On this diagram the Order Entry Clerk is an active actor which starts the use cases 'Enter Order' and 'Cancel Order'. This is denoted by the solid arrows going from the actor to the use cases.
The accounts system is a passive actor for the use case 'Enter Order'. At some point in the use case the system will query the accounts system for customer account information and the accounts system will respond.
For the use case 'Get Address' the accounts system is an active actor, starting the use case and requesting an address from the system being defined.
|This diagram is not a data flow diagram. Interaction still occurs in both directions between the actors and system as part of each use case. The arrow indicates only which party has started the potentially bi-directional interaction.|
|In fact the arrowhead isn't even defined in the UML specification. It has simply become a de-facto standard through common use. The relationship is defined in UML as a static relationship between classifiers called an 'association' and shown on a use case diagram as a solid line with no arrowhead.|
|Solid lines or arrows between use cases are not allowed. In chapter 3 we will look at possible relationships between use cases but they will still not be data flows|
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