx and cached by varnish. Let’s get back to the
point. I wanted varnish to serve a static page with a 503 status code
(for SEO purpose). It took a bit of VCL knowledge and here is how to do
it:

“` text /etc/varnish/maintenance.vcl
backend default {
.host = “127.0.0.1”;
.port = “8080”;
}

sub vcl_recv {
error 503;
}

sub vcl_error {
set obj.http.Content-Type = “text/html; charset=utf-8”;
# you can put absolutely what you want
synthetic {“
<html>
<head>
My maintenance page
</head>
<body>
<h1>This website is under maintenance.</h1>
</body>
</html>
“};
return (deliver);
}


The trick here is the `synthetic` keyword which is described in the
varnish documentation like this:

> The synthetic keyword is used to produce a synthetic response body in
> `vcl_error`. It takes a single string as argument.

Now that you have a cool VCL for your maintenance page, you can load
this vcl in varnish like this:

``` bash
varnishadm -T 127.0.0.1:6082 -S /etc/varnish/secret "vcl.load maintenance /etc/varnish/maintenance.vcl"

and to replace the original rules by those described in maintenance.vcl

varnishadm -T 127.0.0.1:6082 -S /etc/varnish/secret "vcl.use maintenance"

And varnish will serve the maintenance page. Like a boss.

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