Introduction to Software Engineering Dr. Basem Alkazemi bykazemi@uqu.edu.sa http://uqu.edu.sa/bykazemi

Objectives Describe what is Software Requirements and its main types. Identify what need to be elicit and the elicitation techniques. Explore requirement analysis techniques Define the structure of the requirement document. Introduce some System modeling techniques.

What is Requirement? Requirements are the set of things that a customer requires with a number of constraints that must be satisfied on the final system. It can be represented in high-level abstract statement of a service or a detailed mathematical functional specification. Requirements may serve as: The basis for a bid for a contract - therefore must be open to interpretation; The basis for the contract itself - therefore must be defined in detail;

Examples of Requirements “Fire Alarm must go off when a movement is detected before 06:00 and after 19:00” “The system must dial 999 to inform the Police” “Customers should be able to view their remaining balance” “The system should identify employee by face”

Requirements Engineering The process of establishing the activities for interpreting customer requirements to build a complete working system. Include the following processes: Elicitation Analysis and negotiation Specs generation System modeling Validation Management

Why Reqs are non-tech? User cannot understand technical details Maybe they could after system Launched! Reduce restrictions on code developers Satisfy requirements using whatever possible techniques and tools. Customer may specify details that might confuse more than clarify! Fast system, large size, easy to use … etc Requirement might change over time

Characteristics of good Spec. Implementation Free Omit technical details. Complete Should include descriptions of all facilities required. Consistent There should be no conflicts or contradictions in the descriptions of the system facilities. Unambiguous Common vocabularies are used. Ambiguous requirements may be interpreted in different ways by developers and users Concise and Minimal No duplication or unnecessary contents. Clear and Understandable Measurable Example: “Experienced controllers shall 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 shall not exceed two per day”.

Exercise Are The following statements right or wrong Specs? “Write a Java program to provide a personal telephone directory. It should implement functions to look up a number and to enter a new telephone number. The program should provide a friendly user interface.” “ Data should be stored in Magnetic Disk and the system must retrieve data in 1ms”

Forms of requirement User requirements System requirements Statements in natural language plus diagrams of the services the system provides and its operational constraints. Written for customers. System requirements A structured document setting out detailed descriptions of the system’s functions, services and operational constraints. Defines what should be implemented so may be part of a contract between client and contractor.

Types of Requirements Functional Requirements Non-Functional Requirements Domain Requirements

Functional requirements Describe functionality or system services. Depend on the type of software, expected users and the type of system or environment where the software is used. Functional user requirements may be high-level statements of what the system should do but functional system requirements should describe the system services in detail.

Non-functional requirements Define system properties and constraints e.g. reliability, response time and storage requirements. Constraints are I/O device capability, system representations, etc. Process requirements may also be specified authorizing a particular CASE system, programming language or development method. Non-functional requirements may be more critical than functional requirements. If these are not met, the system is useless.

Example of Non-Functional Reqs The user interface for LIBSYS shall be implemented as simple HTML without frames or Java applets. The system development process and deliverable documents shall conform to the process and deliverables defined in XYZCo-SP-STAN-95. The system shall not disclose any personal information about customers apart from their name and reference number to the operators of the system.

Elicit Requirements The activity of eliciting requirements involves the engineer and users talking together, with the former trying to understand the latter by asking questions and writing notes.

Elicitation Techniques Background reading Documents inspection Interviews Questionnaires Brainstorming Ethnography Prototyping Use-cases and scenarios

What to Elicit? Description of problem domain. List of Problems. medical, chemistry, math … etc. List of Problems. Any client-imposed constraints upon behavior or structure of the system. The main motivation behind the development.

Sources of Information Client Documentation of Pre-exiting system Users of pre-existing system Possible users of the new system Competitor’s product Domain Expert Relevant standards

Guidelines for writing requirements Invent a standard format and use it for all requirements. Use language in a consistent way. Use shall for mandatory requirements, should for desirable requirements. Use text highlighting to identify key parts of the requirement. Avoid the use of computer terminology.

Problems with natural language Lack of clarity Precision is difficult without making the document difficult to read. Requirements confusion Functional and non-functional requirements tend to be mixed-up. Requirements amalgamation Several different requirements may be expressed together.

Requirement Document Structure Preface Introduction Objectives Scope statement Glossary User Requirement definition System Architecture System Specification System Models DFD, STD, SD .. System Evolution Appendices

Sequence diagrams These show the sequence of events that take place during some user interaction with a system. You read them from top to bottom to see the order of the actions that take place. Cash withdrawal from an ATM Validate card; Handle request; Complete transaction.

Sequence diagram of ATM withdrawal

System Behavior Modeling Data Flow Diagram (DFD) Graphical representation of the flow of data throughout a system.

System Behavior Modeling State Transition Diagram (STD) Describes the possible states of an object as events occur

Key points Requirements set out what the system should do and define constraints on its operation and implementation. Functional requirements set out services the system should provide. Non-functional requirements constrain the system being developed or the development process. User requirements are high-level statements of what the system should do. User requirements should be written using natural language, tables and diagrams.