Unit 3

Question 1

What is meant by a business rule?

Answer 1

Business rules constrain how a business is run. They are not specific to any project but they influence each project’s development – they typically include policies, physical laws, government laws, etc. Business rules reflect the way the business works, independently of any new system being introduced in that business domain.

Question 2

What is meant by verification?

Answer 2

Verification is the set of activities that ensure that a solution is being correctly developed, in this case that it is taking into account the business rules.

Question 3

What is meant by validation?

Answer 3

Validation ensures that what is being built satisfies the business, i.e. that the business rules implemented are the ones that the business wants.

Question 4

What are the important properties that a representation of business rules should have?

Answer 4

The important properties are as follows:

• Business rules that apply to the whole business should be represented separately from project specific models.
• They should be easy to verify (possibly automatically) and validate.
• They should be represented in a readable language that is easy to verify.

Question 5

Consider whether business rules can be modelled in UML. What are the possible limitations of UML?

Answer 5

UML can support the specification of business rules either as annotations to the models or in a more formal way through OCL. However UML and OCL do not provide a separate explicit notation for business rules. As a consequence, it does not:

• facilitate documentation of the rules independently of other models
• facilitate their analysis, validation and change
• facilitate their traceability from the business needs to the software solution.

Question 6

What is meant by a business process?

Answer 6

Definition of what gets done in the business: by whom, in what order, needing which resources and with what consequences.

Question 7

What causes a transition in an activity diagram?

Answer 7

A transition in an activity diagram is caused by the completion of an activity.

Question 8

What is a synchronisation bar, and when is one used in an activity diagram?

Answer 8

A synchronisation bar is used to mark the point when two or more activities can take place concurrently (a fork) or when a number of concurrent tasks must finish before continuing to the next activity (a join).

Question 9

How does the partitioning of activities into swimlanes help us understand a set of activities?

Answer 9

Swimlanes group activities associated with different roles. The swimlanes show the role that is responsible for each activity.

Question 10

Give one reason for modelling a workflow in an activity diagram.

Answer 10

Activity diagrams represent the sequence of activities. When you are modelling a workflow that involves more than one role, it is possible to identify which role is responsible for a particular activity. An activity diagram can help identify the stages at which each role requires some interaction with the process.

Question 11

What is a UML activity diagram?

Answer 11

An activity diagram shows a process as a set of activities and describes how they must be coordinated – which ones depend on others having been completed first and where activities can be carried out in parallel.

Question 12

When would you use a decision node in an activity diagram?

Answer 12

In an activity diagram a node that is used to indicate alternative transitions that can be made when leaving an activity; it is represented by a diamond shape. A decision node has a single incoming transition; the guards on outgoing transitions are mutually exclusive.

Question 13

What is meant by a guard?

Answer 13

A condition that must be satisfied in order to enable an associated transition to fire within a state machine or activity diagram.

Question 14

When, in an activity diagram would you use a merge node?

Answer 14

In an activity diagram, this is a node that brings together multiple alternative flows; represented by a diamond shape; a merge node has a single outgoing transition.

Question 15

What is meant by an ‘actor’ within UML?

Answer 15

UML employs the term actor to identify a user of a system. An actor can also be an external software system or subsystem that will interact with the proposed system.

Question 16

What is a use case?

Answer 16

A use case is something that an actor needs to do with the help of a software system; it may relate to a business process of the domain that will be now carried out with the software system or it may represent a new process. It should lead to a result that brings some observable value to that actor.

Question 17

What is a use case diagram?

Answer 17

A diagram that shows a set of use cases and actors and their associations.

Question 18

What are the purposes of a use case diagram?

Answer 18

The main purpose of use cases is to help with the requirements phase. Detailed software requirements can follow from a description of a use case.

Use cases define the scope of a system, as they represent the interaction of a system with its environment.

They also help with project planning and quality assurance.

Furthermore, they act as a discussion tool between developer and user, and offer a common language for agreeing on the functions of the software system.

Question 19

Name two aspects of software development where use case modelling can help.

Answer 19

So far we have encountered these two: eliciting requirements; representing requirements. Later in the module we will also discuss planning iterations of development and validating software systems.

