NAME

vmod_xkey - Surrogate keys support for Varnish Cache

SYNOPSIS

import xkey [as name] [from "path"]
INT purge(STRING keys)
INT softpurge(STRING keys)

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 spaces and/or commas as separators. 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.xkey);
            # or: set req.http.n-gone = xkey.softpurge(req.http.xkey)
            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:

PURGE / HTTP/1.1
Host: www.example.com
xkey: 166412

Note the xkey 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 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 keys)

Description: Performs a "soft purge" for all objects hashed on any key found in the keys argument. Returns the number of objects that were purged.
A softpurge differs from a regular purge in that it resets an object's TTL but keeps it available for grace mode and conditional requests for the remainder of its configured grace and keep time.

Counters

#

varnish_vsc_begin:: xkey

XKEY – XKEY COUNTERS

Metrics from vmod_xkey

g_keys – gauge - info

Number of surrogate keys

Number of surrogate keys in use. Increases after a request that includes a new key in the xkey header. Decreases when a key is purged or when all cache objects associated with a key expire.

g_hashhead_bytes – gauge - debug

Bytes used by all xkey_hashhead objects

Total bytes used by hashhead objects. Tracks linearly with the number of surrogate keys in use.

g_ochead_bytes – gauge - debug

Bytes used by all xkey_ochead objects

Total bytes used by ochead objects. Increases when an object is added to a key or a key is added to an object. Decreases when the relationship is removed.

g_oc_bytes – gauge - debug

Bytes used by all xkey_oc objects

Total bytes used by oc objects. Tracks linearly with the number of cached objects that are referenced by surrogate keys.

g_bytes – gauge - info

Bytes used by xkeys

Current number of bytes used by xkeys and their references to the object cache.