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Working Out System-Interaction Dynamics with Scenarios Connie’s Convenience Store 85
Establish responsibility: “who I know.”
An authorization server knows its authorization system.
But what about an authorization system? Does it need to know who its server is?
Not likely.
An object needs to know another object only when:
– “I know that object so others can ask me and I’ll tell them about it.”
(The need to support basic queries is why most object connections are
constrained in both directions.)
– “I know that object so I can send it messages to do some work on my be
half.”
In this case, ask this question: is the system responsible to answer the question:
“For a specific authorization system, what’s its server?” Not really. There’s just one authorization
server in this model. So don’t include a constraint for authorization system
to server; it just doesn’t need to know.
Establish responsibility: “what I do.”
An authorization system knows how to:
– get authorization (payment type, amount ; authorization code).
Add authorization server and its responsibilities to your object model, specifically
to the system interaction component.
Add all of this to your model (Figure 1–59):
AuthorizationServer
getAuthorization
AuthorizationSystem
getAuthorization
Figure 1–59: Authorization server: “what I know; who I know; what I do.”
WORKING OUT SYSTEM-INTERACTION DYNAMICS WITH SCENARIOS
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Both check and credit objects need help when told to “authorize.” This is where the SI
objects come into play.