NAME

vmod_xkey - Surrogate keys support for Varnish Cache

SYNOPSIS

import xkey [from "path"] ;

DESCRIPTION

This vmod adds secondary hashes to objects, allowing fast purging on all objects with this hash key.

You can use this to indicate relationships, a bit like a "tag". Then clear out all object that have this tag set. Two good use cases are news sites, where one might add all the stories mentioned on a particular page by article ID, letting each article referenced create an xkey header.

Similarly with an e-commerce site, where various SKUs are often referenced on a page.

Hash keys are specified in the xkey response header. Multiple keys can be specified per header line with a space separator. Alternatively, they can be specified in multiple xkey response headers.

Preferably the secondary hash keys are set from the backend application, but the header can also be set from VCL in vcl_backend_response.

VCL example:

vcl 4.0;
import xkey;

backend default { .host = "192.0.2.11"; .port = "8080"; }

acl purgers {
    "203.0.113.0"/24;
}

sub vcl_recv {
    if (req.method == "PURGE") {
        if (client.ip !~ purgers) {
            return (synth(403, "Forbidden"));
        }
        if (req.http.xkey) {
            set req.http.n-gone = xkey.purge(req.http.key);
            # or: set req.http.n-gone = xkey.softpurge(req.http.key)

            return (synth(200, "Invalidated "+req.http.n-gone+" objects"));
        } else {
            return (purge);
        }
    }
}

Example

On an e-commerce site we have the backend application issue an xkey header for every product that is referenced on that page. So the header for a certain page might look like this:

HTTP/1.1 OK
Server: Apache/2.2.15
xkey: 8155054
xkey: 166412
xkey: 234323

This requires a bit of VCL to be in place. The VCL can be found above.

Then, in order to keep the web in sync with the database, a trigger is set up in the database. When an SKU is updated this will trigger an HTTP request towards the Varnish server, clearing out every object with the matching xkey header:

GET / HTTP/1.1
Host: www.example.com
xkey-purge: 166412

Note the xkey-purge header. It is probably a good idea to protect this with an ACL so random people from the Internet cannot purge your cache.

Varnish will find the objects and clear them out, responding with:

HTTP/1.1 200 Purged
Date: Thu, 24 Apr 2014 17:08:28 GMT
X-Varnish: 1990228115
Via: 1.1 Varnish

The objects are now cleared.

•: INT purge(STRING)

•: INT softpurge(STRING)

INT purge(STRING)

Prototype: INT purge(STRING keys)

Description: Purges all objects hashed on any key found in the keys argument. Returns the number of objects that were purged.
The keys may contain a list of space-separated ids.

INT softpurge(STRING)

Prototype: INT softpurge(STRING keys)