The HttpKernel Component: HttpKernelInterface
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The HttpKernel Component: HttpKernelInterface
In the conclusion of the second chapter of this book, I've talked about one
great benefit of using the Symfony 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
:
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namespace Symfony\Component\HttpKernel;
// ...
interface HttpKernelInterface
{
/**
* @return Response A Response instance
*/
public function handle(
Request $request,
$type = self::MAIN_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:
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// example.com/src/Framework.php
// ...
use Symfony\Component\HttpKernel\HttpKernelInterface;
class Framework implements HttpKernelInterface
{
// ...
public function handle(
Request $request,
$type = HttpKernelInterface::MAIN_REQUEST,
$catch = true
) {
// ...
}
}
With this change, a little goes a long way! Let's talk about one of the most impressive upsides: transparent HTTP caching support.
The HttpCache
class implements a fully-featured reverse proxy, written in
PHP; it implements HttpKernelInterface
and wraps another
HttpKernelInterface
instance:
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// example.com/web/front.php
// ...
use Symfony\Component\HttpKernel;
$framework = new Simplex\Framework($dispatcher, $matcher, $controllerResolver, $argumentResolver);
$framework = new HttpKernel\HttpCache\HttpCache(
$framework,
new HttpKernel\HttpCache\Store(__DIR__.'/../cache')
);
$response = $framework->handle($request);
$response->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:
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// example.com/src/Calendar/Controller/LeapYearController.php
// ...
public function index(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 you are running your framework from the command line by simulating
requests (Request::create('/is_leap_year/2012')
), you can 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:
1
$response = new Response('Yep, this is a leap year! '.rand());
Note
When deploying to your production environment, keep using the Symfony reverse proxy (great for shared hosting) or even better, switch to a more efficient reverse proxy like Varnish.
Using HTTP cache headers to manage your application cache is very powerful and allows you to tune finely your caching strategy as you can use both the expiration and the validation models of the HTTP specification. If you are not comfortable with these concepts, read the HTTP caching chapter of the Symfony documentation.
The Response class contains methods that let you configure the HTTP cache. One
of the most powerful is setCache()
as it abstracts the most frequently used
caching strategies into a single array:
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$response->setCache([
'must_revalidate' => false,
'no_cache' => false,
'no_store' => false,
'no_transform' => false,
'public' => true,
'private' => false,
'proxy_revalidate' => false,
'max_age' => 600,
's_maxage' => 600,
'immutable' => true,
'last_modified' => new \DateTime(),
'etag' => 'abcdef'
]);
// it is equivalent to the following code
$response->setPublic();
$response->setMaxAge(600);
$response->setSharedMaxAge(600);
$response->setImmutable();
$response->setLastModified(new \DateTime());
$response->setEtag('abcde');
When using the validation model, the isNotModified()
method allows you to
cut on the response time by short-circuiting the response generation as early as
possible:
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$response->setETag('whatever_you_compute_as_an_etag');
if ($response->isNotModified($request)) {
return $response;
}
$response->setContent('The computed content of the response');
return $response;
Using HTTP caching is great, but what if you cannot cache the whole page? What if you can cache everything but some sidebar that is more dynamic that the rest of the content? Edge Side Includes (ESI) to the rescue! Instead of generating the whole content in one go, ESI allows you to mark a region of a page as being the content of a sub-request call:
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This is the content of your page
Is 2012 a leap year? <esi:include src="/leapyear/2012"/>
Some other content
For ESI tags to be supported by HttpCache, you need to pass it an instance of
the ESI
class. The ESI
class automatically parses ESI tags and makes
sub-requests to convert them to their proper content:
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$framework = new HttpKernel\HttpCache\HttpCache(
$framework,
new HttpKernel\HttpCache\Store(__DIR__.'/../cache'),
new HttpKernel\HttpCache\Esi()
);
Note
For ESI to work, you need to use a reverse proxy that supports it like the Symfony implementation. Varnish is the best alternative and it is Open-Source.
When using complex HTTP caching strategies and/or many ESI include tags, it can be hard to understand why and when a resource should be cached or not. To ease debugging, you can enable the debug mode:
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$framework = new HttpKernel\HttpCache\HttpCache(
$framework,
new HttpKernel\HttpCache\Store(__DIR__.'/../cache'),
new HttpKernel\HttpCache\Esi(),
['debug' => true]
);
The debug mode adds a X-Symfony-Cache
header to each response that
describes what the cache layer did:
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X-Symfony-Cache: GET /is_leap_year/2012: stale, invalid, store
X-Symfony-Cache: GET /is_leap_year/2012: fresh
HttpCache has many 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!