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SOFTWARE REQUIREMENTRS

SOFTWARE REQUIREMENTRS. LEC 3. outline. Software Requirement Type of Requirement ( user & system ) Functional Requirement Non Functional Requirement Software Requirements Specification (SRS).

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SOFTWARE REQUIREMENTRS

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  1. SOFTWARE REQUIREMENTRS LEC 3

  2. outline • Software Requirement • Type of Requirement ( user & system ) • Functional Requirement • Non Functional Requirement • Software Requirements Specification (SRS)

  3. The software requirements are description of features and functionalities of the target software . • Descriptions and specifications of a Software. • A statement of a customer need or objective, or of a condition or capability that a product (software) must possess to satisfy such a need or objective . • Requirements explain the expectations of users from the software product Software Requirement

  4. 1- User requirements ( user needs) Statements in natural language plus diagrams of the services the system provides and its operational constraints. Written for customers Type of requirement 2- System requirements (technical requirement) • A structured document setting out detailed descriptions of the system’s functions, services and operational constraints. Defines what should be implemented . • Written as a contract between client and contractor (Developer)

  5. This example from a mental health care patient management system (MHC_PMS) User requirement is quite general The system requirements provide more specific information about the services and functions of the system that is to be implemented

  6. Requirement are often classified as : • Functional requirements Statements of services the system should provide, how the system should react to particular inputs and how the system should behave in particular situations. • Non-functional requirements constraints on the services or functions offered by the system such as timing constraints, constraints on the development process, standards, etc. Functional and non-functional requirements

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

  8. Mentcare system: functional requirements • A user shall be able to search the appointments lists for all clinics. • The system shall generate each day, for each clinic, a list of patients who are expected to attend appointments that day. • Each staff member using the system shall be uniquely identified by his or her 8-digit employee number.

  9. Requirements imprecision • Problems arise when functional requirements are not precisely stated. • Ambiguous requirements may be interpreted in different ways by developers and users. • Consider the term ‘search’ in requirement 1 • User intention – search for a patient name across all appointments in all clinics; • Developer interpretation – search for a patient name in an individual clinic. User chooses clinic then search.

  10. Non-functional requirements • These define system properties and constraints e.g. reliability, response time and storage requirements. Constraints are I/O device capability, system representations, etc. • Non-functional requirements may be more critical than functional requirements. If these are not met, the system may be useless.

  11. Types of nonfunctional requirement

  12. Non-functional classifications • Product requirements • Requirements which specify that the delivered product must behave in a particular way e.g. execution speed, reliability, etc. • Organisational requirements • Requirements which are a consequence of organisational 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.

  13. Examples of nonfunctional requirements in the Mentcare system

  14. Non-functional requirements implementation • Non-functional requirements may affect the overall architecture of a system rather than the individual components. • For example, to ensure that performance requirements are met, you may have to organize the system to minimize communications between components. • A single non-functional requirement, such as a security requirement, may generate a number of related functional requirements that define system services that are required. • It may also generate requirements that restrict existing requirements.

  15. Goals and requirements • Non-functional requirements may be very difficult to state precisely and imprecise requirements may be difficult to verify. • Goal • A general intention of the user such as ease of use. • Verifiable non-functional requirement • A statement using some measure that can be objectively tested. • Goals are helpful to developers as they convey the intentions of the system users.

  16. Usability requirements • The system should be easy to use by medical staff and should be organized in such a way that user errors are minimized. (Goal) • Medical staff shall be able to use all the system functions after four hours of training. After this training, the average number of errors made by experienced users shall not exceed two per hour of system use. (Testable non-functional requirement)

  17. Metrics for specifying nonfunctional requirements

  18. Software Requirements Specification (SRS)

  19. Software Requirements Specification • A software requirements specification (SRS) : • is a comprehensive description of the intended purpose and environment for software under development. The SRS fully describes what the software will do and how it will be expected to perform. • The SRS is the principal deliverable that analysts use to communicate detailed requirements information to developers, testers, and other project stakeholders

  20. Purpose of SRS document? • SRS establishes the basis for agreement between the client and the supplier. • Users needs have to be satisfied, but user may not understand software . • Developers will develop the system, but may not know about problem domain • SRS is • the medium to bridge the communications gap, and • specifies user needs in a manner both can understand .

