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I want to show a...

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more subtle performance problem.

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To even see it, we need to go
back to the prod environment.

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Make sure to run cache:clear: cache:warmup:
And also: composer dump-autoload --

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optimize Let's create a fresh
profile of the homepage.

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I'll call this one: [Recording] Homepage prod.

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Click to view the timeline:

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http://bit.ly/sf-bf-instantiation
Overall, this request is pretty fast.

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Click into the "Memory" dimension.

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The biggest call is
Composer\Autoload\includeFile:

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that's literally Composer
including files that we need...

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not a lot of memory optimization
we can do about that.

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But, if we look closer, the memory
dimension reveals something else.

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See this "Container" thing - the
2nd item on the function list?

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This is related to Symfony's
container, which is responsible

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for instantiating all of our objects.

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This specific function is interesting:
it's highlighting a section of a file

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that lives in our cache directory.

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If you looked in that file, you would see
that this part of the code is responsible

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for including some of the
main files that our app needs.

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It's basically another version of the top node:

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it's code that includes files
for classes we're using.

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Ok, so the first few aren't
really that interesting.

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Things get much more intriguing
down on the 4th function call:

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some Container{BlahBlah}
/getDoctrine_Orm_DefaultEntityManagerService.php

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call.

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What is this?

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Well, the details of how this is organized
are specific to Symfony: but this is evidence

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of something that every app does: this
is showing the amount of resources used

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to instantiate Doctrine's EntityManager object.

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I know, we don't often think about how
much time or how much memory it takes

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to instantiate an object, but
it can sometimes be a problem.

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The next function call is for the
instantiation of Doctrine's Connection service.

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Go down a little bit...

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I'm looking for something specific...

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here it is: getLoginFormAuthenticatorService.

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This is responsible for instantiating a
LoginFormAuthenticator object in our app.

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It's not a particularly problematic
function though: it's 10th on the list...

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only takes 2.56 milliseconds
and uses about 500 kilobytes.

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Let's check out the class:
src/Security/LoginFormAuthenticator.php.

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As its name suggests, this is responsible

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for authenticating the user
when they submit the login form.

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But, there's something special about this class.

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Due to the way the Symfony
security system works,

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Symfony instantiates this
object on every request.

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It does that so it can then call supports()
to figure out if this service should be

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"activated" on this request or not.

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For this class, it only needs to its work when
the URL is /login and this is a POST request.

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In every other situation,
supports() returns false

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and no other methods are called on this class.

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So let's think about this.

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Instantiating this class takes about
3 milliseconds and 500 kilobytes...

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which is not a ton...

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but since all it needs to do for most
requests is check the current URL...

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then exit...

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that is kind of heavy.

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The question is: why does it take
so many resources to instantiate?

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Well, 500 kilobytes is not a ton, but
this is - according to Blackfire -

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one of the most expensive objects
that is created on this request.

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Why? Check out the constructor.

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In order to instantiate this class,

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Symfony needs to make sure the
EntityManager is instantiated...

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and the UrlGenerator..

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and the CsrfTokenManager...

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and the UserPasswordEncoder.

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If any of these services
have their own dependencies,

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even more objects may need to be instantiated.

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In rare situations, creating a service
can be a huge performance problem.

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In the case of the EntityManager
and the UrlGenerator...

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those are pretty core objects that would
probably be needed and thus instantiated

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by something on this request anyways.

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But CsrfTokenManager and
UserPasswordEncoder are not normally needed.

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In other words, we're forcing
Symfony to instantiate both

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of those services on every request...

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even though we only need them when
the user is submitting the login form.

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This is a classic situation where you have an
object that is instantiated on every request...

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but only needs to do real work in rare cases.

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Certain event subscribers - like
our AgreeToTermsSubscriber -

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Symfony security voters & Twig extensions
are other examples from Symfony.

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These services might be quick to instantiate...

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so no problem!

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But they also might be expensive.

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So... how could we make it quicker to
instantiate LoginFormAuthenticator?

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In Symfony, with a service subscriber.