I haven't used the Messenger component yet so I'm probably asking something stupid but --> could we make a radical simplification for this "SimpleSerializer" and just usejson_encode() /json_decode() instead of the Symfony Serializer?

random thought, but what about putting aSerializerInterface in the contracts namespace, and forget about individual encoder/decoder contracts perhaps (or move those as well).

It's a bit more involving than that, but having the serializer as a dependency does not add any complexity, so I don't think we need to make it optional. SimpleSerializer is only needed when you are using the component standalone where you would basically have to do what I've done here. So, it only to get you started faster.

@ro0NL That's my thoughts as well :)