Everyone using Symfony needs to customize a built-in feature at some point, either to fit some specific business requirements or just to perform some extra tasks. Sometimes, that requires to change the way some core services are wired, which often leads to friction at time to upgrade.

Should you just copy/paste the feature code and change it to your needs then?
Fortunately, no.
Most Symfony features are designed in a way that makes it possible to extend them somehow, with little or no tight coupling.

In this talk, we will review the factors that should be taken into account in order to use and extend Symfony's code the right way, based on real-word examples and a deep dive into the backward-compatibility promise, plus some tips pulled out of my maintainer hat.