1. CS 426 CS 791z Topics on Software Engineering
Chapter 3: The Requirements Workflow
[Arlow & Neustadt, 2005]
February 28, 2013

2. Outline The requirements workflow Metamodel for software requirements
Requirements workflow details
The importance of requirements
Defining requirements

3. The Requirements Workflow.
Fig. 3.2 [Arlow & Neustadt 2005] shows that most of the work
in requirements workflow occurs in Inception and Elaboration phases

4. .The Requirements Workflow
The purpose of the requirements workflow is to reach an agreement on what the system should do, expressed in a way accessible to the users of the system
Requirements engineering involves: elicitation, negotiation, conflict resolution, prioritization, documentation, and maintenance of requirements
Various stakeholders are involved in establishing the set of requirements for the system
UML uses cases describe functional requirements, but non-functional requirements need different description

5. Metamodel for Software Requirements
Arlow & Neustadt’s approach for requirements engineering is shown in
Fig. 3.3 [Arlow 2002]. Details can be found in Section 3.3

6. Requirements Workflow Detail.
Specific tasks for UP (Unified Process) requirements workflow
Fig. 3.4 [Arlow & Neustadt 2005]

7. .Requirements Workflow Detail
Arlow and Neustadt extend slightly the UP requirements workflow with
the addition of new tasks: find functional requirements, find non-
functional requirements, prioritize requirements, & trace requirements
to use cases. As such, non-functional requirements can be addressed
as well, along with the traditional UP/UML treatment of functional
requirements via use cases. Fig. 3.5 [Arlow & Neustadt 2005]

8. The Importance of Requirements
Requirements engineering is about establishing what the stakeholders need from the system
Requirements engineering encompasses elicitation, negotiation, conflict resolution, prioritization, documentation, and maintenance of requirements
Poor requirements engineering is the major cause of software project failure
The second major cause of software project failure is lack of user participation

9. Defining Requirements.…
Requirement: “a specification of what should be implemented”
There are two types of requirements:
Functional requirements: describe the desired behaviour of the system
Non-functional requirements: specify particular properties of or constraints on the system
In theory, the set of requirements should be only about “what” the system should do, but in practice it is not possible to avoid “how” aspects of the system

10. .Defining Requirements...
The SRS (Systems Requirements Specification) is the document that contains the set of requirements expected to be satisfied by the system, both functional and non-functional
There are many ways to write an SRS (“company dependent” ways)
The SRS provides the input for the analysis and design phases of the development process
The bottom line regarding the SRS is: “does it help me to understand what the system should do or not?”

11. ..Defining Requirements..
Simple format recommended for well-formed requirements:
<id> The <system> shall <function>
Examples of functional requirements (what the system should do):
1 The ATM shall check the validity of the ATM card inserted.
2 The ATM shall validate the PIN number entered by the client.
3 The ATM shall dispense no more than $500 against any ATM card in any 24-hour period
Examples of non-functional requirements (constraints on or properties of the system):
1 The ATM shall be written in C++.
2 The ATM shall validate the PIN in three seconds or less.

12. …Defining Requirements.
Organizing requirements: a more complex taxonomy can be used when there are many requirements, e.g.
Functional requirements
Customers
Products
Orders
Sales channels
Payments
Non-functional requirements:
Performance
Capacity
Availability
Compliance with standards
Security

13. ….Defining Requirements
Requirements may have attributes, e.g.
Must have
Should have
Could have
Want to have [the MoSCoW system]
Requirement attributes in RUP:
Status (proposed, approved, rejected, incorporated)
Benefit (critical, important, useful)
Effort (measured in person*day or function points)
Risk (high, medium, low)
Stability (high, medium, low)
TargetRelease (product version when implemented)

14. Finding Requirements..
Requirements come from the context of the system:
Direct users
Other stakeholders (e.g., managers, maintainers, installers)
Other systems that interact with the system
Hardware devices attached to the system
Legal and regulatory constraints
Technical constraints
Business goals

15. .Finding Requirements
“The map is not the territory” (that is, a model is not the real thing). When modeling, we apply three cognitive filters that simplify our effort [Chomsky, 1975]:
Deletion (information is filtered out)
Distortion (information is modified)
Generalization (information is abstracted into rules, principles, etc)
In requirements specification we need to identify the application of the above filters and find “challenges” for them to recover information
In particular, universal quantifiers such as all, everyone, always, never, nobody, none should be inspected closely for accuracy

16. ..Finding Requirements Interviews:
Don’t imagine a solution
Don’t mind read
Ask context-free questions
Listen
Have patience!
Questionnaire: they can supplement interviews.
Good at getting answers to closed questions
Requirements workshop
Participants: facilitator, requirements engineer, stakeholders, domain experts
Procedure: 1 Brainstorm (accept all ideas) 2 Identify key requirements 3 Iterate over, refine, and prioritize requirements