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Software Engineering: A Practitioner’s Approach, 6/e Chapter 7 Requirements Engineering copyright © 1996, 2001, 2005 R.S. Pressman & Associates, Inc.

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Presentation on theme: "Software Engineering: A Practitioner’s Approach, 6/e Chapter 7 Requirements Engineering copyright © 1996, 2001, 2005 R.S. Pressman & Associates, Inc."— Presentation transcript:

1 Software Engineering: A Practitioner’s Approach, 6/e Chapter 7 Requirements Engineering copyright © 1996, 2001, 2005 R.S. Pressman & Associates, Inc. NOTE: Some slides referenced from: Ian Sommerville Slides for Software Engineering. Coming up: What is a requirement?

2 What is a requirement? Requirements are used to describe all aspects of a system They may range from a high-level abstract statement of a service or of a system constraint to a detailed mathematical functional specification They serve many roles May be the basis for a bid for a contract - therefore must be open to interpretation May be the basis for the contract itself - therefore must be defined in detail Both these statements may be called requirements Coming up: Requirements Definition

3 Requirements Definition
Should specify external behavior of the system Includes functional and non-functional requirements Functional requirements are statements of the services that the system must provide What must the system do? Non-functional requirements are constraints on the services and functions offered by the system How must it do it? or a constraint on the system Coming up: Typical Requirements

4 Typical Requirements Functional: The system shall display the heart rate, blood pressure and temperature of a patient connected to the patient monitor. Non-Functional: "Display of the patient's vital signs must respond to a change in the patient's status within 2 seconds.” ‘ilities’ - Performance, Scalability, Capacity, Availability Reliability, Recoverability, Maintainability, Serviceability, Security,
Regulatory,Manageability What are some functional requirements on an iPod? Non-functional? Coming up: Why do we care?

5 I’ll destroy you and your little software project
Why do we care? Most large software systems address wicked problems Problems which are so complex that they can never be fully understood and where understanding develops during the system development Therefore, requirements are normally both incomplete and inconsistent Requirements help by giving you the best understanding you can have at the beginning I’ll destroy you and your little software project to! problem Coming up: Requirements Engineering-I

6 Requirements Engineering-I
Inception—ask a set of questions that establish … basic understanding of the problem the people who want a solution the nature of the solution that is desired, and the effectiveness of preliminary communication and collaboration between the customer and the developer Elicitation—elicit requirements from all stakeholders Elaboration—create an analysis model that identifies data, function and behavioral requirements Negotiation—agree on a deliverable system that is realistic for developers and customers Inception - Basic Understanding. Just talking Elicitation - Try to get rqmt from users Elaboration - refined technical models software functions/features/constraints Negotiation - schedule/cost, feature A vs B Coming up: Requirements Engineering-II

7 Requirements Engineering-II
Specification—can be any one (or more) of the following: A written document A set of models A formal mathematical A collection of user scenarios (use-cases) A prototype Validation—a review mechanism that looks for errors in content or interpretation areas where clarification may be required missing information inconsistencies (a major problem when large products or systems are engineered) conflicting or unrealistic (unachievable) requirements. Requirements management Specification - final document Validation - checklists, not violating each other, etc… Management - similar to source code mgmt Coming up: Inception

8 Inception Identify stakeholders Recognize multiple points of view
“who else do you think I should talk to?” Recognize multiple points of view Work toward collaboration The first questions Who is behind the request for this work? Who will use the solution? What will be the economic benefit of a successful solution Is there another source for the solution that you need? Stakeholders - anyone with a valid interest Viewpoints - marketing, management, developers, end-users, etc… Collaboration - assign voting rights, etc… who am I trying to satisfy? (Impossible to answer). Coming up: Eliciting Requirements

9 Eliciting Requirements
meetings are conducted and attended by both software engineers and customers rules for preparation and participation are established an agenda is suggested a "facilitator" (can be a customer, a developer, or an outsider) controls the meeting a "definition mechanism" (can be work sheets, flip charts, or wall stickers or an electronic bulletin board, chat room or virtual forum) is used the goal is to identify the problem propose elements of the solution negotiate different approaches, and specify a preliminary set of solution requirements XP - does only user scenarios, Documenting requirements - in DB or Excel or wherever Coming up: Eliciting Requirements

10 Eliciting Requirements
QFD - look at everything through the “voice of the customer” Coming up: Elicitation Work Products

