repo_zf1/documentation/manual/de/module_specs/Zend_Feed_Pubsubhubbub.xml

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<sect1 id="zend.feed.pubsubhubbub.introduction">
    <title>Zend_Feed_Pubsubhubbub</title>

    <para>
        <classname>Zend_Feed_Pubsubhubbub</classname> ist eine Implementation der PubSubHubbub Core
        0.2 Spezifikation (Working Draft). Sie bietet eine Implementation eines Pubsubhubbub
        Publishers und Subscribers geeignet für den Zend Framework und andere PHP Anwendungen.
    </para>

    <sect2 id="zend.feed.pubsubhubbub.what.is.pubsubhubbub">
        <title>Was ist Pubsubhubbub?</title>

        <para>
            Pubsubhubbub is an open, simple web-scale pubsub protocol. A common use case to enable
            blogs (Publishers) to "push" updates from their RSS or Atom feeds (Topics) to end
            Subscribers. These Subscribers will have subscribed to the blog's RSS or Atom feed via a
            Hub, a central server which is notified of any updates by the Publisher and which then
            distributes these updates to all Subscribers. Any feed may advertise that it supports
            one or more Hubs using an Atom namespaced link element with a rel attribute of "hub".
        </para>

        <para>
            Pubsubhubbub has garnered attention because it is a pubsub protocol which is easy to
            implement and which operates over HTTP. Its philosophy is to replace the traditional
            model where blog feeds have been polled at regular intervals to detect and retrieve
            updates.  Depending on the frequency of polling, this can take a lot of time to
            propagate updates to interested parties from planet aggregators to desktop readers. With
            a pubsub system in place, updates are not simply polled by Subscribers, they are pushed to
            Subscribers, elimenating any delay. For this reason, Pubsubhubbub forms part of what has
            been dubbed the real-time web.
        </para>

        <para>
            The protocol does not exist in isolation. Pubsub systems have been around for a while,
            such as the familiar Jabber Publish-Subscribe protocol, XEP-0060, or the less well known
            rssCloud (described in 2001).  However these have not achieved widespread adoption typically
            due to either their complexity, poor timing or lack of suitability for web applications.
            rssCloud, which was recently revived as a response to the appearance of Pubsubhubbub, has
            also seen its usage increase significantly though it lacks a formal specification and
            currently does not support Atom 1.0 feeds.
        </para>

        <para>
            Perhaps surprisingly given its relative early age, Pubsubhubbub is already in use including
            in Google Reader, Feedburner, and there are plugins available for Wordpress blogs.
        </para>
    </sect2>

    <sect2 id="zend.feed.pubsubhubbub.architecture">
        <title>Architecture</title>

        <para>
            <classname>Zend_Feed_Pubsubhubbub</classname> implements two sides of the Pubsubhubbub
            0.2 Specification: a Publisher and a Subscriber. It does not currently implement a Hub
            Server though this is in progress for a future Zend Framework release.
        </para>

        <para>
            A Publisher is responsible for notifying all supported Hubs (many can be supported to
            add redundancy to the system) of any updates to its feeds, whether they be Atom or RSS
            based. This is achieved by pinging the supported Hub Servers with the URL of the updated
            feed. In Pubsubhubbub terminology, any updatable resource capable of being subscribed
            to is referred to as a Topic. Once a ping is received, the Hub will request the updated
            feed, process it for updated items, and forward all updates to all Subscribers
            subscribed to that feed.
        </para>

        <para>
            A Subscriber is any party or application which subscribes to one or more Hubs to receive
            updates from a Topic hosted by a Publisher. The Subscriber never directly communicates
            with the Publisher since the Hub acts as an intermediary, accepting subscriptions and
            sending updates to subscribed Subscribers. The Subscriber therefore communicates only
            with the Hub, either to subscribe/unsubscribe to Topics, or when it receives updates
            from the Hub. This communication design ("Fat Pings") effectively removes the possibility of
            a "Thundering Herd" issue. This occurs in a pubsub system where the Hub merely informs
            Subscribers that an update is available, prompting all Subscribers to immediately retrieve
            the feed from the Publisher giving rise to a traffic spike. In Pubsubhubbub, the Hub
            distributes the actual update in a "Fat Ping" so the Publisher is not subjected to any
            traffic spike.
        </para>

