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03.

Cache Service

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Parsing markdown on every request is going to make our app unnecessarily slow. So... let's cache that! Of course, caching something is "work"... and as I keep saying, all "work" in Symfony is done by a service.

Finding the Cache Service

So let's use our trusty debug:autowiring command to see if there are any services that include the word "cache". And yes, you can also just Google this and read the docs: we're learning how to do things the hard way to make you dangerous:

php bin/console debug:autowiring cache

And... cool! There is already a caching system in our app! Apparently, there are several services to choose from. But, as we talked about earlier, the blue text is the "id" of the service. So 3 of these type-hints are different ways to get the same service object, one of these is actually a logger, not a cache and the last one - TagAwareCacheInterface - is a different cache object: a more powerful one if you want to do something called "tag-based invalidation". If you don't know what I'm talking about, don't worry.

For us, we'll use the normal cache service... and the CacheInterface is my favorite type-hint because its methods are the easiest to work with.

Using the Cache Service

Head back to the controller and add another argument: CacheInterface - the one from Symfony\Contracts - and call it $cache:

52 lines | src/Controller/QuestionController.php
// ... lines 1 - 8
use Symfony\Contracts\Cache\CacheInterface;
// ... lines 10 - 11
class QuestionController extends AbstractController
{
// ... lines 14 - 31
public function show($slug, MarkdownParserInterface $markdownParser, CacheInterface $cache)
{
// ... lines 34 - 49
}
}

This object makes caching fun. Here's how it works: say $parsedQuestionText = $cache->get(). The first argument is a unique cache key. Let's pass markdown_ and then an md5() of $questionText. This will give every unique markdown text its own unique key.

Now, you might be thinking:

Hey Ryan! Don't you need to first check to see if this key is in the cache already? Something like if ($cache->has())?

Yes... but no. This object works a bit different: the get() function has a second argument, a callback function. Here's the idea: if this key is already in the cache, the get() method will return the value immediately. But if it's not - that's a cache "miss" - then it will call our function, we will return the parsed HTML, and it will store that in the cache.

Copy the markdown-transforming code, paste it inside the callback and return. Hmm, we have two undefined variables because we need to get them into the function's scope. Do that by adding use ($questionText, $markdownParser):

52 lines | src/Controller/QuestionController.php
// ... lines 1 - 11
class QuestionController extends AbstractController
{
// ... lines 14 - 31
public function show($slug, MarkdownParserInterface $markdownParser, CacheInterface $cache)
{
// ... lines 34 - 40
$parsedQuestionText = $cache->get('markdown_'.md5($questionText), function() use ($questionText, $markdownParser) {
return $markdownParser->transformMarkdown($questionText);
});
// ... lines 44 - 49
}
}

It's happy! I'm happy! Let's try it! Move over and refresh. Ok... it didn't break. Did it cache? Down on the web debug toolbar, for the first time, the cache icon - these 3 little boxes - shows a "1" next to it. It says: cache hits 0, cache writes 1. Right click that and open the profiler in a new tab.

Cool! Under cache.app - that's the "id" of the cache service - it shows one get() call to some markdown_ key. It was a cache "miss" because it didn't already exist in the cache. Close this then refresh again. This time on the web debug toolbar... yea! We have 1 cache hit! It's alive!

Where is the Cache Stored?

Oh, and if you're wondering where the cache is being stored, the answer is: on the filesystem - in a var/cache/dev/pools/ directory. We'll to talk more about that in a little while.

In the controller, make a tweak to our question - how about some asterisks around "thoughts":

52 lines | src/Controller/QuestionController.php
// ... lines 1 - 11
class QuestionController extends AbstractController
{
// ... lines 14 - 31
public function show($slug, MarkdownParserInterface $markdownParser, CacheInterface $cache)
{
// ... lines 34 - 38
$questionText = 'I\'ve been turned into a cat, any *thoughts* on how to turn back? While I\'m **adorable**, I don\'t really care for cat food.';
// ... lines 40 - 49
}
}

If we refresh now and check the toolbar... yea! The key changed, it was a cache "miss" and the new markdown was rendered.

So the cache system is working and it's storing things inside a var/cache/dev/pools/ directory. But... that leaves me with a question. Having these "tools" - these services - automatically available is awesome. We're getting a lot of work done quickly.

But because something else is instantiating these objects, we don't really have any control over them. Like, what if, instead of caching on the filesystem, I wanted to cache in Redis or APCu? How can we do that? More generally, how can we control the behavior of services that are given to us by bundles.

That is what we're going to discover next.