Messenger

Symfony Event Dispatcher vs Domain events

Events are mutable in symfony
plus the symfony component relies on strings as event names (but we can use class constants)
and string as method names (but also closures)
NB : The wiring is done in a compiler pass which does not belong to the domain .. ?
See difference between messenger and event dispatcher
- Messenger allows your handlers to be called asynchronously, whereas listeners to events from the EventDispatcher are always synchronous.
- EventDispatcher communicates back
- if you simply want to say “this thing happened” and you don’t need any feedback from possible listeners or handlers, use Messenger.
- NB : So for domain events we could actually use messenger ..!

each command should have exactly one handler: we’re commanding that something perform a specific action
But an event is something that’s usually dispatched after that action is taken, and the purpose is to allow anyone else to take any secondary action - to react to the action.
See symfony’s multiple bus example implementing CQRS
The command bus would use a validation NB : not necessary, validation can be done before event generation) and transaction middleware (don’t need to call flush in the handler ! also not necessary if transactions are handled elsewhere)
The event bus would have default_middleware: allow_no_handlers
Plus we can disable autowiring and tag handlers to work only with a specific bus
The query bus pattern is fine but is not typed in symfony

Transport priority : eg async / async_priority_high : changes the order in which messages are handled
Handler priority : only useful when setting multiple handlers. The priority doesn’t change the order in which messages are handled, only the handlers for each message