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 ..!

Difference between a command bus and an event bus

• 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

Priority

• 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