plone.cachepurging

Table of Contents

• Introduction
• Installation
• Initiating a purge in code
• Purging an object manually
• Purging objects automatically
• Purging dependencies
• Which URLs get purged?
• Virtual hosting and URL rewriting

Introduction

The plone.cachepurging package provides cache purging for Zope 2 applications. It is inspired by (and borrows some code from) Products.CMFSquidTool, but it is not tied to Squid. In fact, it is tested mainly with Varnish, though it should also work with Squid and Enfold Proxy.

This package is not tied to Plone. However, if you intend to use it with Plone, you probably want to install plone.app.caching, which provides Plone-specific configuration and a user interface in Plone's control panel.

plone.cachepurging works with Zope 2.12 and later. If you want to use it with Zope 2.10, you may be able to do so by installing ZPublisherEventsBackport, although this is not a tested configuration.

Installation

To use this package, you must do the following:

• Install it into your Zope instance. This normally means depending on it via install_requires in the setup.py file of your package.

• Load its configuration by adding a ZCML line like the following (or a slug):

  <include package="plone.cachepurging" />
  

• Install a plone.registry IRegistry local utility and create records for the interface plone.cachepurging.interfaces.ICachePurgingSettings. See the plone.registry documentation for details.

If you use plone.app.caching in Plone, it will do all of this for you.

To enable cache purging after installation, you must:

• Set up a caching proxy that supports PURGE requests, such as Varnish, Squid or Enfold Proxy.
• Configure the proxy and your application so that resources are cached in the proxy.
• Set the registry option plone.cachepurging.interfaces.ICachePurgingSettings.enabled to True. See the plone.registry documentation for details.
• Add the URL of at least one caching proxy server capable of receiving PURGE requests to the registry option plone.cachepurging.interfaces.ICachePurgingSettings.cachingProxies. This should be a URL that is reachable from the Zope server. It does not need to be accessible from Zope's clients.
• Make your application send purge notifications - see below.

Initiating a purge in code

The simplest way to initiate a purge is to raise a Purge event:

from z3c.caching.purge import Purge
from zope.event import notify

notify(Purge(context))

Notice how we are actually importing from z3c.caching here. That package defines the event type and a few of the interfaces that plone.cachepurging uses. In most cases, you should be able to define how your own packages' behave in relation to a caching proxy by depending on z3c.caching only. This is a safer dependency, as it in turn depends only on a small set of core Zope Toolkit packages.

Presuming plone.cachepurging is installed, firing the event above will:

• Check whether caching is enabled and configured. If not, it will do nothing.
• Look up paths to purge for the given object. This is done via zero or more IPurgePaths adapters. See "Which URLs get purged?" below.
• Convert the purge paths to URLs by combining them with the URLs of the configured caching proxies.
• Queue these for purging.

It doesn't matter if a particular object or URLs is queued more than once. It will only be executed once.

This operation is relatively quick, and does not involve communication with the caching proxy. At the end of the request, after the Zope transaction has been closed (and presuming the transaction was successful - purging is by default not performed for requests resulting in an error), the following will take place:

• The queued URLs are retrieved from the request.
• A worker thread is established for each caching proxy, allowing asynchronous processing and freeing up Zope to handle the next request.
• The worker thread establishes a connection to the caching proxy and sends a PURGE request.
• Any errors are logged at error level to the logger plone.cachepurging.

If you need more control, you can perform the purging directly. Here is a snippet adapted from the plone.cachepurging.purge view:

from io import StringIO

from zope.component import getUtility

from plone.registry.interfaces import IRegistry

from plone.cachepurging.interfaces import IPurger
from plone.cachepurging.interfaces import ICachePurgingSettings

from plone.cachepurging.utils import getPathsToPurge
from plone.cachepurging.utils import getURLsToPurge
from plone.cachepurging.utils import isCachePurgingEnabled

...

if not isCachePurgingEnabled():
    return 'Caching not enabled'

registry = getUtility(IRegistry)
settings = registry.forInterface(ICachePurgingSettings)

purger = getUtility(IPurger)

out = StringIO()

for path in getPathsToPurge(self.context, self.request):
    for url in getURLsToPurge(path, settings.cachingProxies):
        status, xcache, xerror = purger.purgeSync(url)
        print("Purged", url, "Status", status, "X-Cache", xcache, "Error:", xerror, file=out)

return out.getvalue()

Here, we:

• Check whether caching is enabled. This checks the enabled and cachingProxies properties in the registry.

