Metadata-Version: 1.0
Name: plone.cachepurging
Version: 1.0a1
Summary: Cache purging support for Zope 2 applications
Home-page: http://pypi.python.org/pypi/plone.cachepurging
Author: Plone Foundation
Author-email: plone-developers@lists.sourceforge.net
License: GPL
Description: 
        .. contents:: Table of Contents
        
        
        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 StringIO 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 >>out, "Purged", url, "Status", status, "X-Cache", xcache, "Error:", xerror
        
        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 implements
        from zope.component import adapts
        
        from z3c.caching.interfaces import IPurgePaths
        
        from Products.CMFCore.interfaces import IContentish
        
        class ObjectViewPurgePaths(object):
        """Purge /view for any content object with the content object's
        default URL
        """
        
        implements(IPurgePaths)
        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 thats 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 (e.g.
        ``('http://example.com`, 'http://www.example.com',)``) 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.
        
        .. _Products.CMFSquidTool: http://pypi.python.org/pypi/Products.CMFSquidTool
        .. _Squid: http://squid-cache.org
        .. _Varnish: http://varnish-cache.org
        .. _Enfold Proxy: http://enfoldsystems.com/software/proxy/
        .. _plone.app.caching: http://pypi.python.org/pypi/plone.app.caching
        .. _ZPublisherEventsBackport: http://pypi.python.org/pypi/ZPublisherEventsBackport
        .. _plone.registry: http://pypi.python.org/pypi/plone.registry
        .. _zope.lifecycleevent: http://pypi.python.org/pypi/zope.lifecycleevent
        
        Changelog
        ---------
        
        
        1.0a1 - April 22, 2010
        ~~~~~~~~~~~~~~~~~~~~~~
        
        - Initial release
        [optilude, newbery]
        
Keywords: plone cache purge
Platform: UNKNOWN
Classifier: Framework :: Plone
Classifier: Programming Language :: Python
