Table of Contents

• Introduction
• Installation
• The caching control panel
• Caching profiles
  • Default caching profiles
  • Custom caching profiles
• Rulesets and caching operations
  • Default rulesets
  • Default cache operations
  • Default ruleset/operation mappings
  • Cache operation parameters
  • Caching operation helper functions
  • Debug headers and logging
  • Content-type based rulesets
• Caching proxies
  • Purging a caching proxy
  • Installing and configuring a caching proxy
  • Running Plone behind Apache 2.2 with mod_cache
• The RAM cache
  • Alternative RAM cache implementations
• ETags
• Composite views
• Split views

Introduction

plone.app.caching provides a Plone UI and default rules for managing HTTP response caching in Plone. It builds on z3c.caching, plone.caching and plone.cachepurging.

plone.app.caching requires Plone 4 or later.

Installation

To install plone.app.caching, add it to the eggs list in your buildout.cfg, or as a dependency of one of your own packages in setup.py. ZCML configuration will be automatically loaded via a z3c.autoinclude entry point. You will also need to install the package in Plone's Add-ons control panel as normal.

This package depends on a number of other packages, including z3c.form and plone.app.registry, that do not ship with Plone. You will probably want to lock down the versions for those packages using a known good set. Add this to the the extends line in your buildout.cfg, after the line that includes the Plone KGS:

extends =
    ...
    http://good-py.appspot.com/release/plone.app.caching/1.0a1

Update the version number at the end of the URL as appropriate. You can find an overview of the versions here

The caching control panel

After installation, you will find a Caching control panel in Plone's site setup. This consists of four main tabs:

• Change settings, where you can control caching behaviour
• Import settings, where you can import pre-defined profiles of cache settings
• Purge caching proxy, where you can manually purge content from a caching proxy. This tab only appears if you have purging enabled under Change settings.
• RAM cache, where you can view statistics about and purge the RAM cache.

Under the settings tab, you will find four fieldsets:

• General settings, for global options such as turning caching on or off.
• Caching proxies, where you can control Plone's use of a caching proxy such as Squid or Varnish.
• Caching operation mappings, where caching rulesets (hints about views and resources used for caching purposes) can be associated with caching operations (which either intercept a request to return a cached response, or modifies a response to add cache control headers). This is also where rulesets for legacy page templates (created through the web or the portal_skins tool) are configured.
• Detailed settings, where you can configure parameters for individual caching operations.

Caching profiles

All persistent configuration for the caching machinery is stored in the configuration registry, as managed by plone.app.registry. This can be modified using the registry.xml GenericSetup import step. The Import settings tab of the control panel allows you to import these caching profiles.

Default caching profiles

plone.app.caching includes three default caching profiles. Two of these profiles encapsulate the cache settings that are known to work well with a typical default Plone installation. The third is an example profile for a "split-view" caching setup (see the split-view discussion later in this document).

The three default caching profiles:

• Without caching proxy

  Settings useful for setups without a caching proxy.

• With caching proxy

  Settings useful for setups with a caching proxy such as Squid or Varnish. The only difference from the "without caching proxy" profile are some settings to enable proxy caching of files/images in content space and content feeds.

• With caching proxy (and split-view caching)

  An example profile for a caching proxy setup with split-view caching enabled. This example requires a special proxy setup. See the proxy examples in the "proxy-configs" directory.

Custom caching profiles

Caching policies are often a compromise between speed and freshness. More aggressive caching often comes at the cost of increased risk of stale responses. The default profiles provided tend to err on the side of freshness over speed so there is some room for tweaking if greater speed is desired.

Customization may also be needed if third-party products are installed which require special treatment. Examine the HTTP response headers to determine whether the third-party product requires special treatment. Most simple cases probably can be solved by adding the content type or template to the appropriate mapping. More complicated cases, may require custom caching operations.

A GenericSetup profile used for caching should be registered for the ICacheProfiles marker interface to distinguish it from more general profiles used to install a product. This also hides the profile from Plone's Add-ons control panel.