  21. Purpose of SRS document? • SRS provides a reference for validation of the final product. • Clear understanding about what is expected. • The SRS helps the client determine if the software meets the requirements “ SW satisfies the SRS “ .

  22. Need for Good SRS… • A high-quality SRS is a prerequisite to high-quality software. Many errors are made during the requirements phase. And an error in the SRS will manifest itself as an error in the final system implementing the SRS. • A high-quality SRS reduces the development cost by improving the quality of requirements, we can have a huge savings in the future by having fewer expensive defect removals

  23. Components of an SRS • What should an SRS contain ? • Clarifying this will help ensure completeness • An SRS must specify requirements on: • Functionality • Performance • Design constraints • External interfaces

  24. Functional Requirements • Heart of the SRS document; this forms the bulk of the specs. • Functional requirements specify the expected behavior of the system—which outputs should be produced from the given inputs. • They describe the relationship between the input and output of the system. For each functional requirement, a detailed description of all the data inputs and their source, the units of measure, and the range of valid inputs must be specified.

  25. Functional Requirements • All the operations to be performed on the input data to obtain the output should be specified. And the relationship between them • This includes specifying the validity checks on the input and output data, parameters affected by the operation, and equations . For example, if there is a formula for computing the output, it should be specified. • Specification for the system behavior for abnormal situations (error input) , situations where the input is valid but the normal operation cannot be performed .

  26. Performance Requirements The performance requirements part of an SRS specifies the performance constraints on the software system. All the requirements relating to the performance characteristics of the system must be clearly specified. There are two types of performance requirements: • Static • Dynamic.

  27. Performance Requirements • Static requirements are those that do not impose constraint on the execution characteristics of the system. • These include requirements like the number of terminals to be supported , the number of simultaneous users to be supported, and the number of files that the system has to process and their sizes. • These are also called capacity requirements of the system..

  28. Performance Requirements • Dynamic requirements specify constraints on the execution behavior of the system. • These typically include response time and throughput constraints on the system. • Response time is the expected time for the completion of an operation under specified circumstances. • Throughput is the expected number of operations that can be performed in a unit time. • . Acceptable ranges of the different performance parameters should be specified, as well as acceptable performance for both normal and peak workload conditions

  29. Design Constraints • There are a number of factors in the client’s environment that may restrict the choices of a designer leading to design constraints. that may have an impact on the design of the system . • Such factors include standards that must be followed, resource limits, operating environment, reliability and security requirements, and policies .

  30. Design Constraints • An SRS should identify and specify all such constraints. Some examples of these are: 1) Standards Compliance: • This specifies the requirements for the standards the system must follow. The standards may include the report format and accounting procedures. There may be audit requirements which may require logging of operations.

  31. Design Constraints 2)Hardware Limitations: The software may have to operate on some existing or predetermined hardware, thus imposing restrictions on the design. Hardware limitations can include the type of machines to be used, operating system available on the system, languages supported, and limits on primary and secondary storage.

  32. Design Constraints 3)Reliability and Fault Tolerance: • Fault tolerance requirements can place a major constraint on how the system is to be designed, as they make the system more complex and expensive. Recovery requirements are often an integral part here, detailing what the system should do if some failure occurs to ensure certain properties.

  33. Design Constraints 4)Security: Security requirements are becoming increasingly important. These requirements place restrictions on the use of certain commands, control access to data, provide different kinds of access requirements for different people, require the use of passwords and cryptography techniques, and maintain a log of activities in the system .

  34. External Interface • In the external interface specification part, all the interactions of the software with people, hardware, and other software should be clearly specified. • For the user interface, the characteristics of each user interface of the software product should be specified. A preliminary user manual should be created with all user commands, screen formats, an explanation of how the system will appear to the user, and feedback and error messages. Like other specifications, these requirements should be precise and verifiable

  35. Characteristics of an SRS • Correct • Complete • Unambiguous • Verifiable • Consistent • Ranked for importance and/or stability

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