The Software Development Lifecycle (SDLC)

The Software Development Lifecycle (SDLC) is the overall process of planning, creating, testing, and deploying software. Different development methodologies structure this process differently.

Key stages common to most methodologies:

1. Requirements gathering / analysis
2. Design
3. Implementation (coding)
4. Testing
5. Deployment
6. Maintenance

Waterfall Model

The Waterfall model is a sequential, linear approach. Each stage must be completed before the next begins. Once a stage is finished, the team does not return to it.

Stages:

Requirements → Design → Implementation → Testing → Deployment → Maintenance

Advantages:

• Simple, easy to understand and manage — clear milestones and deliverables at each stage.
• Works well when requirements are well-defined and unlikely to change.
• Documentation is produced at each stage.
• Easy to estimate timescales and costs upfront.

Disadvantages:

• Very difficult to accommodate changes to requirements once development has begun.
• Testing occurs late — bugs discovered late are expensive to fix.
• Client sees no working software until the very end.
• High risk: if requirements were wrong, the entire product is wrong.

Suitable for:

• Projects with well-defined, stable, unlikely-to-change requirements (e.g. safety-critical systems, government contracts).
• Smaller, less complex projects.

Agile Methodology

Agile is an iterative, incremental approach. Work is divided into short cycles called sprints (typically 1–4 weeks). Each sprint produces a potentially shippable product increment. Requirements can change between sprints.

Key Agile principles (from the Agile Manifesto):

• Individuals and interactions over processes and tools.
• Working software over comprehensive documentation.
• Customer collaboration over contract negotiation.
• Responding to change over following a plan.

Advantages:

• Flexible — requirements can evolve as the client's understanding develops.
• Client is involved throughout, seeing working software frequently.
• Testing is continuous — bugs found and fixed earlier.
• Higher client satisfaction — product better matches actual needs.
• Risks identified early due to frequent deliveries.

Disadvantages:

• Difficult to predict total cost and timescale at the start.
• Requires constant client involvement — may not suit all clients.
• Less emphasis on documentation — future maintainers may struggle.
• Scope creep — frequent requirement changes can delay completion.
• Requires experienced, self-organising teams.

Examples of Agile frameworks:

• Scrum: sprints, daily stand-ups, sprint reviews.
• Extreme Programming (XP): pair programming, test-driven development.
• Kanban: visual workflow management.

Spiral Model

The Spiral model combines iterative development with risk analysis. Each iteration (spiral) consists of four quadrants:

1. Determine objectives — identify goals, constraints, alternatives.
2. Identify and resolve risks — analyse risks, create prototypes.
3. Develop and test — implement and verify the solution for this cycle.
4. Plan next iteration — review and plan the next spiral.

Advantages:

• Risk analysis at every stage reduces the chance of project failure.
• Suitable for large, high-risk projects where requirements are not fully known.
• Prototyping in each cycle allows early client feedback.

Disadvantages:

• Can be expensive and time-consuming due to the overhead of risk analysis at each cycle.
• Requires specialist expertise in risk assessment.
• Less suitable for small, low-risk projects.

Prototyping

In a prototyping approach, a working model (prototype) of the system is built quickly to gather feedback before building the full system. There are two main types:

• Throwaway prototyping: prototype is built to gather requirements/feedback, then discarded. The actual system is built from scratch.
• Evolutionary prototyping: the prototype is refined and extended until it becomes the final system.

Advantages:

• Reduces misunderstandings — clients can see and interact with something tangible.
• Requirements are clarified early, reducing rework.

Disadvantages:

• Clients may assume the prototype IS the finished product.
• Poor code quality if the prototype evolves into the final product without restructuring.

Rapid Application Development (RAD)

RAD is an approach that emphasises speed of development over rigorous planning. It uses rapid prototyping, constant user feedback, and intensive parallel small-team development.

• Works best for systems that can be modularised.
• Sacrifices some planning rigour for speed.

Comparison Table