Here is an example from this package:

<genericsetup:registerProfile
    name="with-caching-proxy"
    title="With caching proxy"
    description="Settings useful for setups with a caching proxy such as Squid or Varnish"
    directory="profiles/with-caching-proxy"
    provides="Products.GenericSetup.interfaces.EXTENSION"
    for="plone.app.caching.interfaces.ICacheProfiles"
    />

The directory profiles/with-caching-proxy contains a single import step, registry.xml, containing settings to configure the ruleset to operation mapping, and setting options for various operations. At the time of writing, this includes:

<record name="plone.caching.interfaces.ICacheSettings.operationMapping">
    <value purge="False">
        <element key="plone.resource">plone.app.caching.strongCaching</element>
        <element key="plone.stableResource">plone.app.caching.strongCaching</element>
        <element key="plone.content.itemView">plone.app.caching.weakCaching</element>
        <element key="plone.content.feed">plone.app.caching.moderateCaching</element>
        <element key="plone.content.folderView">plone.app.caching.weakCaching</element>
        <element key="plone.content.file">plone.app.caching.moderateCaching</element>
    </value>
</record>

Default options for the various standard operations are found in the registry.xml file that is part of the standard installation profile for this product, in the directory profiles/default. The custom profile overrides a number of operation settings for specific rulesets (see below). For example:

<record name="plone.app.caching.weakCaching.plone.content.itemView.ramCache">
    <field ref="plone.app.caching.weakCaching.ramCache" />
    <value>True</value>
</record>

This enables RAM caching for the "weak caching" operation for resources using the ruleset plone.content.itemView. The default is defined in the main registry.xml as:

<record name="plone.app.caching.weakCaching.ramCache">
    <field type="plone.registry.field.Bool">
        <title>RAM cache</title>
        <description>Turn on caching in Zope memory</description>
        <required>False</required>
    </field>
    <value>False</value>
</record>

Notice how we use a field reference to avoid having to re-define the field.

It may be useful looking at these bundled registry.xml for inspiration if you are building your own caching profile. Alternatively, you can export the registry from the portal_setup tool and pull out the records under the prefixes plone.caching and plone.app.caching.

Typically, registry.xml is all that is required, but you are free to add additional import steps if required. You can also add a metadata.xml and use the GenericSetup dependency mechanism to install other profiles on the fly.

Rulesets and caching operations

The caching infrastructure works on the principle of rulesets mapped to caching operations. A ruleset is basically just a name, and is normally applied in ZCML by the author of a particular view. There are also some default rulesets applied to general resources - see below.

Please note that plone.app.caching places the caching ruleset registry into "explicit" mode. This means that you must declare a caching rulset (with the <cache:rulesetType /> directive) before you can use it.

Caching operations are components written in Python which either interrupt rendering to provide a cached response (such as a 304 NOT MODIFIED response), or add caching information to a response (such as setting the Cache-Control HTTP response header).

For more details on how to use these components, see the documentation for plone.caching.

Once rulesets and caching operations have been registered, they will appear in the caching control panel.

Default rulesets

plone.app.caching declares a few default rulesets. They are listed with descriptions in the control panel.

• Content feed (plone.content.feed)

  A dynamic feed, e.g. using RSS or ATOM.

• Content files and images (plone.content.file)

  Includes files and images in content space usually either downloaded or included as an inline element in one of the other public-facing views.

• Content folder view (plone.content.folderView)

  A public-facing view for a content item that is a folder or container for other items.

• Content item view (plone.content.itemView)

  A public-facing view for a content item that is not a folder or container for other items.

• File and image resources (plone.resource)

  Includes images and files created or customised through the ZMI, those exposed in the portal_skins tool, and images registered in resource directories on the filesystem.

• Stable file and image resources (plone.stableResource)

  Stable resources like the css, javascript, and kss files registered with the Resource Registries. These are resources which can be cached 'forever'. Normally this means that if the object does change, its URL changes too.

Default cache operations

plone.app.caching also declares a number of default operation types. These are listed in the control panel as available operations for the various rulesets. Hover your mouse over an operation in the drop-down list to view its description.

• Strong caching (plone.app.caching.strongCaching)

  Cache in browser and proxy (default: 24 hours). Caution: Only use for stable resources that never change without changing their URL, or resources for which temporary staleness is not critical.