11 Elicitation Work Products
a statement of need and feasibility. a bounded statement of scope for the system or product. a list of customers, users, and other stakeholders who participated in requirements elicitation a description of the system’s technical environment. a list of requirements (preferably organized by function) and the domain constraints that apply to each. a set of usage scenarios that provide insight into the use of the system or product under different operating conditions. any prototypes developed to better define requirements. Coming up: Use-Cases

12 Use-Cases A collection of user scenarios that describe the thread of usage of a system Each scenario is described from the point-of-view of an “actor”—a person or device that interacts with the software in some way Each scenario answers the following questions: Who is the primary actor, the secondary actor (s)? What are the actor’s goals? What preconditions should exist before the story begins? What main tasks or functions are performed by the actor? What extensions might be considered as the story is described? What variations in the actor’s interaction are possible? What system information will the actor acquire, produce, or change? Will the actor have to inform the system about changes in the external environment? What information does the actor desire from the system? Does the actor wish to be informed about unexpected changes? Coming up: Use-Case Diagram

13 Use-Case Diagram Coming up: Building the Analysis Model

14 Building the Analysis Model
Elements of the analysis model Scenario-based elements Functional—processing narratives for software functions Use-case—descriptions of the interaction between an “actor” and the system Class-based elements Implied by scenarios Behavioral elements State diagram Flow-oriented elements Data flow diagram Coming up: State Diagram

15 State Diagram Coming up: Negotiating Requirements

16 Negotiating Requirements
Identify the key stakeholders These are the people who will be involved in the negotiation Determine each of the stakeholders’ “win conditions” Win conditions are not always obvious Negotiate Work toward a set of requirements that lead to “win-win” Coming up: Reasons we need to negotiate

17 Reasons we need to negotiate
Discuss Large software systems must improve the current situation. It is hard to anticipate the effects that the new system will have on the organization Discuss Prototyping is often required to clarify requirements Negotiate Different users have different requirements and priorities. There is a constantly shifting compromise in the requirements Negotiate System end-users and organizations who pay for the system have different requirements Coming up: Requirements Document Structure

18 Requirements Document Structure
Purpose Overall Description System Features (Functional Requirements/Use Cases) External Interface Requirements User interface requirements or standards Hardware interfaces to other systems Software interfaces to other systems Communication Interfaces Non-functional Requirements Appendices See SRS Template on the CS421 project page (provided by … but essentially from IEEE). Coming up: Requirements Rationale

19 Requirements Rationale
It is important to provide rationale with requirements This helps the developer understand the application domain and why the requirement is stated in its current form Particularly important when requirements have to be changed. The availability of rationale reduces the chances that change will have unexpected effects In this class we’ll provide the requirement’s source and use case instead of an explicit “rationale” Coming up: Non-Functional Requirement Types

20 Non-Functional Requirement Types
Remember: Non Functional = How the system should do something Product requirements Requirements which specify that the delivered product must behave in a particular way e.g. execution speed, reliability, etc. Organizational requirements Requirements which are a consequence of organizational policies and procedures e.g. process standards used, implementation requirements, etc. External requirements Requirements which arise from factors which are external to the system and its development process e.g. interoperability requirements, legislative requirements, etc. Coming up: Non-Functional Requirement Examples

21 Non-Functional Requirement Examples
Product requirement 4.C.8 It shall be possible for all necessary communication between the APSE and the user to be expressed in the standard Ada character set. Organizational requirement The system development process and deliverable documents shall conform to the process and deliverables defined in XYZCo-SP-STAN-95. External requirement The system shall provide facilities that allow any user to check if personal data is maintained on the system. A procedure must be defined and supported in the software that will allow users to inspect personal data and to correct any errors in that data. constraint: interoperability constraint: quality security, maintainability Coming up: Requirements must be testable

22 Requirements must be testable
Requirements must be written so that they can be objectively verified The problem with this requirement is its use of vague terms such as ‘errors shall be minimized” The system should be easy to use by experienced controllers and should be organized in such a way that user errors are minimized. The error rate should be been quantified Experienced controllers should be able to use all the system functions after a total of two hours training. After this training, the average number of errors made by experienced users should not exceed two per day. Coming up: Requirements: testable metrics

23 Requirements: testable metrics
Coming up: System Level Requirements

24 System Level Requirements
Some requirements place constraints on the system as a whole rather than specific system functions Example The time required for training a system operator to be proficient in the use of the system must not exceed 2 working days. These may be emergent requirements which cannot be derived from any single sub-set of the system requirements Coming up: Requirements Validation