        <para>
            <classname>Zend_Feed_Pubsubhubbub</classname> implements Pubsubhubbub Publishers and
            Subscribers with the
            classes <classname>Zend_Feed_Pubsubhubbub_Publisher</classname> and
            <classname>Zend_Feed_Pubsubhubbub_Subscriber</classname>. In addition, the Subscriber
            implementation may handle any feed updates forwarded from a Hub by using
            <classname>Zend_Feed_Pubsubhubbub_Subscriber_Callback</classname>.  These classes, their
            use cases, and APIs are covered in subsequent sections.
        </para>
    </sect2>

    <sect2 id="zend.feed.pubsubhubbub.zend.feed.pubsubhubbub.publisher">
        <title>Zend_Feed_Pubsubhubbub_Publisher</title>

        <para>
            In Pubsubhubbub, the Publisher is the party who publishes a live feed and frequently updates
            it with new content. This may be a blog, an aggregator, or even a web service with a public
            feed based API. In order for these updates to be pushed to Subscribers, the Publisher
            must notify all of its supported Hubs that an update has occured using a simple HTTP POST
            request containing the URI or the updated Topic (i.e the updated RSS or Atom feed). The Hub
            will confirm receipt of the notification, fetch the updated feed, and forward any updates to
            any Subscribers who have subscribed to that Hub for updates from the relevant feed.
        </para>

        <para>
            By design, this means the Publisher has very little to do except send these Hub pings
            whenever its feeds change. As a result, the Publisher implementation is extremely
            simple to use and requires very little work to setup and use when feeds are updated.
        </para>

        <para>
            <classname>Zend_Feed_Pubsubhubbub_Publisher</classname> implements a full Pubsubhubbub
            Publisher. Its setup for use is also simple, requiring mainly that it is configured with
            the URI endpoint for all Hubs to be notified of updates, and the URIs of all Topics to
            be included in the notifications.
        </para>

        <para>
            The following example shows a Publisher notifying a collection of Hubs about updates to
            a pair of local RSS and Atom feeds. The class retains a collection of errors which
            include the Hub URLs, so the notification can be re-attempted later and/or logged if any
            notifications happen to fail. Each resulting error array also includes a "response" key
            containing the related HTTP response object. In the event of any errors, it is strongly
            recommended to attempt the operation for failed Hub Endpoints at least once more at a
            future time. This may require the use of either a scheduled task for this purpose or
            a job queue though such extra steps are optional.
        </para>

        <programlisting language="php"><![CDATA[
$publisher = new Zend_Feed_Pubsubhubbub_Publisher;
$publisher->addHubUrls(array(
    'http://pubsubhubbub.appspot.com/',
    'http://hubbub.example.com',
));
$publisher->addUpdatedTopicUrls(array(
    'http://www.example.net/rss',
    'http://www.example.net/atom',
));
$publisher->notifyAll();

if (!$publisher->isSuccess()) {
    // check for errors
    $errors     = $publisher->getErrors();
    $failedHubs = array()
    foreach ($errors as $error) {
        $failedHubs[] = $error['hubUrl'];
    }
}

// reschedule notifications for the failed Hubs in $failedHubs
]]></programlisting>

        <para>
            If you prefer having more concrete control over the Publisher, the methods
            <methodname>addHubUrls()</methodname> and <methodname>addUpdatedTopicUrls()</methodname>
            pass each array value to the singular <methodname>addHubUrl()</methodname> and
            <methodname>addUpdatedTopicUrl()</methodname> public methods.  There are also matching
            <methodname>removeUpdatedTopicUrl()</methodname> and
            <methodname>removeHubUrl()</methodname> methods.
        </para>

        <para>
            You can also skip setting Hub URIs, and notify each in turn using the
            <methodname>notifyHub()</methodname> method which accepts the URI of a Hub endpoint as
            its only argument.
        </para>

        <para>
            There are no other tasks to cover. The Publisher implementation is very simple since
            most of the feed processing and distribution is handled by the selected Hubs. It is
            however important to detect errors and reschedule notifications as soon as possible
            (with a reasonable maximum number of retries) to ensure notifications reach all
            Subscribers. In many cases as a final alternative, Hubs may frequently poll your
            feeds to offer some additional tolerance for failures both in terms of their own
            temporary downtime or Publisher errors/downtime.
        </para>
    </sect2>

    <sect2 id="zend.feed.pubsubhubbub.zend.feed.pubsubhubbub.subscriber">
        <title>Zend_Feed_Pubsubhubbub_Subscriber</title>

