Message exchange patterns

Send Message, No Response:
- One-Way MEP
Send Message, Immediate Response:
- Request-Response MEP
- Response can be Application Ack, Application Response (Query)

Send Message, Deferred Response
- Two One-Way MEP
Send Message with both Immediate and Deferred Responses
- One Request-Response MEP and one One-Way MEP between receiver and sender

Rene Spronk, January 2008 "HL7 Implementation Mechanics"