Question 20

Suggest a reason why use case diagrams are an aid to communication between user and developer.

Answer 20

Use cases offer users an opportunity to understand the system since the use case notation is relatively simple and doesn’t require an understanding of UML. This provides a mechanism that enables developer and client to share a common understanding of the system, as long as the developer provides some text to demonstrate their understanding of the problem.

Question 21

What is the purpose of a system boundary? Is it always necessary to draw one in a use case diagram?

Answer 21

The purpose of a system boundary is to identify a single system, distinguishing between the internal and external components. Typically, the external components are the actors and the internal components are the use cases. UML says that the system boundary is optional.

Question 22

Explain why the actors in a use case diagram do not represent actual individuals.

Answer 22

An actor in a use case diagram represents a particular role that an individual might play when interacting with a software system. For example, a receptionist checks guests into and out of a hotel. But it could be that the person who works as a receptionist at one hotel becomes a guest at another hotel in the chain and hence takes on another role. Actors can also represent other systems, rather than people/roles.

Question 23

Suggest a guideline that will help you decide whether or not to include an interaction with an external system on your use case diagram.

Answer 23

One possible guideline would be to show interaction with an external system if the use case needs to communicate with the actors that represent the external system.

Question 24

Are roles in business process models the same as actors in use cases?

Answer 24

Roles in business process models may not correspond directly to actors in use cases as although they interact with a business process they may not interact with a proposed system. Some roles may become irrelevant when introducing a new system or may not need to interact with the system.

Question 25

What is meant by a precondition in the context of a use case?

Answer 25

It is a statement that must be true before the use case can be carried out.

Question 26

What is meant by a postcondition in the context of a use case?

Answer 26

It is a statement that will be true once the use case has been completed.

Question 27

What is the relationship between a use case and a scenario?

Answer 27

For each use case there is a set of possible scenarios. A scenario is an instance of a use case. A scenario describes a sequence of interactions between the system and some actors.

Question 28

What is meant by a main success scenario?

Answer 28

The main success scenario shows the steps normally followed to achieve the stated goal of the use case. But there can be other scenarios for the same use case, each one having different outcomes depending upon circumstances.

Question 29

How do use cases help with requirements capture?

Answer 29

Use cases help with requirements capture through the identification of actors and tasks in the system. For each actor, the set of use cases establishes what that actor requires from the software system. The association between an actor and a use case is about communication.

Question 30

How do use cases help with the elicitation of detailed software requirements?

Answer 30

Detailed software requirements can be associated with each step in a use case scenario. There may be more than one requirement for each step.

Question 31

How do use cases help with development?

Answer 31

One of the difficulties that developers face is planning delivery times. Often a customer can put pressure on the developer to meet a particular deadline. It is part of the developer’s job to elicit from the users the use cases that have the highest priority and to indicate what functionality in the software system can be met under such constraints. The use case descriptions help the developer to:

• understand the complexity of each use case
• determine which actors interact with each use case and to what extent
• establish which use cases carry the most risk
• estimate how long each use case is likely to take to implement.

Understanding these aspects of the system can help developers plan the order in which the use cases should be developed, and provide an appropriate time frame. Several criteria – such as risk, coverage and criticality – can be used to help establish priorities of use cases.

Question 32

How do use cases help with the system’s architecture?

Answer 32

Use cases, as standalone chunks of system specification, dictate the sorts of functionality that need to be provided by the system and constitute an aid for identifying interfaces in an architecture. Use cases can also be grouped in terms of similar functionality, therefore influencing the architecture of the system. Scenarios can be used to check how an architecture meets non-functional requirements, in particular those that can be affected by the architecture, such as security and safety requirements.

Question 33

How do use cases help with system validation (checking that the system actually supports the functionality required by the users)?

Answer 33

One way to validate a system is to use the walk-through technique, checking the functionality related to each use case in turn. The walk-through technique can also be used to elicit system tests where each use case is required to deal with a number of scenarios – a process known as verification. For each software requirement generated from a step of a scenario, the fit criterion helps to devise the test.

Question 34

What are some ways in which use cases can be used in an agile approach?

