Use Cases - Finding Use Cases - Actors

16 important questions on Use Cases - Finding Use Cases - Actors

What are pass-though actors?

A pass-through Actor is usually a device that just passes along input and outputs from other Actors.

For example:
consider a computer keyboard and the monitor; they act as intermediaries between the end user and the computer system.

Unless you are designing the hardware, these devices are transparent, and they do not need to be Actors.

Instead, consider the end user as the Actor and consider the pass-through Actors as internal devices (or design elements) of the system under consideration.
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How do we display Actors?

as stick figures

 
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How can we arrange and organize actors?

We can arrange Actors into hierarchical generalization structures and organize them with packages.

Though uncommon, we can give Actors properties and give them associations to other model elements
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What is an alternative format for the Actor symbol ?

This format is a Class box with the «Actor» stereotype.

The box and the stick figure notation are otherwise equivalent diagram elements.

Most modelers use the stick figure predominately

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When is the box approach useful?

The box approach is useful if you wish to emphasize that the Actor is not a person—perhaps it is an external system or database.

It is also useful if we wish to show the Actor’s properties.

As a Class box, it can be divided into compartments, showing attributes, operations, signals, and constraints.
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How do we connect the Actor to the use cases on a use case diagram?

On a Use Case diagram, we connect the Actor to the Use Cases that they participate in using an association line
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If we connect more than one Actor to a Use Case, then...?

then all the Actors are required for the Use Case to run
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Because Use Case diagrams are usually notional and aimed at a relatively nontechnical audience, we tend not to indicate the typical adornments that an association can have.

What adornments can we have on a use case diagram?


However, we can use:
  • multiplicity and
  • arrowheads if they are necessary to clarify the situation.



For example:
  • the arrowheads indicate which Actor contacts the system to start the Use Cases
  • or if the Use Case contacts an Actor to start the behavior.

Note that once the Use Case is initiated, data and messages can flow in both directions irrespective of the direction of the arrow
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Who initiates or triggers the Use Case?

A Use Case often has a primary Actor that initiates or triggers the Use Case.

We consider the other Actors as secondary Actors.
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When there is no multiplicity shown on the side of Use Case, we normally assume a 0…1 value.

Why is it 0...1  and not 1?

If we had assumed exactly 1, it would require every instance of the Actor to be participating in an instance of the Use Case, which is overwhelmingly false.

As this is probably not legitimate, it would be preferable to show the multiplicity explicitly.
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How many chess players are required to play a chess match?

Two Chess Players are required to Play Chess Match, but not every Chess Player is currently participating in a match.
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What are the participating roles of the Driver and Rider?

We show both the Driver and Rider as participants in the Hail Car Use Case, though neither is required to always be participating.
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Give an example why it is better to use roles instead of names:

For example
in the United States, the President signs bills into laws.
You would not use the President’s birth name as the name of the Actor, because as time goes on, we get new individuals in that role.


Second example:

People may change their roles.
Manager at a company can change department or function.
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What are also usually not good names for Actors?

Job titles are not usually good names for Actors because they are too political and not sufficiently descriptive.
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An external system that interacts with our system during the execution of our Use Case is also an ...

Actor
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Though such systems don’t have explicit goals or wishes, they do have a perspective.

How can we determine the goal of the external system?

Their goals can be determined by investigating the purposes of the external systems and why it might be designed to interact with your system.

Often, there is a representative of the external system that can answer questions to determine its goals as an Actor.
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