• Look up the registry and cache purging settings to find the list of caching proxies.

• Obtain an IPurger utility. This has three main methods:

  def purgeAsync(url, httpVerb='PURGE'):
      """Send a PURGE request to a particular URL asynchronously in a
      worker thread.
      """
  
  def purgeSync(url, httpVerb='PURGE'):
      """Send a PURGE request to a particular URL synchronosly.
  
      Returns a triple ``(status, xcache, xerror)`` where ``status`` is
      the HTTP status of the purge request, ``xcache`` is the contents of
      the ``x-cache`` response header, and ``x-error`` is the contents
      of the first header found from the list of headers in
      ``errorHeaders``.
      """
  
  def stopThreads(wait=False):
      """Attempts to stop all threads.  Threads stop immediately after
      the current item is being processed.
  
      Returns True if successful, or False if threads are still running
      after waiting 5 seconds for each one.
      """
  

• Get all paths to purge for the current context using the helper function getPathsToPurge(). Paths are relative to the domain root, i.e. they start with a '/'.

• Obtain a full PURGE URL for each caching proxy, using the helper function getURLsToPurge()

• Send a synchronous caching request. This blocks until the caching proxy has responded (or timed out).

Purging an object manually

The code above illustrates how to initiate asynchronous and synchronous purges. If you simply want to do this through the web, you can invoke one of the following views, registered for any type of context:

@@plone.cachepurging.purge

Performs an immediate purge of the context, using code similar to that shown above.

@@plone.cachepurging.queue

Queues the context for purging.

Both of these views require the permission plone.cachepurging.InitiatePurge, which by default is granted to the Manager role only.

Purging objects automatically

Quite commonly, you will want to purge objects in three scenarios:

• When the object is modified
• When the object is moved or renamed
• When the object is removed

These are of course all described by standard Zope event types from the zope.lifecycleevent package. If the standard IObjectModifiedEvent, IObjectMovedEvent and IObjectRemovedEvent event types are fired for your context, you can mark it with the IPurgeable interface to automatically purge the object.

One way to do this without changing the code of your content object is to do this in ZCML, e.g. with:

<class class=".content.MyContent">
    <implements interface="z3c.caching.interfaces.IPurgeable" />
</class>

(Again notice how we are using a generic interface from z3c.caching).

This is equivalent to registering an event handler for each of the events above and doing notify(Purge(object)) in each one. That is, a z3c.caching.interfaces.IPurgeEvent will be raised in a handler for the lifecycle events, which in turn will cause purging to take place.

Purging dependencies

Sometimes, purging one object implies that other objects should be purged as well. One way to do this is to register an event handler for the IPurgeEvent event type, and dispatch further purge events in response. For example, here is some code to purge the parent of the purged object:

from zope.component import adapter
from z3c.caching.interfaces import IPurgeEvent
from z3c.caching.purge import Purge

@adapter(IMyContent, IPurgeEvent)
def purgeParent(object, IPurgeEvent):
    parent = object.__parent__
    if parent is not None:
        notify(Purge(parent))

This could be registered in ZCML like so:

<subscriber handler=".events.purgeParent" />

If the parent is also of type IMyContent (or you replace that interface with a more generic one), then its parent will be purged too, recursively.

Which URLs get purged?

The Purge event handler calculates the URLs to purge for the object being passed via named z3c.caching.interfaces.IPurgePaths adapters. Any number of such adapters may be registered. plone.cachepurging ships with one, for OFS.interfaces.ITraversable (i.e. most objects that you can find through the ZMI), which purges the object's absolute_url_path().

The IPurgePaths interface looks like this:

class IPurgePaths(Interface):
    """Return paths to send as PURGE requests for a given object.

    The purging hook will look up named adapters from the objects sent to
    the purge queue (usually by an IPurgeEvent being fired) to this interface.
    The name is not significant, but is used to allow multiple implementations
    whilst still permitting per-type overrides. The names should therefore
    normally be unique, prefixed with the dotted name of the package to which
    they belong.
    """

    def getRelativePaths():
        """Return a list of paths that should be purged. The paths should be
        relative to the virtual hosting root, i.e. they should start with a
        '/'.

        These paths will be rewritten to incorporate virtual hosting if
        necessary.
        """

    def getAbsolutePaths():
        """Return a list of paths that should be purged. The paths should be
        relative to the domain root, i.e. they should start with a '/'.

