symfony
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+Create your own framework... on top of the Symfony2 Components (part 10)
+========================================================================
+
+In the conclusion of the second part of this series, I've talked about one
+great benefit of using the Symfony2 components: the *interoperability* between
+all frameworks and applications using them. Let's do a big step towards this
+goal by making our framework implement ``HttpKernelInterface``::
+
+    namespace Symfony\Component\HttpKernel;
+
+    interface HttpKernelInterface
+    {
+        /**
+         * @return Response A Response instance
+         */
+        function handle(Request $request, $type = self::MASTER_REQUEST, $catch = true);
+    }
+
+``HttpKernelInterface`` is probably the most important piece of code in the
+HttpKernel component, no kidding. Frameworks and applications that implement
+this interface are fully interoperable. Moreover, a lot of great features will
+come with it for free.
+
+Update your framework so that it implements this interface::
+
+    <?php
+
+    // example.com/src/Framework.php
+
+    // ...
+
+    use Symfony\Component\HttpKernel\HttpKernelInterface;
+
+    class Framework implements HttpKernelInterface
+    {
+        // ...
+
+        public function handle(Request $request, $type = HttpKernelInterface::MASTER_REQUEST, $catch = true)
+        {
+            // ...
+        }
+    }
+
+Even if this change looks trivial, it brings us a lot! Let's talk about one of
+the most impressive one: transparent `HTTP caching`_ support.
+
+The ``HttpCache`` class implements a fully-featured reverse proxy, written in
+PHP; it implements ``HttpKernelInterface`` and wraps another
+``HttpKernelInterface`` instance::
+
+    // example.com/web/front.php
+
+    use Symfony\Component\HttpKernel\HttpCache\HttpCache;
+    use Symfony\Component\HttpKernel\HttpCache\Store;
+
+    $framework = new Simplex\Framework($dispatcher, $matcher, $resolver);
+    $framework = new HttpCache($framework, new Store(__DIR__.'/../cache'));
+
+    $framework->handle($request)->send();
+
+That's all it takes to add HTTP caching support to our framework. Isn't it
+amazing?
+
+Configuring the cache needs to be done via HTTP cache headers. For instance,
+to cache a response for 10 seconds, use the ``Response::setTtl()`` method::
+
+    // example.com/src/Calendar/Controller/LeapYearController.php
+
+    public function indexAction(Request $request, $year)
+    {
+        $leapyear = new LeapYear();
+        if ($leapyear->isLeapYear($year)) {
+            $response = new Response('Yep, this is a leap year!');
+        } else {
+            $response = new Response('Nope, this is not a leap year.');
+        }
+
+        $response->setTtl(10);
+
+        return $response;
+    }
+
+.. tip::
+
+    If, like me, you are running your framework from the command line by
+    simulating requests (``Request::create('/is_leap_year/2012')``), you can
+    easily debug Response instances by dumping their string representation
+    (``echo $response;``) as it displays all headers as well as the response
+    content.
+
+To validate that it works correctly, add a random number to the response
+content and check that the number only changes every 10 seconds::
+
+    $response = new Response('Yep, this is a leap year! '.rand());
+
+.. note::
+
+    When deploying to your production environment, keep using the Symfony2
+    reverse proxy (great for shared hosting) or even better, switch to a more
+    efficient reverse proxy like `Varnish`_.
+
+Using HTTP cache headers to manage your application cache is very powerful and
+allows you to finely tuned your caching strategy as you can use both the
+expiration and the validation models of the HTTP specification. If you are not
+comfortable with these concepts, I highly recommend you to read the `HTTP
+caching`_ chapter of the Symfony2 documentation.
+
+The Response class contains many other methods that let's you configure the
+HTTP cache very easily. One of the most powerful is ``setCache()`` as it
+abstracts the most frequently used caching strategies into one simple array::
+
+    $date = date_create_from_format('Y-m-d H:i:s', '2005-10-15 10:00:00');
+
+    $response->setCache(array(
+        'public'        => true,
+        'etag'          => 'abcde',
+        'last_modified' => $date,
+        'max_age'       => 10,
+        's_maxage'      => 10,
+    ));
+
+    // it is equivalent to the following code
+    $response->setPublic();
+    $response->setEtag('abcde');
+    $response->setLastModified($date);
+    $response->setMaxAge(10);
+    $response->setSharedMaxAge(10);
+
+When using the validation model, the ``isNotModified()`` method allows you to
+easily cut on the response time by short-circuiting the response generation as
+early as possible::
+
+    $response->setETag('whatever_you_compute_as_an_etag');
+
+    if ($response->isNotModified($request)) {
+        return $response;
+    }
+    $response->setContent('The computed content of the response');
+
+    return $response;
+
+Using HTTP caching is great, but what if you cannot cache the whole page? What
+if you can cache everything but some sidebar that is more dynamic that the
+rest of the content? Edge Side Includes (`ESI`_) to the rescue! Instead of
+generating the whole content in one go, ESI allows you to mark a region of a
+page as being the content of a sub-request call::
+
+    This is the content of your page
+
+    Is 2012 a leap year? <esi:include src="/leapyear/2012" />
+
+    Some other content
+
+For ESI tags to be supported by HttpCache, you need to pass it an instance of
+the ``ESI`` class. The ``ESI`` class automatically parses ESI tags and makes
+sub-requests to convert them to their proper content::
+
+    use Symfony\Component\HttpKernel\HttpCache\ESI;
+
+    $framework = new HttpCache($framework, new Store(__DIR__.'/../cache'), new ESI());
+
+.. note::
+
+    For ESI to work, you need to use a reverse proxy that supports it like the
+    Symfony2 implementation. `Varnish`_ is the best alternative and it is
+    Open-Source.
+
+When using complex HTTP caching strategies and/or many ESI include tags, it
+can be hard to understand why and when a resource should be cached or not. To
+ease debugging, you can enable the debug mode::
+
+    $framework = new HttpCache($framework, new Store(__DIR__.'/../cache'), new ESI(), array('debug' => true));
+
+The debug mode adds a ``X-Symfony-Cache`` header to each response that
+describes what the cache layer did:
+
+.. code-block:: text
+
+    X-Symfony-Cache:  GET /is_leap_year/2012: stale, invalid, store
+
+    X-Symfony-Cache:  GET /is_leap_year/2012: fresh
+
+HttpCache has many some features like support for the
+``stale-while-revalidate`` and ``stale-if-error`` HTTP Cache-Control
+extensions as defined in RFC 5861.
+
+With the addition of a single interface, our framework can now benefit from
+the many features built into the HttpKernel component; HTTP caching being just
+one of them but an important one as it can make your applications fly!
+
+.. _`HTTP caching`: http://symfony.com/doc/current/book/http_cache.html
+.. _`ESI`:          http://en.wikipedia.org/wiki/Edge_Side_Includes
+.. _`Varnish`:      https://www.varnish-cache.org/