Requirements Engineering CSCI 5801: Software Engineering

Requirements Engineering

What are Requirements? The services that the customer needs a system to perform The constraints under which the system operates and is developed

Standish survey of software development projects (1994) Factors Reported for Failure 13.1% - Incomplete Requirements 10.6% - Lack of Resources 9.9 % - Unrealistic expectations 9.3 % - Lack of Executive support 8.7 % - Changing requirements and specification 8.1 % - Lack of Planning 7.5 % - System no longer Needed Most common cause of project failure!

Major Activities Requirement Elicitation Requirement Documentation Identify stakeholders Communicate with them to attempt to understand their requirements Requirement Documentation Creating representation of requirements understood and agreed to by all Requirement Validation Making sure requirements correct May involve negotiation with stakeholders

Major Activities May be handled differently in different models Waterfall  Requirement Specification Document Extreme Programming  User stories Usually cyclical Elicitation Validation Documentation

What Must be Specified? Functional requirements Interactions between system and environment independent of implementation Most common way to think of requirements Nonfunctional requirements Constraints on how system performs its functional requirements (speed, reliability, usability, etc.) Context in which constraints must be met Often as important as functional requirements

Functional Requirements Can be treated like mapping between system inputs and outputs Example: password system User enters name, password (name, password) found in database  go to main menu Otherwise  Error message and re-prompt User input, sensor readings, data in databases… System Output displays, reports, updated data in databases…

FURPS+ One definition of major requirement types Functional Usability Reliability Performance Sustainability + (anything else)

Usability Ease of user: Based on user knowledge Operation of system Acquiring and entering inputs Interpreting outputs Learning to use Based on user knowledge Knowledge of domain Expertise in computing in general “Enter patient TH level” “Error: improper TH level entered” context

Reliability Doing specified tasks while minimizing error Durability: Errors do not exceed specified level Mean time to failure, % of transactions with error, … Security: Ability to resist given types of attack Robustness: Ability to handle illegal input Safety: Avoidance of catastrophic results to environment (erasure of database, etc.) Negative number? 100 digits (overflow) SQL injection… “Enter patient TH level”

Performance Ability to perform specified tasks within time/ space constraints Response time “Must show available courses within 5 seconds” Throughput “Must process 50 requests/second” Memory usage Context: Must specify conditions under which system operates for constraints to be meaningful …when running on X hardware and Y operating system …with Z users running simultaneously

Sustainability Ease with which specified types of expected maintenance can be done Expected future increments to system “May need to be able to enforce prerequisite constraints” Portability Hardware (PC  Mac) Software (MySQL  Oracle, JSP  ASP, etc.) Internationalization English  Spanish, Chinese, …

All Others (the “+”) Organizational requirements What your organization requires/usually does “Implemented in Java using RAD and standard UI” Interface requirements External entities in customer environment Existing databases Legacy software Legal requirements “Must meet FERPA and ADA requirements”

Measurable Requirements Must have some way to verify that delivered system meets requirements Waterfall: has company met contract? XP/Agile: how can test be created with customer? Functional requirements: System generally does or does not give desired response

Measurable Requirements Nonfunctional requirements: Many must be quantifiable in some way Otherwise have no way of proving have been met! Examples of bad requirements “The interfaces must be user-friendly” “The system must rarely crash” “Student records must be stored securely” “The system must respond within 1 second” Why is this last one bad?

Requirements in Context Quantified requirements must be defined in terms of rest of system Hardware and software (OS, etc.) Load on system (other programs, number of users) Knowledge levels of users

Usability Requirements Average errors/task “On average a student who has registered for one semester previously should make mo more than 3 mistakes during an entire session… Average time to complete task …and should be able to add all of their courses within 15 minutes” Training time “Registration personnel should require no more that 2 hours of training to reach 99% correctness rate” Can test with user focus groups

Reliability Requirements Mean time to failure “The system should be able to run for at least 100 consecutive hours on average without failure, under expected load of 50 simultaneous registrations” Transactions per error “There shall be no more than 1 failure per 1000 transactions on average” Algorithm security “Student information will be encrypted with AES-128”

Performance Requirements Response Time “For requests for open sections, the system response time should be less than 2 seconds in 90% of cases under normal load” Throughput “The server should be able to handle 100 requests per second on a Dell T-130 running Windows 7 and Apache Tomcat” Can usually test empirically