  In the caching profiles without-caching-proxy and with-caching-proxy, this operation is mapped to the rulesets plone.resource and plone.stableResource, which causes the following headers to be added to the response:

  Last-Modified: <last-modified-date>
  Cache-Control: max-age=<seconds>, proxy-revalidate, public
  

• Moderate caching (plone.app.caching.moderateCaching),

  Cache in browser but expire immediately (same as weak caching), and cache in proxy (default: 24 hours). Use a purgable caching reverse proxy for best results. Caution: If proxy cannot be purged reliably (for example, in the case of composite pages where it may be difficult to track when a dependency has changed) then stale responses might be seen until the cached entry expires. A similar caution applies even if in the purgeable case, if the proxy cannot be configured to disallow caching in other intermediate proxies that may exist between the local proxies and the browser (see the example proxy configs included with this package for some solutions to this problem).

  In the caching profile with-caching-proxy, this operation is mapped to the rulesets plone.content.feed and plone.content.file, which causes the following headers to be added to the response:

  [plone.content.feed]
  ETag: <etag-value>
  Cache-Control: max-age=0, s-maxage=<seconds>, must-revalidate
  
  [plone.content.file]
  Last-Modified: <last-modified-date>
  Cache-Control: max-age=0, s-maxage=<seconds>, must-revalidate
  

• Weak caching (plone.app.caching.weakCaching)

  Cache in browser but expire immediately and enable 304 responses on subsequent requests. 304's require configuration of the Last-Modified and/or ETags settings. If Last-Modified header is insufficient to ensure freshness, turn on ETag checking by listing each ETag component that should be used to to construct the ETag header. To also cache public responses in Zope memory, set the RAM cache parameter to True.

  In the caching profile without-caching-proxy, this operation is mapped to the rulesets plone.content.itemView, plone.content.folderView, plone.content.feed, and plone.content.file, which causes the following headers to be added to the response:

  [plone.content.itemView, plone.content.folderView, plone.content.feed]
  ETag: <etag-value>
  Cache-Control: max-age=0, must-revalidate, private
  
  [plone.content.file]
  Last-Modified: <last-modified-date>
  Cache-Control: max-age=0, must-revalidate, private
  

  In the caching profile with-caching-proxy, this operation is mapped only to the rulesets plone.content.itemView and plone.content.folderView.

• No caching (plone.app.caching.noCaching)

  Use this operation to keep the response out of all caches. The default settings generate an IE-safe no-cache operation. Under certain conditions, IE chokes on no-cache and no-store Cache-Control tokens, so instead we just exclude caching in shared caching proxies with the private token, expire immediately in the browser, and disable validation. This emulates the usual behavior expected from the no-cache token. If the nominally more secure, but occasionally troublesome, no-store token is also desired, set the No store parameter to True. [XXX - 'no store' option not done yet]

• Chain (plone.caching.operations.chain)

  Allows multiple operations to be chained together. When intercepting the response, the first chained operation to return a value will be used. Subsequent operations are ignored. When modifying the response, all operations will be called, in order.

These operation descriptions are a bit simplified as several of these operations also include tests to downgrade caching depending on various parameter settings, workflow state, and access privileges. For more detail, it's best to review the operation code itself.

Default ruleset/operation mappings

To recap, plone.app.caching defines three default cache policies containing the cache operation mappings for each of the six rulesets. The default mappings are as follows:

	without-caching-proxy	with-caching-proxy	with-caching-proxy-splitviews
itemView	weakCaching	weakCaching	moderateCaching
folderView	weakCaching	weakCaching	moderateCaching
feed	weakCaching	moderateCaching	moderateCaching
file	weakCaching	moderateCaching	moderateCaching
resource	strongCaching	strongCaching	strongCaching
stableResource	strongCaching	strongCaching	strongCaching

Cache operation parameters

Much of the cache operation behavior is controlled via user-adjustable parameters. In fact, three of the default caching operations (strong caching, moderate caching, and weak caching) are essentially all the same operation but with different default parameter settings and with some parameters hidden from the UI.

• Maximum age (maxage)

  Time (in seconds) to cache the response in the browser or caching proxy. Adds a "Cache-Control: max-age=<value>" header and a matching "Expires" header to the response.

• Shared maximum age (smaxage)

  Time (in seconds) to cache the response in the caching proxy. Adds a "Cache-Control: s-maxage=<value>" header to the response.

• ETags (etags)

  A list of the names of the ETag components to include in the ETag header. Also turns on "304 Not Modified" responses for "If-None-Match" conditional requests.

