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Software Requirements Specification CS 4310 Fall 2012. Davis, A., Software Requirements. Prentice Hall, 1993. Peters, J. and W. Pedrycz, Software Engineering An Engineering Approach. John Wiley and Sons, 2000. Requirement.
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Software Requirements SpecificationCS 4310Fall 2012 Davis, A., Software Requirements. Prentice Hall, 1993. Peters, J. and W. Pedrycz, Software Engineering An Engineering Approach. John Wiley and Sons, 2000.
Requirement • A requirement is a user need or a necessary feature, function, or attribute of a system that can be sensed from a position external to that system. • Describes what and not how. • Uses the word shall. • Examples: • The system shall display the current location of a ship. • The system shall generate a dial tone within 5 seconds after a person picks up the receiver.
Software Requirements Specification (SRS) • SRS is a document containing a complete yet concise description of the entire external interface of the system with its environment including other software, communication ports, hardware, and human users. • Carves universe into two sets • All systems satisfying user’s real needs • All systems that do not satisfy user’s real needs
SRS-1 • Communication among customers, users, analysts, and designers • Defines external behavior of system (cannot be ambiguous) • May give design to help with understanding but developers are not bound by design
SRS-2 • Support for system testing activities • Primary input to system test planning and generation • Test to check if system meets requirements • Means of controlling evolution of system • Check if modification is new refinement or existing
Structure of an SRS • IEEE Std. 830-1993 • Review of template
Types of Requirements • Behavioral: define what the system does; inputs, outputs, and transformation of inputs to outputs • Nonbehavioral: define the attributes of the system as it performs its job, e.g., efficiency, reliability, security, maintainability, portability, and standards of compliance
Groupings of Requirements • Related to same class of user • Related to same real-world object • Related to same external stimulus/response • Related to same system feature • Related to same class of function • Weakest of all groups
Attributes of a Well-Written SRS-1 • Correct iff every requirement stated therein represents something required of the system to be built. • Unambiguous iff every requirement stated therein has only one interpretation.
Attributes of a Well-Written SRS-2 • Complete if it possesses the following: • Everything that the software is suppose to do is included in the SRS. • Definitions of responses of software of all realizable classes of input data in all realizable classes of situations are included • All pages are numbered, all figures and tables are numbered, named, and referenced; all terms and units of measures are provided. • No sections are marked TBD
Attributes of a Well-Written SRS-3 • Verifiable (SRS) iff every requirement stated therein is verifiable. A requirement is verifiable iff there exists some finite cost-effective process with which a person or machine can check that an actual as-built software product meets the requirements. • Example requirements that are not verifiable: • The product shall have an easy-to-use interface. • The program shall not enter an infinite loop. • Avoid words such as “usually”, “generally,” or “often”
Attributes of a Well-Written SRS-4 • Consistent iff no requirement stated therein is in conflict with other preceding documents and no subset of requirements stated therein conflict. • Conflicting Behavior: Specify different stimuli to induce a response or different responses to the same stimuli • Conflicting Terms: Terms used in different contexts to mean the same thing. • Conflicting Characteristics: Demand the product to exhibit contradictory traits • Temporal inconsistency: Demand the product to obey contradictory timing characteristics
Attributes of a Well-Written SRS-5 • Traced iforigin of requirements is clear. • Traceable if the SRS is written in a manner that facilitates the referencing of each individual requirement stated therein.
Non-behavioral Characteristics • Portability • Reliability • Efficiency • Human Engineering • Testability • Understandability • Modifiability
Portability • Degree to which software running on one host computer environment can be converted to run on another. • Not necessary for all applications (embedded systems, single-use systems). • Some applications, it is essential.
Problems with specification • May be impossible to quantify: “The maximum time to port to host system X shall be …” • We don’t know what the next generation will be; • The maximum time is not useful: it doesn’t affect the design of the system.
Approaches to Specifying Portability • Source language • Java: JVM ported to lots of platforms • Ada: DoD certifies – no extentions, no subsets • Ideally, language is a design issue, but if its effect on portability is critical, it’s a requirement • Language selection tends to be political in organizations • Host operating systems • Say which ones up front, if you know • Compiler selection • Ansi Standard compilers
Reliability • This is difficult in software: • “The system shall be 99.999% reliable.” • What does this mean? • It could mean that the phone system may lose a call now and again, but the entire system must not fail for more than 5 minutes a year. • In a patient monitoring system, it may mean that if it does go down, it alerts staff. It must not make a mistake in monitoring more than one patient in 100,000.