        <para>
            In Pubsubhubbub, the Subscriber is the party who wishes to receive updates to any Topic (RSS
            or Atom feed). They achieve this by subscribing to one or more of the Hubs advertised by
            that Topic, usually as a set of one or more Atom 1.0 links with a rel attribute of "hub". The
            Hub from that point forward will send an Atom or RSS feed containing all updates to that
            Subscriber's Callback URL when it receives an update notification from the Publisher. In
            this way, the Subscriber need never actually visit the original feed (though it's still
            recommended at some level to ensure updates are retrieved if ever a Hub goes offline). All
            subscription requests must contain the URI of the Topic being subscribed and a Callback URL
            which the Hub will use to confirm the subscription and to forward updates.
        </para>

        <para>
            The Subsciber therefore has two roles. To create and manage subscriptions, including
            subscribing for new Topics with a Hub, unsubscribing (if necessary), and periodically
            renewing subscriptions since they may have a limited validity as set by the Hub. This is handled
            by <classname>Zend_Feed_Pubsubhubbub_Subscriber</classname>.
        </para>

        <para>
            The second role is to accept updates sent by a Hub to the Subscriber's Callback URL, i.e.
            the URI the Subscriber has assigned to handle updates. The Callback URL also handles events
            where the Hub contacts the Subscriber to confirm all subscriptions and unsubscriptions.
            This is handled by using an instance of
            <classname>Zend_Feed_Pubsubhubbub_Subscriber_Callback</classname> when the Callback URL is
            accessed.
        </para>

        <important>
            <para>
                <classname>Zend_Feed_Pubsubhubbub_Subscriber</classname> implements the Pubsubhubbub 0.2
                Specification. As this is a new specification version not all Hubs currently implement
                it. The new specification allows the Callback URL to include a query string which is
                used by this class, but not supported by all Hubs. In the interests of maximising
                compatibility it is therefore recommended that the query string component of the
                Subscriber Callback URI be presented as a path element, i.e. recognised as a
                parameter in the route associated with the Callback URI and used by the application's
                Router.
            </para>
        </important>

        <sect3 id="zend.feed.pubsubhubbub.zend.feed.pubsubhubbub.subscriber.subscribing.and.unsubscribing">
            <title>Subscribing and Unsubscribing</title>

            <para>
                <classname>Zend_Feed_Pubsubhubbub_Subscriber</classname> implements a full Pubsubhubbub
                Subscriber capable of subscribing to, or unsubscribing from, any Topic via any Hub
                advertised by that Topic.  It operates in conjunction with
                <classname>Zend_Feed_Pubsubhubbub_Subscriber_Callback</classname> which accepts requests
                from a Hub to confirm all subscription or unsubscription attempts (to prevent
                third-party misuse).
            </para>

            <para>
                Any subscription (or unsubscription) requires the relevant information before
                proceeding, i.e. the URI of the Topic (Atom or RSS feed) to be subscribed to for
                updates, and the URI of the endpoint for the Hub which will handle the subscription and
                forwarding of the updates. The lifetime of a subscription may be determined by the
                Hub but most Hubs should support automatic subscription refreshes by checking with
                the Subscriber. This is supported by <classname>Zend_Feed_Pubsubhubbub_Subscriber_Callback</classname>
                and requires no other work on your part. It is still strongly recommended that you use
                the Hub sourced subscription time to live (ttl) to schedule the creation of new subscriptions
                (the process is identical to that for any new subscription) to refresh it with the Hub.
                While it should not be necessary per se, it covers cases where a Hub may not support
                automatic subscription refreshing and rules out Hub errors for additional redundancy.
            </para>

            <para>
                With the relevant information to hand, a subscription can be attempted as
                demonstrated below:
            </para>

            <programlisting language="php"><![CDATA[
    $storage = new Zend_Feed_Pubsubhubbub_Model_Subscription;

    $subscriber = new Zend_Feed_Pubsubhubbub_Subscriber;
    $subscriber->setStorage($storage);
    $subscriber->addHubUrl('http://hubbub.example.com');
    $subscriber->setTopicUrl('http://www.example.net/rss.xml');
    $subscriber->setCallbackUrl('http://www.mydomain.com/hubbub/callback');
    $subscriber->subscribeAll();
    ]]></programlisting>

            <para>
                In order to store subscriptions and offer access to this data for general use,
                the component requires a database (a schema is provided later in this section).
                By default, it is assumed the table name is "subscription" and it utilises
                <classname>Zend_Db_Table_Abstract</classname> in the background meaning it
                will use the default adapter you have set for your application. You may also
                pass a specific custom <classname>Zend_Db_Table_Abstract</classname> instance
                into the associated model <classname>Zend_Feed_Pubsubhubbub_Model_Subscription</classname>.
                This custom adapter may be as simple in intent as changing the table name to use or as
                complex as you deem necessary.
            </para>

