Waterfall, In the Large, and In the Small

There’s lots of ways to slice and dice software process. In Scenarios, Stories, Use Cases: Through the Systems Development Life-Cycle, Ian F. Alexander and Neil Maiden show the waterfall model, systems engineering ‘in the large’ , and systems engineering ‘in the small.’

Waterfall Life Cycle Elements

Alexander and Maiden outline the key sequence of steps in the waterfall model based on Royce (1970).

• Requirements Discovery
• Requirements Validation
• System Specification
• System Design
• Coding, First of Class
• Integration & Testing
• Operations & Maintenance

Systems Engineering ‘In the Large’ Phase Sequence

Alexander and Maiden propose a set of  macro activities and project phases that represent systems engineering ‘in the large’ to distinguish from the waterfall sequence of steps:

• Explore Concept
• Proof of Concept
• Full Development
• Production
• Installation
• In-service Support
• Disposal

Systems Engineering ‘In the Small’ Activity Sequence

Alexander and Maiden suggest a set of micro activities that represent systems engineering ‘in the small.’

• User Requirements Definition
• System Requirements Definition
• System Design/Architecture
• Preliminary Design
• Detailed Design
• Implementation
• Test and Integration
• Acceptance and Certification

In the Large Process Models

Alexander and Maiden provide an example of ‘in the large’ systems engineering process models:

• Evolutionary model
• Incremental model
• US DoD model
• UK Ministry of Defense CADMID model
• RUP (Rational Unified Process) – (RUP 2003)
• ‘Agile Methods’ (e.g. Extreme Programming (XP))

Evolutionary, Incremental, and High-Risk Model

Alexander and Maiden suggest that you can group the ‘in the large’  process models into the following three models:

• Evolutionary model
• Incremental model
• High-Risk model

Combining Models

Alexander and Maiden suggest that it’s possible to combine models ‘ in the large’ as necessary:

• Evolutionary and Incremental
• Evolutionary and High-Risk
• Incremental and High Risk
• Evolutionary, Incremental, and High Risk

Key Take Aways

Here’s my key take aways:

• By identifying and labeling systems engineering ‘in the small’ and ‘in the large’ you can more accurately and more precisely discuss the two levels of software process.
• Don’t confuse the systems engineering ‘in the large’ with the Waterfall sequence of stages.
• You can use the systems engineering ‘in the large’ and ‘in the small’ activities to define software processes.
• Waterfall Life-Cycle stages include: Requirements Discovery, Requirements Validation, System Specification, System Design, Coding, First of Class, Integration & Testing, Operations & Maintenance
• Systems engineering ‘in the large’ includes the following activities/phases: Explore Concept, Proof of Concept, Full Development, Production, Installation, In-service Support, Disposal
• Systems engineering ‘in the small’ includes: User Requirements Definition, System Requirements Definition, System Design/Architecture, Preliminary Design, Detailed Design, Implementation, Test and Integration, Acceptance and Certification