Traditional reliability (hardware) • Mean Time to Failure (MTTF) • MTTF of system is well defined in terms of MTTF of components. • Redundant components improve reliability because failure of components is independent. • Hardware degrades in its environment. • Bathtub curve for electronics over entire population of products. Burn-in failures time
MTTF and Software • Software doesn’t degrade over time. • Suppose you run a program for 10 years without failure, then it suddenly fails. Why? • Software was changed. • Software was used a new way.
Bugs • Failure: Software does not do what is required (specified). Behavior is different from that needed. • Fault: A cause of a failure. • Not all faults result in failure. • All failures result from faults. • A state in which there is a fault without a failure is a hazard state. • Error: A design or implementation flaw.
Testing • The purpose of testing is not to demonstrate correct execution of the program. • The purpose of testing is to discover faults.
Problems specifying reliability in terms of bugs • Assume quality is designed in from the start: • The software testing shall require no more than two months. • Software testing shall discover no more than 10 bugs. • The software shall have no more than one bug per thousand lines of code. • We want zero bugs, so this must be better than 5 per 1000. • How do we know how many there are? Wait until software is retired, then count them.
Fault Seeding • Before testing, insert some bugs • Track how many of these are found in testing. • Total bugs in system = (#seeded * total_detected)/seeded_detected
Example • Secretly seed 10 bugs. • Test team discovers 120 bugs, 6 of which are seeds. • Bugs = 10 * 120 / 6 = 200 bugs • # bugs remaining = 200 – 120 = 80, 4 of which are “known”.
Notes • Not all bugs are equal • Equally difficult to find • Equally difficult to repair • Seeding is harder than it looks. • Intuitive • Measure of testing effectiveness
Reliability • Levels of criticality • Cause mild inconvenience • Cause minor financial loss • Cause major embarrassment • Cause major financial loss • Injure people • Kill a few people • Kill many people • Destroy humankind
Example Reliability Requirement • No more than five bugs per 10K lines of executable code shall be detected during integration and system testing. • No more than ten bugs per 10K lines of executable code shall remain in the system after delivery, as calculated by the Monte Carlo seeding technique defined in Appendix III. • The system shall be 100% operational 99.9% of the calendar time during its first year of operation.
Efficiency • The utilization of scarce resources. • Memory • CPU • Disk • Communication • Easy to specify if limits are given • Example: Air Traffic Control system: The system shall trace the movements of up to fifty aircraft.
Need to say how it will degrade: • What if there are 51 aircraft? Possibilities: • Software fails. • Track first 50, ignore 51st. • Software notifies 51st pilot to leave area.
Human Engineering: Levels of Specification • The system shall have an easy-to-use human interface. • The system shall be menu driven. • The system shall be menu-driven. Appendix A shows sample menus. • The system shall be menu-driven. Appendix A shows all menus to be used.
Human Engineering: Error Messages • Unless there is a sound understanding of the types of error messages the system can generate, there is insufficient knowledge of the system’s expected normal behavior. • It is a good idea to have an appendix that specifies the text of all error messages.
Testability, Modifiability, Understandability • Very difficult to quantify. • These are important contributors to cost (maintenance). • One suggestion is to specify conformity to a set of programming standards.
Programming standards specify • Naming conventions • Invocation conventions • Calls, interrupts, synchronization • Message formats • Component headers • Format and content • In-line documentation style • Use of global constructs and variables • Use of named constructs • Modularity standards
TEAM WORK (modified from IEEE SE Problems) • Irbis is gathering the requirements for a software-controlled furnace. After interviewing several users, Irbis obtained the following requirements: • R1: Gas intlet valves shall always be open when furnace is heating. • R2: Heating shall stop when furnace temperature reaches 150°C. • R3: Furnace temperature should increase gradually when heating. • R4: The gas inlet valves shall be closed when the temperature goes above 200°C. • In teams of 3, for each of the requirements: • identify the requirement’s defects. • Provide a fix to address the requirement’s defects. • Indicate in which section of the SRS will you place the requirement and the reasoning for your decision.