Software Engineering basics - Formal Requirement

In Software Engineering, a formal requirement is a type of requirement that is expressed in a precise, mathematical, or formally defined language rather than natural language. These requirements are unambiguous, rigorously defined, and can be analyzed or verified using formal methods.

1. Definition

Formal Requirement:

A requirement that is specified using a formal notation or mathematical logic to remove ambiguity and allow precise verification.

2. Characteristics

  • Precise: Leaves no room for interpretation.

  • Unambiguous: Clear meaning for both humans and machines.

  • Verifiable: Can be checked or proven mathematically or through formal analysis.

  • Suitable for critical systems: Often used in safety-critical domains like aviation, medical, or nuclear software.

3. Examples

  1. Mathematical Expression:

    • “For all input values x, where 0 ≤ x ≤ 100, the output y = 2x + 5.”*

  2. Temporal Logic:

    • “Whenever the emergency button is pressed, the alarm must sound within 1 second.”

  3. Set-based Specification:

    • “The system shall allow only registered users u ∈ Users to access the dashboard.”

4. Advantages

  • Eliminates ambiguity found in natural language requirements.

  • Allows formal verification and proof of correctness.

  • Reduces errors in critical systems.

5. Disadvantages

  • Requires specialized knowledge in formal methods.

  • Can be time-consuming and complex to create.

  • Not always practical for small or non-critical projects.

Summary

Formal requirements are rigorously defined requirements written in mathematical or formal notation, ensuring precision, verifiability, and lack of ambiguity, commonly used in safety-critical and high-reliability systems.