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.TH VMOD_XKEY 3 "" "" ""
.SH NAME
vmod_xkey \- Surrogate keys support for Varnish Cache
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.\" Edit ./vmod_xkey.vcc and run make instead
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.SH SYNOPSIS
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.nf
.ft C
import xkey [as name] [from "path"]

INT purge(STRING keys)

INT softpurge(STRING keys)
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.fi
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.SH DESCRIPTION
.sp
This vmod adds secondary hashes to objects, allowing fast purging on
all objects with this hash key.
.sp
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.
.sp
Similarly with an e\-commerce site, where various SKUs are often
referenced on a page.
.sp
Hash keys are specified in the \fBxkey\fP response header. Multiple keys
can be specified per header line with spaces and/or commas as
separators. Alternatively, they can be specified in multiple \fBxkey\fP
response headers.
.sp
Preferably the secondary hash keys are set from the backend
application, but the header can also be set from VCL in
\fBvcl_backend_response\fP\&.
.\" vcl-start
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.sp
VCL example:
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.ft C
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);
        }
    }
}
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.\" vcl-end
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.SS Example
.sp
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:
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HTTP/1.1 OK
Server: Apache/2.2.15
xkey: 8155054
xkey: 166412
xkey: 234323
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.sp
This requires a bit of VCL to be in place. The VCL can be found above.
.sp
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:
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GET / HTTP/1.1
Host: www.example.com
xkey\-purge: 166412
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.sp
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.
.sp
Varnish will find the objects and clear them out, responding with:
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HTTP/1.1 200 Purged
Date: Thu, 24 Apr 2014 17:08:28 GMT
X\-Varnish: 1990228115
Via: 1.1 Varnish
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.sp
The objects are now cleared.
.SS INT purge(STRING keys)
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.TP
.B Description
Purges all objects hashed on any key found in the \fBkeys\fP argument.
Returns the number of objects that were purged.
.sp
The \fBkeys\fP may contain a list of space\-separated ids.
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.SS INT softpurge(STRING keys)
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.TP
.B Description
Performs a "soft purge" for all objects hashed on any key found in the
\fBkeys\fP argument.  Returns the number of objects that were purged.
.sp
A softpurge differs from a regular purge in that it resets an
object\(aqs TTL but keeps it available for grace mode and conditional
requests for the remainder of its configured grace and keep time.
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.SS Counters
.SS #
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varnish_vsc_begin:: xkey
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.SH XKEY – XKEY COUNTERS
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Metrics from vmod_xkey
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.sp
\fBg_keys\fP – \fIgauge\fP \- info
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Number of surrogate keys
.sp
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.
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.sp
\fBg_hashhead_bytes\fP – \fIgauge\fP \- debug
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Bytes used by all xkey_hashhead objects
.sp
Total bytes used by hashhead objects. Tracks linearly with the number of surrogate keys in use.
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.sp
\fBg_ochead_bytes\fP – \fIgauge\fP \- debug
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Bytes used by all xkey_ochead objects
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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.
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.sp
\fBg_oc_bytes\fP – \fIgauge\fP \- debug
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Bytes used by all xkey_oc objects
.sp
Total bytes used by oc objects. Tracks linearly with the number of cached objects that are referenced by surrogate keys.
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.sp
\fBg_bytes\fP – \fIgauge\fP \- info
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Bytes used by xkeys
.sp
Current number of bytes used by xkeys and their references to the object cache.
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.\" varnish_vsc_end:: xkey
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