            <para>
                While this Model is offered as a default ready-to-roll solution, you may create your
                own Model using any other backend or database layer (e.g. Doctrine) so long as the
                resulting class implements the interface
                <classname>Zend_Feed_Pubsubhubbub_Model_SubscriptionInterface</classname>.
            </para>

            <para>
                Behind the scenes, the Subscriber above will send a request to the Hub endpoint containing the
                following parameters (based on the previous example):
            </para>

            <table id="zend.feed.pubsubhubbub.zend.feed.pubsubhubbub.subscriber.subscribing.and.unsubscribing.table">
                <title>Subscription request parameters</title>

                <tgroup cols="3">
                    <thead>
                        <row>
                            <entry>Parameter</entry>

                            <entry>Value</entry>

                            <entry>Explanation</entry>
                        </row>
                    </thead>

                    <tbody>
                        <row>
                            <entry>hub.callback</entry>

                            <entry>http://www.mydomain.com/hubbub/callback?xhub.subscription=5536df06b5dcb966edab3a4c4d56213c16a8184</entry>

                            <entry>
                                <para>
                                    The URI used by a Hub to contact the Subscriber and either request
                                    confirmation of a (un)subscription request or send updates from
                                    subscribed feeds. The appended query string contains a custom
                                    parameter (hence the xhub designation). It is a query string
                                    parameter preserved by the Hub and resent with all Subscriber
                                    requests. Its purpose is to allow the Subscriber to identify and
                                    look up the subscription associated with any Hub request in a
                                    backend storage medium.  This is a non-standard parameter used by
                                    this component in preference to encoding a subscription key in the
                                    URI path which is more difficult to implement in a Zend Framework
                                    application.
                                </para>

                                <para>
                                    Nevertheless, since not all Hubs support query string parameters,
                                    we still strongly recommend adding the subscription key as a path component
                                    in the form http://www.mydomain.com/hubbub/callback/5536df06b5dcb966edab3a4c4d56213c16a8184.
                                    To accomplish this, it requires defining a route capable of parsing out the final
                                    value of the key and then retrieving the value and passing it to the Subscriber
                                    Callback object. The value would be passed into the method
                                    <methodname>Zend_Pubsubhubbub_Subscriber_Callback::setSubscriptionKey()</methodname>.
                                    A detailed example is offered later.
                                </para>
                            </entry>
                        </row>

                        <row>
                            <entry>hub.lease_seconds</entry>

                            <entry>2592000</entry>

                            <entry>
                                <para>
                                    The number of seconds for which the Subscriber would like a new
                                    subscription to remain valid for (i.e. a TTL). Hubs may enforce their own maximum
                                    subscription period. All subscriptions should be renewed by simply
                                    re-subscribing before the subscription period ends to ensure
                                    continuity of updates. Hubs should additionally attempt to automatically
                                    refresh subscriptions before they expire by contacting Subscribers (handled
                                    automatically by the Callback class).
                                </para>
                            </entry>
                        </row>

                        <row>
                            <entry>hub.mode</entry>

                            <entry>subscribe</entry>

                            <entry>
                                <para>
                                    Simple value indicating this is a subscription request.
                                    Unsubscription requests would use the "unsubscribe" value.
                                </para>
                            </entry>
                        </row>

                        <row>
                            <entry>hub.topic</entry>

                            <entry>http://www.example.net/rss.xml</entry>

                            <entry>
                                <para>
                                    The URI of the topic (i.e. Atom or RSS feed) which the Subscriber
                                    wishes to subscribe to for updates.
                                </para>
                            </entry>
                        </row>

                        <row>
                            <entry>hub.verify</entry>

                            <entry>sync</entry>

                            <entry>
                                <para>
                                    Indicates to the Hub the preferred mode of verifying subscriptions
                                    or unsubscriptions. It is repeated twice in order of preference. Technically
                                    this component does not distinguish between the two modes and treats both
                                    equally.
                                </para>
                            </entry>
                        </row>

                        <row>
                            <entry>hub.verify</entry>

                            <entry>async</entry>

                            <entry>
                                <para>
                                    Indicates to the Hub the preferred mode of verifying subscriptions
                                    or unsubscriptions. It is repeated twice in order of preference. Technically
                                    this component does not distinguish between the two modes and treats both
                                    equally.
                                </para>
                            </entry>
                        </row>

