100% with you that messages are a better way for microservices to receive their input. I don't see how a service could receive its input over HTTP and still retain its autonomy.
A message store like Message DB can at the same time serve as record of system state and communication channel. Writing a message to the store is the same as publishing it for consumers to pick up. Messages are written to streams, and interested parties subscribe to those streams by polling them for new messages.
The store itself isn't aware of which components are subscribing or managing their read positions.
So to your question, the first action of a component receiving a message is do whatever it does to handle the message and then write new messages (specifically events) to record and signal what happened in response to the original message.
A message store like Message DB can at the same time serve as record of system state and communication channel. Writing a message to the store is the same as publishing it for consumers to pick up. Messages are written to streams, and interested parties subscribe to those streams by polling them for new messages.
The store itself isn't aware of which components are subscribing or managing their read positions.
So to your question, the first action of a component receiving a message is do whatever it does to handle the message and then write new messages (specifically events) to record and signal what happened in response to the original message.