collective.purgebyid

collective.purgbyid is a new method for cache invalidation of Plone based web sites. It uses the idea of adding an extra header, called X-Ids-Involved, which contains thee uuids of the objects involved in the construction of the resources. For example, an image contains just its uuid:

% wget -S http://localhost:8080/Plone/image01
...
  X-Ids-Involved: #c8d7c0bc2b794325b916d990de91d7ee#

Other pages may be more complicated. Then a new purge rewrite rule adds a custom URL to the set of URLs to purge: the "purge by id" custom URL is in the form /@@purgebyid/<UUID> where UUID is the object's uuid to be purged.

Last, Varnish is configured so that, when an URL /@@purgebyid/<UUID> is purged, it will ban all the objects that match an X-Ids-Involved header of the right type (i.e. containing the uuid of the resource to purge). This means that when a resources is purged, it is enough to purge also it /@@purgebyid/<UUID> URL because it will be Varnish responsibily to also catch all of the occurrencies of the resources whenever the URL which is used to access it.

Varnish

Config example:

sub vcl_recv {
    if (req.request == "PURGE") {
        if (!client.ip ~ purge) {
            error 405 "Not allowed.";
        }
        if (req.url ~ "^/@@purgebyid/") {
            ban("obj.http.x-ids-involved ~ #" + regsub(req.url, "^/@@purgebyid/", "") + "#");
            error 200 "Ban added";
        }
    }
}

sub vcl_deliver {
    unset resp.http.x-ids-involved;
}

References

• Blog post http://www.biodec.com/it/blog/migliorare-la-gestione-del-purge-caching-in-plone-collective-purgebyid (italian language)