1VMOD_XKEY(3)                                                      VMOD_XKEY(3)
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NAME

6       vmod_xkey - Surrogate keys support for Varnish Cache
7

SYNOPSIS

9          import xkey [as name] [from "path"]
10
11          INT purge(STRING keys)
12
13          INT softpurge(STRING keys)
14

DESCRIPTION

16       This  vmod  adds  secondary hashes to objects, allowing fast purging on
17       all objects with this hash key.
18
19       You can use this to indicate relationships, a bit like  a  "tag".  Then
20       clear  out  all  object  that have this tag set. Two good use cases are
21       news sites, where one might add all the stories mentioned on a particu‐
22       lar  page by article ID, letting each article referenced create an xkey
23       header.
24
25       Similarly with an e-commerce site, where various SKUs are often  refer‐
26       enced on a page.
27
28       Hash  keys are specified in the xkey response header. Multiple keys can
29       be specified per header line with spaces and/or commas  as  separators.
30       Alternatively, they can be specified in multiple xkey response headers.
31
32       Preferably  the  secondary  hash keys are set from the backend applica‐
33       tion, but the header can also be set from VCL in vcl_backend_response.
34
35       VCL example:
36
37          vcl 4.0;
38          import xkey;
39
40          backend default { .host = "192.0.2.11"; .port = "8080"; }
41
42          acl purgers {
43              "203.0.113.0"/24;
44          }
45
46          sub vcl_recv {
47              if (req.method == "PURGE") {
48                  if (client.ip !~ purgers) {
49                      return (synth(403, "Forbidden"));
50                  }
51                  if (req.http.xkey) {
52                      set req.http.n-gone = xkey.purge(req.http.xkey);
53                      # or: set req.http.n-gone = xkey.softpurge(req.http.xkey)
54
55                      return (synth(200, "Invalidated "+req.http.n-gone+" objects"));
56                  } else {
57                      return (purge);
58                  }
59              }
60          }
61
62   Example
63       On an e-commerce site we have the backend  application  issue  an  xkey
64       header for every product that is referenced on that page. So the header
65       for a certain page might look like this:
66
67          HTTP/1.1 OK
68          Server: Apache/2.2.15
69          xkey: 8155054
70          xkey: 166412
71          xkey: 234323
72
73       This requires a bit of VCL to be in place. The VCL can be found above.
74
75       Then, in order to keep the web in sync with the database, a trigger  is
76       set  up  in  the  database. When an SKU is updated this will trigger an
77       HTTP request towards the Varnish server, clearing out every object with
78       the matching xkey header:
79
80          GET / HTTP/1.1
81          Host: www.example.com
82          xkey-purge: 166412
83
84       Note  the xkey-purge header. It is probably a good idea to protect this
85       with an ACL so random people from the Internet cannot purge your cache.
86
87       Varnish will find the objects and clear them out, responding with:
88
89          HTTP/1.1 200 Purged
90          Date: Thu, 24 Apr 2014 17:08:28 GMT
91          X-Varnish: 1990228115
92          Via: 1.1 Varnish
93
94       The objects are now cleared.
95
96   INT purge(STRING keys)
97       Description
98              Purges all objects hashed on any key found in the keys argument.
99              Returns the number of objects that were purged.
100
101              The keys may contain a list of space-separated ids.
102
103   INT softpurge(STRING keys)
104       Description
105              Performs  a "soft purge" for all objects hashed on any key found
106              in the keys argument.  Returns the number of objects  that  were
107              purged.
108
109              A  softpurge  differs  from a regular purge in that it resets an
110              object's TTL but keeps it available for grace  mode  and  condi‐
111              tional  requests  for  the remainder of its configured grace and
112              keep time.
113
114   Counters
115   #
116          varnish_vsc_begin:: xkey
117

XKEY – XKEY COUNTERS

119          Metrics from vmod_xkey
120
121       g_keys gauge - info
122          Number of surrogate keys
123
124          Number of surrogate keys in use. Increases after a request that  in‐
125          cludes  a new key in the xkey header. Decreases when a key is purged
126          or when all cache objects associated with a key expire.
127
128       g_hashhead_bytes gauge - debug
129          Bytes used by all xkey_hashhead objects
130
131          Total bytes used by hashhead objects. Tracks linearly with the  num‐
132          ber of surrogate keys in use.
133
134       g_ochead_bytes gauge - debug
135          Bytes used by all xkey_ochead objects
136
137          Total  bytes  used  by  ochead  objects. Increases when an object is
138          added to a key or a key is added to an object.  Decreases  when  the
139          relationship is removed.
140
141       g_oc_bytes gauge - debug
142          Bytes used by all xkey_oc objects
143
144          Total  bytes  used by oc objects. Tracks linearly with the number of
145          cached objects that are referenced by surrogate keys.
146
147       g_bytes gauge - info
148          Bytes used by xkeys
149
150          Current number of bytes used by xkeys and their  references  to  the
151          object cache.
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156                                                                  VMOD_XKEY(3)
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