Cleanroom Software Engineering - Advantages, Principles and Process Teams

Cleanroom software engineering involves the integrated use of software engineering modeling, program verification and statistical software quality assurance.
- Cleanroom software engineering verifies design specification using mathematically-based proof of correctness.
- Cleanroom software engineering relies heavily on statistical use testing to uncover high impact errors.
- Cleanroom software engineering generally follows an incremental development process.

CLEANROOM PRINCIPLES

- Small Teams include independent specification, development, and certification sub-teams.
- Incremental development under statistical quality control.
- Software development is based on mathematical principles. The box principle is used for specification and design. The formal verification is used to confirm correctness of implementation of specification. The program correctness is verified by team reviews using questionnaires.
- Testing is based on statistical principles.

CLEANROOM PROCESS TEAMS

- Specification team develops and maintains the system specification.
- Development team develops and verifies software. The software is not compiled or executes during verification.
- Certification team develops set of statistical test to exercise software after development. Reliability growth models are used to assess reliability.

BENEFITS OF CLEANROOM SOFTWARE ENGINEERING

- Zero failures in the field which is a goal but a realistic expectation is<5 failures per KLOC on first program execution in the first team project.
- Short development cycles
- Longer product life.

Cleanroom Testing - Statistical Use Testing and Certfication

Clean room testing is different from conventional testing approaches. The goal of clean room testing is to validate software requirements by demonstrating that a statistical sample of use-cases have been executed successfully.

Statistical use testing tests the software in a way the users intend to use it. the clean room testing teams determine the usage probability distribution or the software. Each increment's specification is analyzed to define a set of inputs or events that cause the software to change its behavior. A probability of use is assigned to each input or event based on the interviews with users, the creation of usage scenarios and a general understanding o application domain. The testing team executes these use cases and verifies software behavior against the specification for the system. Using the recorded interval times, the certification team can compute mean time to failure.

Within the clean room software engineering approach, certification implies that the reliability can be specified for each component. Reusable software components can be stored along with their usage scenarios, program stimuli, and probability distributions. The certification approach involves :
- usage scenarios must be created.
- usage profile is specified.
- test cases are generated from profile.
- tests are executed and failure data are recorded and analyzed.
- reliability is computed and certified.

Certification for clean room software engineering requires creation of three models:
- Sampling Model : Certification is given if no failure or a specified number of failures occur after executing m random test cases. The value of m is derived mathematically.
- Component Model : This model enables the analyst to determine the probability that component i will fail prior to completion.
- Certification Model : The overall reliability of the system is projected and certified.