The actors of a system
The actor in the UML sense: a role outside the system that interacts with it, primary or secondary, human or external system, generalisation between actors and the system boundary.
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Actor: what is it?
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A role played with respect to the system, by a person or by another system, that lies outside it and interacts with it.
An actor is a role and not a person: Marie can be both “Cashier” and “Stock-taker”, and several people can play “Member”.
On an online shop, the banking service validates the payment requested by the customer. What role does it play for the use case “Pay for the order”?
- Secondary actor
- Primary actor
- Part of the system
- Use case
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Secondary actor
The bank is outside the site and answers a request from the system: the goal (paying) belongs to the customer, who is the primary actor.
An actor is a …, not a person: the same person can play several actors, and an actor can be played by several people.
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An actor is a role, not a person: the same person can play several actors, and an actor can be played by several people.
So an actor is named after its function with respect to the system (“Member”), never after a first name or a position on the org chart.
Primary actor: what is it?
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The role that starts a use case to reach its own goal and gets the benefit from it.
The test is: “who wants this outcome?”. The customer wants their order; the bank does not have that goal.
A database stores the orders of the site being developed. Why is it not an actor?
- No human handles it directly within the organisation
- It is part of the system we are building
- It communicates with no other element of the system
- An actor must necessarily be a physical person
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It is part of the system we are building
The criterion is the boundary, not being human: an external system can be an actor, a database we build cannot. It does, moreover, communicate with the rest of the system.