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.

                                                                                                    VMOD_XKEY(3)