messenger_prop1.yaml

    messenger:
        # these 3 confs are mutually exclusive! cannot use them together
        transports:
            normal_transport:
                dsn: '%env(MESSENGER_TRANSPORT_DSN)%/some_exchange_1'
                # if not provided at all: defaults to `messenger.serializer.default_serializer`
                # can be used with the short notation: works the same way as now
                serializer: transport_serializer_service_id
                # or as an array, everything is optional: if "service_id" not provided if it uses `messenger.serializer.default_serializer`
                # context + format override `messenger.serializer.symfony_serializer` conf
                serializer:
                    service_id: transport_serializer_service_id
                    context: {}
                    format: json
                    serializer: symfony_serializer_service_id

            incoming_message_transport:
                dsn: '%env(MESSENGER_TRANSPORT_DSN)%/some_exchange_2'
                # one of "messageClass" OR "messageResolver" is required (not both)
                serializer_incoming_message:
                    # not sure if or how we should propose to the user a "transport_serializer_id" option here
                    # IMO we should directly use a new service/class `messenger.transport.incoming_message_symfony_serializer`
                    messageClass: App\Some\Message
                    messageResolver: message_resolver_service_id # Symfony will provide a new "MessageResolverInterface", (no default implementation will be provided?)
                    context: {}
                    format: json
                    serializer: symfony_serializer_service_id

                # proposition: could have a short notation. will use `messenger.serializer.symfony_serializer` conf
                serializer_incoming_message: App\Some\Message # or "message_resolver_service_id"

            outgoing_message_transport:
                dsn: '%env(MESSENGER_TRANSPORT_DSN)%/some_exchange_2'
                # uses a new service/class `messenger.transport.outgoing_message_symfony_serializer`
                serializer_outgoing_message:
                    context: {}
                    format: json
                    serializer: symfony_serializer_service_id
                # no field required, so we could use short notation:
                serializer_outgoing_message: true

messenger_prop2.yaml

    messenger:
        transports:
            normal_transport:
                dsn: '%env(MESSENGER_TRANSPORT_DSN)%/some_exchange_1'
                serializer:
                    # depending on the type, possible array keys are different
                    type: normal # normal|incoming|outgoing
                    
                    # concern all types
                    context: {}
                    format: json
                    serializer: symfony_serializer_service_id
                    
                    # only for "type: normal"
                    service_id: transport_serializer_service_id
                    
                    # only for "type: incoming"
                    messageClass: App\Some\Message
                    messageResolver: message_resolver_service_id