requirements engineering

Requirement

A description/specification of what a system should do or a constraint it must satisfy.

Functional requirement

Services the system should provide.

Non-functional requirement

Constraints on services/functions or development (e.g., performance, reliability, location).

Domain requirement

Requirements derived from the application domain, may be functional or non-functional.

Software requirements document

Official statement of what developers must build.

User requirements

Natural language + diagrams for customers.

System requirements

Structured detailed descriptions as contract.

Software specification

Detailed design basis for developers.

Happy day scenario

Everything works smoothly.

Rainy day scenario

Errors, exceptions, or unusual flows.

Requirements elicitation

Asking stakeholders what they need.

Requirements analysis

Checking requirements for sense and conflicts.

Requirements validation

Ensuring they solve the user's problem.

Requirements management

Handling requirement changes over time.

Requirements confusion

Mixing functional and non-functional.

Requirements amalgamation

Several requirements expressed together.

Ambiguity

Words interpreted differently by readers/writers.

Over-flexibility

Same requirement expressed in many ways.

Structured natural language

Using templates to reduce ambiguity.

Form-based specification

Fill-in forms with inputs/outputs/preconditions.

Program Description Language (PDL)

Pseudo-code style operational specification.

Mathematical specification

Using formal notations (finite-state machines, sets).

Enduring requirements

Stable, long-term requirements (e.g., hospitals always need doctors).

Volatile requirements

Change during development or system use.

Mutable requirements

Change due to environmental shifts.

Emergent requirements

Appear as understanding evolves.

Consequential requirements

Emerge from introducing the system.

Compatibility requirements

Depend on other systems or processes.

Use case

Scenario-based interaction description in UML.

Sequence diagram

Shows sequence of events for use cases.

Ethnography

Observing people in their real environment to understand requirements.