• Last-modified validation (lastModified)

  Adds a "Last-Modified" header to the response and turns on "304 Not Modified" responses for "If-Modified-Since" conditional requests.

• RAM cache (ramCache)

  Turn on caching in Zope memory. If the URL is not specific enough to ensure uniqueness then either ETags or Last-Modified should also be added to the list of parameters in order to generate a unique cache key.

• Vary (vary)

  Name(s) of HTTP headers in the request that must match (in addition to the URL) for a caching proxy to return a cached response.

• Anonymous only (anonOnly)

  Set this to True if you want to force logged-in users to always get a fresh copy. This works best with the "moderate caching" operation, and will not work well with a "Max age" (to cache content in the browser) greater than zero. By setting this option, you can focus the other cache settings on the anonymous use case. Note that if you are using a caching proxy, you will need to set a Vary header of "X-Anonymous" or similar, and ensure that such a header is set in the proxy for logged in users (a blunter alternative is to use "Cookie" as the header, although this can have false positives). See the example Varnish and Squid configurations that come with this package for more details.

• Request variables that prevent caching (cacheStopRequestVariables)

  A list of variables in the request (including Cookies) that prevent caching if present. Note, unlike the others above, this global parameter is not directly visible in the plone.app.caching UI. There should unlikely be any need to change this list but, if needed, it can be edited via the Configuration Registry control panel.

Caching operation helper functions

If you will find the implementations of the default caching operations in the package plone.app.caching.operations. If you are writing a custom caching operation, the utils module contains helper functions which you may find useful.

Debug headers and logging

It can sometimes be useful to see which rulesets and operations (if any) are being applied to published resources. There are two ways to see this: via debug response headers and via debug logging.

Several debug response headers are added automatically by plone.app.caching and plone.cahing. These headers include:

• X-Cache-Rule: <matching rule id>
• X-Cache-Operation: <matching operation id>
• X-Cache-Chain-Operations: <list of chain operation ids>
• X-RAMCache: <ram cache id>

Viewing these headers is relatively easy with tools like the Firebug and LiveHTTPHeaders add-on for the Firefox browser. Similar tools for inspecting response headers exist for Safari and IE.

If you enable the DEBUG logging level for the plone.caching logger, you will get additional debug output in your event log. One way to do that is to set the global Zope logging level to DEBUG in zope.conf:

<eventlog>
    level DEBUG
    <logfile>
        path <file path here>
        level DEBUG
    </logfile>
</eventlog>

If you are using plone.recipe.zope2instance to create your Zope instances, you can set the logging level with the event-log-level option.

You should see output in the log like:

2010-01-11 16:44:10 DEBUG plone.caching Published: <ATImage at /test/i> Ruleset: plone.download Operation: None
2010-01-11 16:44:10 DEBUG plone.caching Published: <ATImage at /test/i> Ruleset: plone.download Operation: plone.caching.operations.chain

The None indicates that no ruleset or operation was mapped.

It is probably not a good idea to leave debug logging on for production use, as it can produce a lot of output, filling up log files and adding unnecessary load to your disks.

Content-type based rulesets

Normally, you declare caching rulesets for a view, e.g. with:

<cache:ruleset
    ruleset="plone.content.itemView"
    for=".browser.MyItemView"
    />

See plone.caching for details.

plone.app.caching installs a special ruleset lookup adapter that is invoked for skin layer page templates and browser views not assigned a more specific rule set. This adapter allows you to declare a ruleset for the default view of a given content type by supplying a content type class or interface to the <cache:rulset /> directive:

<cache:ruleset
    ruleset="plone.content.itemView"
    for=".interfaces.IMyContentType"
    />

or for a class:

<cache:ruleset

ruleset="plone.content.itemView" for=".content.MyContentType" />

There are two reasons to want to do this:

• Your type uses a skin layer page template for its default view, instead of a browser view. In this case, you can either declare the ruleset on the type as shown above (in ZCML), or map the type name in the registry, using the GUI or GenericSetup. The former is more robust and certainly more natural if you are declaring other, more conventional rulesets in ZCML already.
• You want to set the ruleset for a number of content types. In fact, plone.app.caching already does this for you: The Archetypes base classes BaseContent and BaseFolder are assigned the rulesets plone.content.itemView and plone.content.folderview, respectively. Ditto for Dexterity's IDexterityItem and IDexterityContainer interfaces.

