You say the handler is removed, but below is „allow_no_handlers“... it is a bit confusing

Actually the middleware is removed in favor of a flag in the HandleMessageMiddleware. I believe this information isn't really useful from the config POV so I omit it and just talk about setting the framework.messenger.[bus].default_middleware config option to allow_no_handlers to set this flag.
How can I improve this?

if it's too hard to explain.. i suggested #28945 (comment) 👼

@ro0NL : At least, I agreeallow_no_handlers at the bus node level would be self-explanatory.

it also brings more flexibility, i.e. keep logging but disable other defaults.

Being able to control it per case, IMHO would be more straightforward. Currently it's a bit all or nothing, and you need to know what default_middleware=true actually means.

That's why I was never really convinced by the default middleware config over explicit middleware stack configuration.

but then we cant provide sensible defaults (including default ordering).. so having a node per default + custom middleware stack seems like best of both.

So the only "weird" case is, if you want re-order defaults

bus:
  logging: false # log before
  middleware: [custom, logging] # log after

That still requires knowledge about what the defaults are and their order.

We could add a framework.messenger.default_middleware with before and after entries (or a [middleware] placeholder for specific bus middleware), with same format as middlewares & default values set in the Configuration allowing these to be dumped on debug:config and changed in userland so anyone can provide their own defaults, but not sure it is worth.

framework:
    messenger:
        default_middleware: ~
        # Which by default is the same as:
        # default_middleware:
        #   before:
        #     - logging
        #   after:
        #     - route_messages
        #     - call_message_handler
        buses:
            messenger.bus.command:
                middleware:
                    - doctrine_transaction
            messenger.bus.query:
                default_middleware: false
                middleware:
                    - cache
                    - call_message_handler
            messenger.bus.event:
                allow_no_handlers: true

But defaults may also be a PITA if one day we need to deprecate a middleware part of defaults in favor of another one. We cannot either add more or remove one in the future.

IMHO, explicit is always better.

maybe you're right 👍 the defaults are not super complex to declare anyway...