        These paths will *not* be rewritten to incorporate virtual hosting.
        """

Most implementations will use getRelativePaths() to return a path relative to the virtual hosting root (i.e. what the absolute_url_path() method returns). This is subject to rewriting for virtual hosting (see below).

getAbsolutePaths() is useful if you have a path that is not subject to change no matter how Zope is configured. For example, you could use this if your caching proxy supports "special" URLs to invoke a particular type of purge. (Such behaviour can be implemented in Varnish using VCL, for example.) This is not subject to rewriting for virtual hosting.

Let's say you wanted to always purge the URL ${object_url}/view for any object providing IContentish from CMF. A simple implementation may look like this:

from zope.interface import implementer
from zope.component import adapts

from z3c.caching.interfaces import IPurgePaths

from Products.CMFCore.interfaces import IContentish

@implementer(IPurgePaths)
class ObjectViewPurgePaths(object):
    """Purge /view for any content object with the content object's
    default URL
    """

    adapts(IContentish)

    def __init__(self, context):
        self.context = context

    def getRelativePaths(self):
        return [self.context.absolute_url_path() + '/view']

    def getAbsolutePaths(self):
        return []

This adapter could be registered with a ZCML statement like:

<adapter factory=".paths.ObjectViewPurgePaths" name="my.package.objectview" />

The name is not significant, but should be unique unless it is intended to override an existing adapter. By convention, you should prefix the name with your package's dotted name unless you have a reason not to.

The default adapter that simply returns absolute_url_path() is called default.

Virtual hosting and URL rewriting

Zope 2 uses "magic" URLs for virtual hosting. A common scenario is to set the virtual host root to a Plone site object at the root of the Zope instance. This is usually done through URL rewriting. The user sees a URL like http://example.com/front-page. A web server like Apache (or a proxy like Squid or Varnish) changes this into a URL like this:

http://localhost:8080/VirtualHostBase/http/example.com:80/Plone/VirtualHostRoot/front-page

Here, the Zope server is running on http://localhost:8080, the external domain is http://example.com:80 (the :80 part is normally not shown by web browsers, since that is the default protocol for the http URL scheme), and the virtual hosting root is /Plone.

Zope sees these tokens in the URL and understands how to incorporate the external domain and virtual host root into the results of methods like absolute_url() and absolute_url_path(), thus allowing URLs generated in the site to show the correct external URL.

So far so good. The challenge comes when you put a caching proxy into the mix. There are two scenarios:

1. The caching proxy is "behind" whatever performs the URL rewrite. In this case, the inbound URL (which the proxy may choose to cache, and which may therefore need to be purged) contains the virtual hosting tokens.
2. The caching proxy is "in front of" whatever performs the URL rewrite, or performs the rewrite before passing the request off to the Zope backend. In this case, the inbound URL does not contain the virtual hosting tokens.

Purging works by sending the proxy server a PURGE request with the same path as that of a cached resource. Thus, in scenario 1, that URL needs to contain the virtual hosting tokens. Since these are not part of any URL generated by Zope (though they are retained in the PATH_INFO request variable), the paths returned by getRelativePaths() of the IPurgePaths adapters need to be rewritten (in reverse, as it were) to include them.

This is done using an IPurgePathRewriter adapter on the request. The default implementation will deal with any valid VirtualHostMonster URL, including setups using "inside-out" hosting (with _vh_ type path segments), although you can write your own adapter if you have truly unique needs.

If you perform URL rewriting in front of the caching proxy (scenario 1 above), you need to configure two registry options, since there is no way for plone.cachepurging to know how the web and/or proxy cache server(s) in front of Zope are configured:

plone.cachepurging.interfaces.ICachePurgingSettings.virtualHosting

Set this to True to incorporate virtual hosting tokens in the PURGE paths. This is applicable in scenario 1 above.

plone.cachepurging.interfaces.ICachePurgingSettings.domains

Set this to a tuple of domains including ports (e.g. ('http://example.com:80`, 'http://www.example.com:80',)) if your site is served on multiple domains. This is useful because the virtual hosting URL contains the "external" domain name. If your site is hosted such that it can be reached via multiple domains (e.g. http://example.com vs. http://www.example.com), the virtual hosting path will be different depending on which one the user happened to use. Most likely, you will want to purge both variants.

Note that it is probably better to normalise your paths in the fronting web server, so that Zope only ever sees a single external domain. If you only have one domain, or if the virtualHosting option is false, you do not need to set this option.