Caching proxies

It is common to place a so-called caching reverse proxy in front of Zope when hosting large Plone sites. On Unix, a popular option is Varnish, although Squid is also a good choice. On Windows, you can use Squid or the (commercial, but better) Enfold Proxy.

It is important to realise that whilst plone.app.caching provides some functionality for controlling how Plone interacts with a caching proxy, the proxy itself must be configured separately.

Some operations in plone.app.caching can set response headers that instruct the caching proxy how best to cache content. For example, it is normally a good idea to cache static resources (such as images and stylesheets) and "downloadables" (such as Plone content of the types File or Image) in the proxy. This content will then be served to most users straight from the proxy, which is much faster than Zope.

The downside of this approach is that an old version of a content item may returned to a user, because the cache has not been updated since the item was modified. There are three general strategies for dealing with this:

• Since resources are cached in the proxy based on their URL, you can "invalidate" the cached copy by changing an item's URL when it is updated. This is the approach taken by Plone's ResourceRegistries (portal_css, portal_javascript & co): in production mode, the links that are inserted into Plone's content pages for resource managed by ResourceRegistries contain a time-based token, which changes when the ResourceRegistries are updated. This approach has the benefit of also being able to "invalidate" content stored in a user's browser cache.
• All caching proxies support setting timeouts. This means that content may be stale, but typically only up to a few minutes. This is sometimes an acceptable policy for high-volume sites where most users do not log in.
• Most caching proxies support receiving PURGE requests for paths that should be purged. For example, if the proxy has cached a resource at /logo.jpg, and that object is modified, a PURGE request could be sent to the proxy (originating from Zope, not the client) with the same path to force the proxy to fetch a new version the next time the item is requested.

The final option, of course is to avoid caching content in the proxy altogether. The default policies will not allow standard content pages to be cached in the proxy, because it is too difficult to invalidate cached instances. For example, if you change a content item's title, that may require invalidation of a number of pages where that title appears in the navigation tree, folder listings, Collections, portlets, and so on. Tracking all these dependencies and purging in an efficient manner is impossible unless the caching proxy configuration is highly customised for the site.

Purging a caching proxy

Synchronous and asynchronous purging is enabled via plone.cachepurging. In the control panel, you can configure the use of a proxy via various options, such as:

• Whether or not to enable purging globally.
• The address of the caching server to which PURGE requests should be sent.
• Whether or not virtual host rewriting takes place before the caching proxy receives a URL or not. This has implications for how the PURGE path is constructed.
• Any domain aliases for your site, to enable correct purging of content served via e.g. http://example.com and http://www.example.com.

The default purging policy is geared mainly towards purging file and image resources, not content pages, although basic purging of content pages is included. The actual paths to purge are constructed from a number of components providing the IPurgePaths interface. See plone.cachepurging for details on how this works, especially if you need to write your own.

The default purge paths include:

• ${object_path}, -- the object's canonical path
• ${object_path}/ -- in case the object is a folder
• ${object_path}/view -- the view method alias
• ${object_path}/${default-view} -- in case a default view template is used
• The download URLs for any Archetypes object fields, in the case of Archetypes content. This includes support for the standard File and Image types.

Files and images created (or customised) in the ZMI are purged automatically when modified. Files managed through the ResourceRegistries do not need purging, since they have "stable" URLs. To purge Plone content when modified (or removed), you must select the content types in the control panel. By default, only the File and Image types are purged.

You should not enable purging for types that are not likely to be cached in the proxy. Although purging happens asynchronously at the end of the request, it may still place unnecessary load on your server.

Finally, you can use the Purge tab in the control panel to manually purge one or more URLs. This is a useful way to debug cache purging, as well as a quick solution for the awkward situation where your boss walks in and wonders why the "about us" page is still showing that old picture of him, before he had a new haircut.

Installing and configuring a caching proxy

The plone.app.caching package includes some example buildout configurations in the proxy-configs directory. Two versions are included: one demonstrating a Squid-behind-Apache proxy setup and another demonstrating a Varnish-behind-Apache proxy setup. Both examples also demonstrate how to properly configure split-view caching.

