Metadata-Version: 1.0
Name: plone.app.caching
Version: 1.0b1
Summary: Plone UI and default rules for plone.caching/z3c.caching
Home-page: http://pypi.python.org/pypi/plone.app.caching
Author: Plone Foundation
Author-email: plone-developers@lists.sourceforge.net
License: GPL
Description: .. contents:: Table of Contents
        
        
        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 <http://good-py.appspot.com/release/plone.app.caching>`_
        
        
        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. 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``.
        
        It may be useful looking at these files 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.
        
        
        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 plone.memoize.ram import MemcacheAdapter
        from plone.memoize.interfaces import ICacheChooser
        
        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)
        
        * resourceRegistry
        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``.
        
        
        .. _plone.caching: http://pypi.python.org/pypi/plone.caching
        .. _plone.cachepurging: http://pypi.python.org/pypi/plone.cachepurging
        .. _plone.recipe.zope2instance: http://pypi.python.org/pypi/plone.recipe.zope2instance
        .. _Varnish: http://varnish-cache.org
        .. _Squid: http://squid-cache.org
        .. _Enfold Proxy: http://enfoldsystems.com/software/proxy/
        .. _memcached: http://memcached.org
        .. _plone.recipe.squid: http://pypi.python.org/pypi/plone.recipe.squid
        .. _plone.recipe.varnish: http://pypi.python.org/pypi/plone.recipe.varnish
        
        Changelog
        ---------
        
        1.0b1 - August 4, 2010
        ~~~~~~~~~~~~~~~~~~~~~~
        
        * Add an option for "anonymous only" caching to the default operations.
        This is a simple way to switch off caching for logged-in users. See
        the README for more details.
        [optilude]
        
        * Add basic plone.namedfile caching rules, if plone.namedfile is installed
        [optilude]
        
        * Implement lookup based on portal type class/interface as well as name,
        and set up defaults for items and folders.
        [optilude]
        
        * template fixes for cmf.pt compatibility
        [pilz]
        
        
        1.0a1 - April 24, 2010
        ~~~~~~~~~~~~~~~~~~~~~~
        
        * Initial release.
        [optilude, newbery, smcmahon]
        
Keywords: plone caching
Platform: UNKNOWN
Classifier: Framework :: Plone
Classifier: Programming Language :: Python
Classifier: Topic :: Software Development :: Libraries :: Python Modules