                        <row>
                            <entry>hub.verify_token</entry>

                            <entry>3065919804abcaa7212ae89.879827871253878386</entry>

                            <entry>
                                <para>
                                    A verification token returned to the Subscriber by the Hub when it
                                    is confirming a subscription or unsubscription. Offers a measure of
                                    reliance that the confirmation request originates from the correct
                                    Hub to prevent misuse.
                                </para>
                            </entry>
                        </row>
                    </tbody>
                </tgroup>
            </table>

            <para>
                You can modify several of these parameters to indicate a different preference.  For
                example, you can set a different lease seconds value using
                <methodname>Zend_Pubsubhubbub_Subscriber::setLeaseSeconds()</methodname> or show a
                preference for the async verify mode by using <code>
                    setPreferredVerificationMode(Zend_Feed_Pubsubhubbub::VERIFICATION_MODE_ASYNC)</code>.
                However the Hubs retain the capability to enforce their own preferences and for this
                reason the component is deliberately designed to work across almost any set of options
                with minimum end-user configuration required. Conventions are great when they work!
            </para>

            <note>
                <para>
                    While Hubs may require the use of a specific verification mode (both are supported
                    by <classname>Zend_Pubsubhubbub</classname>), you may indicate a specific preference
                    using the <methodname>setPreferredVerificationMode()</methodname> method. In "sync"
                    (synchronous) mode, the Hub attempts to confirm a subscription as soon as it is
                    received, and before responding to the subscription request. In "async"
                    (asynchronous) mode, the Hub will return a response to the subscription request
                    immediately, and its verification request may occur at a later time. Since
                    <classname>Zend_Pubsubhubbub</classname> implements the Subscriber verification role
                    as a separate callback class and requires the use of a backend storage medium, it
                    actually supports both transparently though in terms of end-user performance,
                    asynchronous verification is very much preferred to eliminate the impact of a
                    poorly performing Hub tying up end-user server resources and connections for
                    too long.
                </para>
            </note>

            <para>
                Unsubscribing from a Topic follows the exact same pattern as the previous example, with
                the exception that we should call <methodname>unsubscribeAll()</methodname> instead. The
                parameters included are identical to a subscription request with the exception that
                "hub.mode" is set to "unsubscribe".
            </para>

            <para>
                By default, a new instance of <classname>Zend_Pubsubhubbub_Subscriber</classname> will
                attempt to use a database backed storage medium which defaults to using the default
                <classname>Zend_Db</classname> adapter with a table name of "subscription".
                It is recommended to set a custom storage solution where these defaults are not apt either
                by passing in a new Model supporting the required interface or by passing a new instance
                of <classname>Zend_Db_Table_Abstract</classname> to the default Model's constructor to change
                the used table name.
            </para>
        </sect3>

        <sect3 id="zend.feed.pubsubhubbub.zend.feed.pubsubhubbub.subscriber.handling.hub.callbacks">
            <title>Handling Subscriber Callbacks</title>

            <para>
                Whenever a subscription or unsubscription request is made, the Hub must verify the
                request by forwarding a new verification request to the Callback URL set in the
                subscription/unsubscription parameters. To handle these Hub requests, which will include
                all future communications containing Topic (feed) updates, the Callback URL should trigger the
                execution of an instance of <classname>Zend_Pubsubhubbub_Subscriber_Callback</classname>
                to handle the request.
            </para>

            <para>
                The Callback class should be configured to use the same storage medium as the Subscriber
                class. Using it is quite simple since most of its work is performed internally.
            </para>

            <programlisting language="php"><![CDATA[
    $storage = new Zend_Feed_Pubsubhubbub_Model_Subscription;
    $callback = new Zend_Feed_Pubsubhubbub_Subscriber_Callback;
    $callback->setStorage($storage);
    $callback->handle();
    $callback->sendResponse();

    /**
     * Check if the callback resulting in the receipt of a feed update.
     * Otherwise it was either a (un)sub verification request or invalid request.
     * Typically we need do nothing other than add feed update handling - the rest
     * is handled internally by the class.
     */
    if ($callback->hasFeedUpdate()) {
        $feedString = $callback->getFeedUpdate();
        /**
         *  Process the feed update asynchronously to avoid a Hub timeout.
         */
    }
    ]]></programlisting>