These configurations are provided for instructional purposes but with a little modification they can also be used in production. To use in a real production instance, you will need to adjust the configuration to match your setup. For a simple standard setup, you might only need to change the hostname value in the buildout.cfg. Read the README.txt files in each example for more instructions.

There are also some alternative buildout recipes for building and configuring proxy configs: plone.recipe.squid and plone.recipe.varnish. The examples in this package do not use these recipes in favor of using a more explicit, and hopefully more educational, template-based approach. Even if you decide to use one of the automated recipes, it will probably be worth your while to study the examples included in this package to get a few pointers.

Running Plone behind Apache 2.2 with mod_cache

Apache 2.2 has a known bug around its handling of the HTTP response header CacheControl with value max-age=0 or headers Expires with a date in the past. In these scenarios mod_cache will not cache the response no matter what value of s-maxage is set.

https://issues.apache.org/bugzilla/show_bug.cgi?id=35247

One possible workaround for this is to use mod_headers directives in your Apache configuration to set max-age=1 if s-maxage is positive and max-age is 0 and also to drop the Expires header

Header edit Cache-Control max-age=0(.*s-maxage=[1-9].*) max-age=1$1 Header unset Expires

Dropping the Expires header has the disadvantage that HTTP 1.0 clients and proxies may not cache your responses as you wish.

The RAM cache

In addition to caching content in users' browsers (through setting appropriate response headers) and a caching proxy, Plone can cache certain information in memory. This is done in two main ways:

• Developers may use the plone.memoize package's ram module to cache the results of certain functions in RAM. For example, some viewlets and portlets cache their rendered output in RAM for a time, alleviating the need to calculate them every time.
• Some caching operations may cache an entire response in memory, so that they can later intercept the request to return a cached response..

Caching in RAM in Zope is not as efficient as caching in a proxy, for a number of reasons:

• Zope still has to perform traversal, security, transaction management and so on before serving a request with a RAM-cached response.
• Zope's use of memory is not as efficient as that of a finely optimised caching proxy.
• Storing lots of content in RAM may compete with the standard ZODB object cache and other memory pools used by Zope, thus slowing down Zope overall.
• In multi-client ZEO setups, the RAM cache is (by default at least) not shared among instances (though it is shared among threads in that instance). Thus, each ZEO client process will maintain its own cache.

You can use the RAM cache tab in the caching control panel to view statistics about the use of the RAM cache. On the Change settings tab, you can also control the size of the cache, and the frequency with which it is purged of old items.

Alternative RAM cache implementations

The RAM cache exposed through plone.memoize.ram is looked up via an ICacheChoser utility. The default implementation looks up a zope.ramcache.interfaces.ram.IRAMCache utility. Plone installs a local such utility (to allows its settings to be persisted - the cache itself is not persistent), which is shared by all users of the cache.

You can provide your own ICacheChooser utility to change this policy, by installing this as a local utility or overriding it in overrides.zcml. One reason to do this may be to back the cache with a memcached server, which would allow a single cache to be shared among multiple Zope clients.

Below is a sketch of such a cache chooser, courtesy of Wojciech Lichota:

from threading import local
from pylibmc import Client
from zope.interface import implements
from plone.memoize.interfaces import ICacheChooser
from plone.memoize.ram import MemcacheAdapter

class MemcachedCacheChooser(object):
    implements(ICacheChooser)
    _v_thread_local = local()

    def getClient(self):
        """
        Return thread local connection to memcached.
        """
        connection = getattr(self._v_thread_local, 'connection', None)
        if connection is None:
            connection = Client(['127.0.0.1:11211'])
            self._v_thread_local.connection = connection

        return connection

    def __call__(self, fun_name):
        """
        Create new adapter for plone.memoize.ram.
        """
        return MemcacheAdapter(client=self.getClient(), globalkey=fun_name)

You could install this with the following lines in an overrides.zcml:

<utility factory=".memcached.MemcachedCacheChooser" />

ETags

ETags are used in to check whether pages need to be re-calculated or can be served from cache. An ETag is simply a string. Under plone.app.caching, it is a string of tokens separated by pipe characters. The tokens hold values such as a user id, the current skin name, or a counter indicating how many objects have been added to the site. The idea is that the browser sends a request with the ETag included in an If-None-Match header. Plone can then quickly calculate the current ETag for the requested resource. If the ETag is the same, Plone can reply with 304 NOT MODIFIED response, telling the browser to use its cached copy. Otherwise, Plone renders the page and returns it as normal.

