Conventional waterfall model

The conventional waterfall model is a linear sequential software development model in which the project moves through phases in order, and each phase must be completed and approved before the next begins. The flow goes downward like a waterfall. It was described by Winston Royce in 1970 and is a predictive life cycle.

Requirements
     |
     v
   Design
     |
     v
 Implementation
     |
     v
   Testing
     |
     v
 Deployment
     |
     v
 Maintenance

Phases

1. Requirements analysis: all requirements are collected and documented (SRS).
2. System design: architecture, database, interface design.
3. Implementation: coding of units and unit testing.
4. Integration and testing: system and acceptance testing.
5. Deployment: delivery to customer, installation and training.
6. Maintenance: bug fixes, enhancements.

Each phase produces documents, which are reviewed and signed off.

Advantages

• Simple and easy to understand and manage.
• Clear milestones, deliverables and documentation.
• Good for projects with stable, well-understood requirements.
• Easy to estimate and schedule.

Disadvantages

• Not flexible: changing requirements are costly.
• Working software is seen only late in the project.
• High risk and uncertainty if requirements are wrong.
• Testing occurs late; problems are found late.
• Poor for long, complex or evolving projects.

When to use

Small to medium projects with fixed requirements, such as payroll systems, or regulated environments where documentation is mandatory.

Modified versions allow feedback to the previous phase (waterfall with feedback) and the V-model adds testing activities for each phase.