Key Concepts
There is no single correct process. Choose based on your project type, team size, and how well requirements are understood.
Waterfall Model
Sequential phases: Requirements → Design → Implementation → Testing → Maintenance. Move down, never up.
Incremental Development
Develop the system in increments. Each increment delivers a working slice of functionality.
Reuse-Oriented Engineering
Base the system on integrating existing components — COTS, open-source, services.
4 Process Activities
Every software process — regardless of model — contains these 4 core activities.
Concept Deep Dives
Click each concept to expand — real examples, diagrams, pros & cons.
Waterfall Model
When to Use
Requirements are well-understood upfront. Safety-critical systems. Fixed contracts.
Real-World Example
Defense systems, medical device firmware, large government contracts where requirements are locked.
✓ Advantages
- Easy to manage — clear phase boundaries
- Good for stable, well-understood requirements
- Strong documentation
⚠ Watch Out
- Inflexible to change
- Customer only sees product at the end
- Late discovery of problems = expensive fixes
Incremental Development
When to Use
Requirements likely to change. Customer needs early delivery. Internet-based systems.
Real-World Example
Spotify — shipped a minimal music player first, then added playlists, recommendations, podcasts incrementally.
✓ Advantages
- Early value delivery
- Easier to accommodate change
- Customer feedback improves each increment
⚠ Watch Out
- Process less visible (no clear phases)
- Structure degrades without refactoring
- Hard to manage large teams
Reuse-Oriented Engineering
When to Use
Suitable components exist. Budget-constrained. Faster time-to-market needed.
Real-World Example
A startup building a SaaS app: uses Stripe (payments), Auth0 (auth), Twilio (SMS) — all existing services.
✓ Advantages
- Reduced cost and risk
- Faster delivery
- Battle-tested components
⚠ Watch Out
- Loss of control over evolution
- Components may not fit exactly
- Security risk from third-party dependencies
4 Process Activities
When to Use
Always applicable — waterfall runs them sequentially, agile interleaves them.
Real-World Example
Even a solo weekend project: you decide what to build (spec), write it (dev), test it (validate), then update it (evolve).
✓ Advantages
- Universal framework for understanding any process
- Maps to team roles and responsibilities
⚠ Watch Out
- Oversimplification — real projects are messier
Quick Reference
- 1Software processes are the coherent sets of activities for producing a software system.
- 2Waterfall: plan-driven, sequential phases. Good for stable requirements.
- 3Incremental development: interleaves specification, development, validation. Cheaper to accommodate changes.
- 4Reuse-oriented: based on systematic reuse. Reduces development costs and risks but may not meet exact user needs.
- 5All processes include: specification, development, validation, and evolution.
- 6Process improvement: CMMI framework measures process maturity on a 5-level scale.
From the Book & Beyond
Case Study — The Waterfall Model (Royce, 1970)
Sommerville traces waterfall — the first published software process model — to engineering processes used in large military systems (Royce, 1970). Each stage produces documents that are "signed off" before the next phase starts, from requirements definition down to operation and maintenance, normally the longest phase of all. The classic failure mode: teams prematurely freeze the specification, so the finished system won't do what users want and design flaws get patched with implementation tricks. Sommerville says waterfall really fits only embedded systems, critical systems needing up-front safety analysis, and large multi-company projects.
2026 Perspective — CI/CD Ate the Process Diagram
The dominant process in 2026 isn't in this chapter by name: trunk-based development plus CI/CD, where every commit is automatically built, tested, and often deployed — incremental development pushed to its logical extreme, with releases measured in hours, not months. But Sommerville's plan-driven vs. incremental split still earns its keep: avionics, medical devices, and multi-vendor government contracts still demand signed-off specifications. So most real organizations run hybrids — plan-driven at the architecture and compliance level, continuous delivery inside the team. Knowing when each fits is the actual skill.
Quiz — Test Yourself
Think through your answer first, then reveal.