Many caching operations use ETags. The tokens to include are typically listed in an etags tuple in the operation's options.

The ETag names tokens supported by default are:

• userid

  The current user's id

• roles

  A list of the current user's roles in the given context

• language

  The language(s) accepted by the browser, in the ACCEPT_LANGUAGE header

• userLanguage

  The current user's preferred language

• gzip

  Whether or not the content is going to be served compressed

• lastModified

  A timestamp indicating the last-modified date of the given context

• catalogCounter

  A counter that is incremented each time the catalog is updated, i.e. each time content in the site is changed.

• locked

  Whether or not the given context is locked for editing.

• skin

  The name of the current skin (theme)

• resourceRegistries

  A timestamp indicating the most recent last-modified date for all three Resource Registries. This is useful for avoiding requests for expired resources from cached pages.

It is possible to provide additional tokens by registering an IETagValue adapter. This should be a named adapter on the published object (typically a view, file resource or Zope page template object) and request, with a unique name. The name is used to look up the component. Thus, you can also override one of the tokens above for a particular type of context or request (e.g. via a browser layer), by registering a more specific adapter with the same name.

As an example, here is the language adapter:

from zope.interface import implements
from zope.interface import Interface

from zope.component import adapts
from plone.app.caching.interfaces import IETagValue

class Language(object):
    """The ``language`` etag component, returning the value of the
    HTTP_ACCEPT_LANGUAGE request key.
    """

    implements(IETagValue)
    adapts(Interface, Interface)

    def __init__(self, published, request):
        self.published = published
        self.request = request

    def __call__(self):
        return self.request.get('HTTP_ACCEPT_LANGUAGE', '')

This is registered in ZCML like so:

<adapter factory=".etags.Language" name="language" />

Composite views

A composite view is just a general term for most page views you see when you visit a Plone site. It includes all content item views, content folder views, and many template views. For our purposes, the distinguishing characteristic of composite views is the difficulty inherent in keeping track of all changes that might affect the final composited view. Because of the difficulty of dependancy tracking, composite views are often notoriously difficult to purge reliably from caching proxies so the default caching profiles set headers which expire the cache immediately (i.e. weak caching).

However, most of the inline resources linked to from the composite view (css, javascript, images, etc.) can be cached very well in proxy so the overall speed of most composite views will always be better with a caching proxy in front despite the page itself not being cached.

Also, when using Squid as a caching proxy, we can still see some additional speed improvement as Squid supports conditional requests to the backend and 304 responses from plone.app.caching are relatively quick. This means that even though the proxy cache will expire immediately, Squid can revalidate its cache relatively quickly. Varnish does not currently support conditional requests to the backend.

For relatively stable composite views or for those views for which you can tolerate some potential staleness, you might be tempted to try switching from weak caching to moderate caching with the s-maxage expiration value set to some tolerable value but first make sure you understand the issues regarding "split view" caching (see below).

Split views

A non-zero expiration for proxy or browser caching of a composite view will often require some special handling to deal with "split view" caching.

Caching proxies and browsers keep track of cached entries by using the URL as a key. If a Vary header is included in the response then those request headers listed in Vary are also included in the cache key. In most cases, this is sufficient to uniquely identify all responses. However, there are exceptions. We call these exceptions "split views". Anytime you have multiple views sharing the same cache key, you have a split view problem. Split views cannot be cached in proxies or browsers without mixing up the responses.

In the Plone case, most composite views are also split views because they provide different views to anonymous and authenticated requests. In Plone, authenticated requests are tracked via cookies which are not usually used in cache keys.

One solution to this problem is to add a Vary:Cookie response header but, unfortunately, since cookies are used for all sorts of state maintenance and web tracking, this will usually result in very inefficient caching.

Another solution is to enforce a different domain name, different path, or different protocol (https/http) for authenticated versus anonymous responses.

Yet another solution involves intercepting the request and dynamically adding a special X-Anonymous header to the anonymous request and then adding Vary:X-Anonymous to the split view response so that this header will added to the cache key. Examples of this last solution for both Squid and Varnish are included in the proxy-configs directory of this package, which are intended to be used in concert with something like the split-view caching profile of plone.app.caching.