25 Requirements Validation
Concerned with demonstrating that the requirements define the system that the customer really wants Requirements error costs are high so validation is very important Fixing a requirements error after delivery may cost up to 100 times the cost of fixing an implementation error Prototyping is an important technique of requirements validation Coming up: Validating Requirements-I

26 Validating Requirements-I
Is each requirement consistent with the overall objective for the system/product? Work with all parties on this question! Have all requirements been specified at the proper level of abstraction? That is, do some requirements provide a level of technical detail that is inappropriate at this stage? Is the requirement really necessary or does it represent an add-on feature that may not be essential to the objective of the system? Is each requirement bounded and unambiguous? Does each requirement have attribution? That is, is a source (generally, a specific individual) noted for each requirement? Do any requirements conflict with other requirements? Unneccessary example with our ATM system? Buy Stamps! :-) Coming up: Validating Requirements-II

27 Validating Requirements-II
Is each requirement achievable in the technical environment that will house the system or product? Is each requirement testable, once implemented? Does the requirements model properly reflect the information, function and behavior of the system to be built. Has the requirements model been “partitioned” in a way that exposes progressively more detailed information about the system. Have requirements patterns been used to simplify the requirements model. Have all patterns been properly validated? Are all patterns consistent with customer requirements? Coming up: Requirements Reviews

28 Requirements Reviews Regular reviews should be held while the requirements definition is being formulated Both client and contractor staff should be involved in reviews Reviews may be formal (with completed documents) or informal. Good communications between developers, customers and users can resolve problems at an early stage Coming up: Requirements Reviews Check

29 Requirements Reviews Check
Verifiability. Is the requirement realistically testable? Comprehensibility. Is the requirement properly understood? Traceability. Is the origin of the requirement clearly stated? Adaptability. Can the requirement be changed without a large impact on other requirements? Coming up: Requirements final thoughts…

30 Requirements final thoughts…
Requirements should specify one and only one thing Requirements can only pass or fail, they can’t partially pass Requirements should be testable Requirements should specify what YOUR system must do, not what other (external) systems do To make this easier, in this class all functional requirements must begin with “The system shall…” Should have a source (generally same source as the use-case), and for the same reason Coming up: Requirements final thoughts…

31 Requirements final thoughts…
Should not be a design choice (this is hard to get right). The system shall store user information including name, DOB, address and SSN. <-- Good! The system shall store user information in an Oracle database including name, DOB, address, SSN. <-- bad Is Oracle really REQUIRED? Hard to say… maybe, but probably not. This is a decision you would make at implementation design time. Question: Does the customer care that you use Oracle? MySQL? Etc.. Maybe someone found some other MUCH BETTER approach storing the data on moon rocks. Again: This is hard to avoid… and I’m not to concerned with it on the SRS, but I want you to be very aware of when you are making design choices instead of required features. Coming up: Requirements must have a unique ID

32 Requirements must have a unique ID
When testing you need to reference REQ-1 or REQ-287. Multiple things cannot be labeled REQ-1. Later our test cases will say: This test case validates requirements REQ-1, REQ-27, and REQ-56. Coming up: Bad Examples

33 Bad Examples Bad requirements examples:
The system shall validate and accept credit cards and cashier’s checks. High priority. The system shall process all mouse clicks very fast to ensure user’s do not have to wait. The user must have Adobe Acrobat installed. These don’t have a source or unique ID, but what else is wrong? Coming up: Bad Examples

34 Bad Examples The system shall validate and accept credit cards and cashier’s checks. High priority. Problem: two requirements instead of one. If the credit card processing works, but the cashier’s check validation does not… is this requirement pass or fail? Has to be fail, but that is misleading. Maybe only credit cards are high priority and cashier’s checks are low priority. The system shall process all mouse clicks very fast to ensure user’s do not have to wait. Problem: This is not testable. Quantify how fast is acceptable? The user must have Adobe Acrobat installed. Problem: This is not something our system must do. It could be in the constraints/assumptions or maybe operating environment sections, but is not a functional requirement of our system Coming up: The Result of Good Requirements

35 The Result of Good Requirements
Your wicked problems shrink into small manageable problems …. almost as if they are melting away! I’m melting! Coming up: The Result of Good Requirements

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