z3c.caching 2.0a1

Caching infrastructure for web apps

Strict mode

By default, you are not required to declare the type of a ruleset before using it. This is convenient, but increases the risk of typos or a proliferation of rulesets that are semantically equivalent. If you want to guard against this case, you can put the ruleset into explicit mode, like this:

from z3c.caching.registry import setExplicitMode setExplicitMode(True)

Last modified date/time

The ILastModified adapter interface can be used to describe the last modified date of a given published object:

class ILastModified(Interface): """An abstraction to help obtain a last-modified date for a published resource.

Should be registered as an unnamed adapter from a published object (e.g. a view). """

def __call__(): """Return the last-modified date, as a Python datetime object.

The datetime returned must be timezone aware and should normally be in the local timezone.

May return None if the last modified date cannot be determined. """

One implementation for this interface is provided by default: When looked up for a Zope browser view, it will delegate to an ILastModified adapter on the view's context. Higher level packages may choose to implement this adapter for other types of publishable resources, and/or different types of view context.

Cache purging

High-traffic sites often put a caching proxy such as Squid or Varnish in front of the web application server to offload the caching of resources. Such proxies can be controlled via response headers (perhaps set via caching operations looked up based on z3c.caching rulesets). Most caching proxies also support so-called PURGE requests, where the web application sends a request directly to the caching proxy asking it to purge (presumably old) copies it may hold of a resource (e.g. because that resource has changed).

This package does not implement any communication with caching proxies. If you need that in a Zope 2 context, consider plone.cachepurging. However, a few components are included to help packages declare their behaviour in relation to a caching proxy that supports purging.

Firstly, z3.caching defines a Purge event, described the interface z3c.caching.interfaces.IPurgeEvent:

class IPurgeEvent(IObjectEvent): """Event which can be fired to purge a particular object.

This event is not fired anywhere in this package. Instead, higher level frameworks are expected to fire this event when an object may need to be purged.

It is safe to fire the event multiple times for the same object. A given object will only be purged once. """

If an object has been changed so that it may need to be purged, you can fire the event, like so:

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

notify(Purge(context))

A higher level framework such as plone.cachepurging can listen to this event to queue purge requests for the object.

Of course, the most common reason to purge an object's cached representations is that it has been modified or removed. z3c.caching provides event handlers for the standard IObjectModifiedEvent, IObjectMovedEvent and IObjectRemovedEvent events, which re-broadcasts a Purge event for the modified/moved/removed object.

To opt into these event handlers, simply mark your content object with the IPurgeable interface, e.g.:

from z3c.caching.interfaces import IPurgeable

class MyContent(Persistent): implements(IPurgeable)

...

You can also do this declaratively in ZCML, even for classes not under your control:

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

These helpers can signal to a framework like plone.cachepurging that the object needs to be purged, but this is not enough to know how to construct the PURGE request. The caching proxy also needs to be told which path or paths to purge. This is the job of the IPurgePaths adapter interface:

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. """

The difference between the "relative" and "absolute" paths only comes into effect if virtual hosting is used. In most cases, you want to implement getRelativePaths() to return a path that is relative to the current virtual hosting root. In Zope 2, you can get this via the absolute_url_path() function on any traversable item. Alternatively, you can look up an IAbsoluteURL adapter and discard the domain portion.

getAbsolutePaths() is mainly useful for paths that are "special" to the caching proxy. For example, you could configure Varnish to purge the entire cache when sending a request to /_purge_all, and then implement getAbsolutePaths() to return an iterable with that string in it.

Here is the default implementation from plone.cachepurging, which purges the default path of an object derived from Zope 2's OFS.Traversable:

class TraversablePurgePaths(object): """Default purge for OFS.Traversable-style objects """

implements(IPurgePaths) adapts(ITraversable)

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

def getRelativePaths(self): return [self.context.absolute_url_path()]

def getAbsolutePaths(self): return []

In ZCML, this is registered as:

<adapter factory=".paths.TraversablePurgePaths" name="default" />

The Plone-specific plone.app.caching implements further adapters (with other, unique names) for things like the default view method alias (/view) and downloadable paths for Archetypes image and file fields.

2.0a1 - April 22, 2010

• Added Purge` event and IPurgeable` and ``IPurgePaths interfaces.

Although this package doesn't provide any purging support, it is convenient to keep the interfaces necessary for other packages to describe their cache purging behaviour here. This allows a relatively harmless dependency on z3c.caching, even in generic code, as distinct from a higher-level, application server specific framework. [optilude]

• Added concept of an explicitly declare ruleset type. Optional by default,

but can be made required by setting explicit to True. [optilude]

• Added ILastModified implementation for a view which delegates to the

view's context. [optilude]

• Added enumerateTypes() method to the registry, used to list all currently

used cache rule ids. [optilude]

• Made the registry use the ZCA more directly.

[matthewwilkes]

1.0b1 - October 15, 2008

• Initial release

[wichert]