            <note>
                <para>
                    It should be noted that
                    <classname>Zend_Feed_Pubsubhubbub_Subscriber_Callback</classname> may independently
                    parse any incoming query string and other parameters.  This is necessary since PHP
                    alters the structure and keys of a query string when it is parsed into the
                    <varname>$_GET</varname> or <varname>$_POST</varname> superglobals.  For example,
                    all duplicate keys are ignored and periods are converted to underscores.
                    Pubsubhubbub features both of these in the query strings it generates.
                </para>
            </note>

            <important>
                <para>
                    It is essential that developers recognise that Hubs are only concerned with sending
                    requests and receiving a response which verifies its receipt. If a feed update is
                    received, it should never be processed on the spot since this leaves the Hub waiting
                    for a response. Rather, any processing should be offloaded to another process or
                    deferred until after a response has been returned to the Hub. One symptom of a
                    failure to promptly complete Hub requests is that a Hub may continue to attempt
                    delivery of the update/verification request leading to duplicated update attempts
                    being processed by the Subscriber. This appears problematic - but in reality a
                    Hub may apply a timeout of just a few seconds, and if no response is received within
                    that time it may disconnect (assuming a delivery failure) and retry later. Note that
                    Hubs are expected to distribute vast volumes of updates so their resources are
                    stretched - please do process feeds asynchronously (e.g. in a separate process or
                    a job queue or even a cron scheduled task) as much as possible.
                </para>
            </important>
        </sect3>

        <sect3 id="zend.feed.pubsubhubbub.zend.feed.pubsubhubbub.subscriber.setting.up.and.using.a.callback.url.route">
            <title>Setting Up And Using A Callback URL Route</title>

            <para>
                As noted earlier, the <classname>Zend_Feed_Pubsubhubbub_Subscriber_Callback</classname>
                class receives the combined key associated with any subscription from the Hub via one
                of two methods. The technically preferred method is to add this key to the Callback
                URL employed by the Hub in all future requests using a query string parameter with
                the key "xhub.subscription". However, for historical reasons, primarily that this was
                not supported in Pubsubhubbub 0.1 (it was recently added in 0.2 only), it is strongly
                recommended to use the most compatible means of adding this key to the Callback URL
                by appending it to the URL's path.
            </para>

            <para>Thus the URL http://www.example.com/callback?xhub.subscription=key would become
            http://www.example.com/callback/key.</para>

            <para>Since the query string method is the default in anticipation of a greater level
            of future support for the full 0.2 specification, this requires some additional work
            to implement.</para>

            <para>The first step to to make the <classname>Zend_Feed_Pubsubhubbub_Subscriber_Callback</classname>
            class aware of the path contained subscription key. It's manually injected therefore
            since it also requires manually defining a route for this purpose. This is achieved simply by
            called the method <methodname>Zend_Feed_Pubsubhubbub_Subscriber_Callback::setSubscriptionKey()</methodname>
            with the parameter being the key value available from the Router. The example below
            demonstrates this using a Zend Framework controller.</para>

            <programlisting language="php"><![CDATA[
class CallbackController extends Zend_Controller_Action
{

    public function indexAction()
    {
        $storage = new Zend_Feed_Pubsubhubbub_Model_Subscription;
        $callback = new Zend_Feed_Pubsubhubbub_Subscriber_Callback;
        $callback->setStorage($storage);
        /**
         * Inject subscription key parsing from URL path using
         * a parameter from Router.
         */
        $subscriptionKey = $this->_getParam('subkey');
        $callback->setSubscriptionKey($subscriptionKey);
        $callback->handle();
        $callback->sendResponse();

        /**
         * Check if the callback resulting in the receipt of a feed update.
         * Otherwise it was either a (un)sub verification request or invalid request.
         * Typically we need do nothing other than add feed update handling - the rest
         * is handled internally by the class.
         */
        if ($callback->hasFeedUpdate()) {
            $feedString = $callback->getFeedUpdate();
            /**
             *  Process the feed update asynchronously to avoid a Hub timeout.
             */
        }
    }

}
    ]]></programlisting>

        <para>Actually adding the route which would map the path-appended key
        to a parameter for retrieval from a controller can be accomplished using
        a Route configuration such as the INI formatted example below for use
        with <classname>Zend_Application</classname> bootstrapping.</para>

        <programlisting language="dosini"><![CDATA[
; Callback Route to enable appending a PuSH Subscription's lookup key
resources.router.routes.callback.route = "callback/:subkey"
resources.router.routes.callback.defaults.module = "default"
resources.router.routes.callback.defaults.controller = "callback"
resources.router.routes.callback.defaults.action = "index"
]]></programlisting>

        </sect3>
    </sect2>
</sect1>