Answer 34

• write use cases as needed instead of writing all use cases upfront
• write just enough content rather than complete descriptions
• write use cases that are useful for communication, not heavy ones that are difficult to understand and change.

Question 35

What is the purpose of identifying relationships between actors?

Answer 35

The purpose of identifying relationships between actors is to indicate generalisations and establish which use cases can be performed by which actors.

Question 36

What is a stereotype in UML?

Answer 36

A stereotype is a way of attaching extra classifications to a model adding to its basic language. Stereotypes can be user defined – this is a way of extending UML.

Question 37

What is the meaning of the «include» stereotype?

Answer 37

The «include» stereotype indicates a situation where a use case is reused. The purpose is to demonstrate commonality between tasks so that reuse can be achieved. The additional use case is included unconditionally in the original (base) use case.

Question 38

What is the meaning of the «extend» stereotype?

Answer 38

The «extend» stereotype indicates a conditional extension to the original use case, known as alternative behaviour. This is used to illustrate a case where there are two or more significantly different scenarios, so that the main case and the additional subsidiary cases are clearly differentiated. The main purpose of this classification is to separate out a special case. You should add a condition to each extension – with either a note or an extension point – to specify when the variant behaviour will be included.

Question 39

Is it necessary to place the «include» and «extend» stereotypes on all diagrams?

Answer 39

No, it is not necessary to place the «include» stereotype and the «extend» stereotype on all diagrams. In fact, in some situations they can cause confusion since they will not be understood by everyone.

Question 40

How would you modify a use case model to show that you intend to employ a component that already exists?

Answer 40

Each use case that benefits from the component must have a relationship to that component shown on the diagram. This relationship should have the «include» stereotype attached to it.

Question 41

What problems may arise when developing a software system from a set of use cases?

Answer 41

One problem is that the focus may end up being top-down and function-oriented, resulting in an inflexible and difficult-to-maintain system. Focusing on use cases may cause the developer to sacrifice the object-oriented nature of the system, thus losing any advantage that UML offers.

Another danger lies in mistaking design for requirements, where a design decision is mistaken for a constraint. Focusing on the requirements in a use case may cause the developer to view the system too operationally, where a sequence of events is assumed to be the only answer. Developers need to distinguish between requirements and preferred designs.

Use cases need to be used in a way that is understandable to the customer but also useful to the developer.

Question 42

What are the tasks involved in preparing a use case model (for a moderately large system) intended for the development team?

Answer 42

The tasks comprise:

• defining the context for the model by identifying the actors involved in the aspect of the system in question
• analysing the behaviour that each actor expects from the proposed system, and identifying the use cases (as units of functionality within the overall requirements)
• identifying the common behaviour that may be reused by other actors, and the variations on common behaviour (the stereotypes «include» and «extend»)
• drawing a model that shows the use cases, the actors and the relationships between them
• annotating the use cases as you learn more about the requirements.

For large projects, you will need to record separately any constraints that affect more than one use case diagram. One way is to produce a use case model at a higher level of abstraction.

Question 43

Who initiates the prototyping process?

Answer 43

The developers would normally start the prototyping process because they have detected or identified a particular problem in their requirements analysis. (But note that the project manager would need to get approval from the customer for the additional effort.)

Question 44

Who should test a prototype?

Answer 44

The intended users should test it. For example, if you developed a series of interfaces as part of a prototype for the borrowing and returning of books, the library members would be the testers.

Question 45

What is the main benefit of identifying user interfaces in your activity diagrams?

Answer 45

The main benefit of recording user (or any other) interfaces in an activity diagram is traceability. To the users, the interface is the software system: an unacceptable interface can lead to failure. The user interface is the link between what the users want and what the developer produces in response.

Also, the developer can identify the relative importance of each user interface for the project plan, particularly when resources are needed for a prototype.

Question 46

Suppose the developer built a prototype interface that could deal with all identified use cases in the first iteration of the library system. Suggest a criterion that a librarian might use when evaluating the prototype, which goes beyond the completion of the underlying tasks within each use case.

Answer 46

This was the example we thought of, but you may have thought of others. Librarians may find it essential that the system makes it very easy to switch between different tasks. They will expect the prototype to provide evidence that the eventual system will meet this criterion.