Imagine B is a web services performing a travel booking, and C is an airline reservation service and D is a hotel booking service.
The Message logic is:responses are correlated to requests,
b,c, d happen after a.
a might be "book itinerary foo", b might be "your itinerary is being booked", c might be "book ticket", d might be "book hotel".
e might be "here's the airline info", f might be "here's the hotel info".
g is a message to A once e and f have arrived. Like "here's your consolidated itinerary".
h is a message to A from D about D specific information. Like, "here's your flight details and amounts that we are charging to your credit card."
m returns an n (error) until d and f happens, then m returns o. m might be "get itinerary foo".
Interaction Diagram
State Chart for B
