I am facing an issue when decoupling two systems by an event/message broker like Apache Kafka. The issue is related to a frontend triggering actions in a backend:

How does the producer (frontend service) know, that the published event has been properly handled by all the backend services (as consumers), if the publisher does not know neither the "identities" nor the count of consuming backends?

To be precise: Users can change for example their email address using a frontend UI. An associated service publishes that "change request" event to an appropriate topic within Kafka. The UI form is then "locked" to prevent subsequent change requests, until the change event has been fully processed by every consumer. But it's unclear how to detect this state.