Software Development Life Cycle (SDLC) is a structured process used by software engineers to design, develop, test, and deploy software applications efficiently. It defines the phases of software development to ensure high-quality software is delivered on time and within budget.
Phases of SDLC
1️⃣ Planning
- Define project goals and scope
- Identify risks and feasibility study
- Allocate resources and create a project schedule
2️⃣ Requirement Analysis
- Gather functional and non-functional requirements
- Document Software Requirement Specification (SRS)
- Identify business and technical needs
3️⃣ Design
- System Architecture and Database Design
- UI/UX design and wireframing
- Design models: UML diagrams, Flowcharts
4️⃣ Implementation (Coding)
- Developers write code based on design documents
- Follow coding standards and best practices
- Use version control (Git, GitHub)
5️⃣ Testing
- Perform Unit Testing, Integration Testing, System Testing
- Identify and fix bugs before deployment
- Automated & Manual testing
6️⃣ Deployment
- Release the software to users (on-premise or cloud)
- Deploy updates and patches if necessary
7️⃣ Maintenance
- Fix bugs and security vulnerabilities
- Add new features based on user feedback
- Optimize performance over time
SDLC Models
🔹 Waterfall Model – Linear & Sequential (Best for simple projects)
🔹 Agile Model – Iterative & flexible (Best for evolving projects)
🔹 Spiral Model – Risk-focused & iterative (Used in complex projects)
🔹 V-Model – Testing is planned in parallel with development
🔹 RAD (Rapid Application Development) – Quick development with prototypes
Waterfall Model in SDLC
The Waterfall Model is a linear and sequential software development methodology where each phase of the software development life cycle (SDLC) must be completed before moving to the next phase. It follows a top-down approach and does not allow returning to previous stages once a phase is completed.
Phases of the Waterfall Model
1️⃣ Requirement Gathering & Analysis
- Collect all functional and non-functional requirements.
- Create a Software Requirement Specification (SRS) document.
- Requirements are fixed at this stage, and no changes are allowed later.
2️⃣ System Design
- Based on requirements, system architecture and design are planned.
- High-Level Design (HLD): Defines overall system architecture.
- Low-Level Design (LLD): Focuses on module-level details.
3️⃣ Implementation (Coding)
- Developers write code based on the design documents.
- Each module is developed and tested individually.
4️⃣ Testing
- All modules are integrated and tested as a complete system.
- Types of testing: Unit Testing, Integration Testing, System Testing, User Acceptance Testing (UAT).
- Bugs are fixed, but new features cannot be added.
5️⃣ Deployment
- The software is deployed to the production environment for users.
- Final adjustments are made, if necessary.
6️⃣ Maintenance
- Fixing bugs reported by users.
- Providing updates and patches for performance improvements.
Advantages of Waterfall Model
✅ Simple & Easy to Manage – Clear structure with defined phases.
✅ Well-Documented Process – Each phase has proper documentation.
✅ Best for Small, Well-Defined Projects – Works well when requirements are fixed.
✅ Testing is Systematic – Testing happens only after development is complete.
Disadvantages of Waterfall Model
❌ Rigid & Inflexible – No scope for changing requirements after the project starts.
❌ Late Testing – Bugs are detected late in the process, making them costly to fix.
❌ Slow & Time-Consuming – Entire development must be completed before testing begins.
❌ Not Suitable for Complex or Evolving Projects – Does not accommodate feedback during development.
When to Use the Waterfall Model?
✅ When requirements are well-defined and unlikely to change.
✅ When the project is small and simple.
✅ When proper documentation is required.
✅ When the technology used is well-understood.
Waterfall Model vs. Agile Model
| Feature | Waterfall Model | Agile Model |
|---|---|---|
| Approach | Linear & Sequential | Iterative & Flexible |
| Requirement Changes | Not Allowed | Allowed Anytime |
| Testing Phase | After Development | Continuous |
| Best For | Small, Fixed Projects | Dynamic, Evolving Projects |
| Delivery Time | At the End of Development | After Every Sprint (2-4 weeks) |
Agile Model in SDLC
The Agile Model is an iterative and incremental software development methodology that focuses on flexibility, collaboration, and customer feedback. Unlike the Waterfall Model, where each phase is completed before moving forward, Agile breaks the project into small cycles (Sprints), allowing continuous improvements and frequent releases.
Key Features of Agile Model
✅ Iterative Development – Software is built in small, manageable parts called Sprints (usually 2-4 weeks).
✅ Customer Collaboration – Frequent interactions with clients for feedback and requirement adjustments.
✅ Flexibility – Requirements can change at any stage.
✅ Continuous Testing & Integration – Testing happens throughout development.
✅ Faster Delivery – Delivers working software frequently instead of waiting for a complete product.
Phases of Agile Model (Using Scrum Framework)
1️⃣ Concept / Requirement Gathering
- Define high-level requirements and business objectives.
- Prioritize features in the Product Backlog.
2️⃣ Sprint Planning
- Select features from the backlog for the next Sprint.
- Break tasks into small development items.
3️⃣ Design & Development (Sprint Execution)
- Developers build the selected features within the sprint.
- Daily stand-up meetings for progress updates.
4️⃣ Testing & Quality Assurance
- Continuous testing (Unit, Integration, System Testing).
- Identify and fix issues before release.
5️⃣ Sprint Review & Feedback
- Deliver a working product increment at the end of the sprint.
- Gather customer feedback and improve in the next sprint.
6️⃣ Deployment & Maintenance
- Deploy the new version to production.
- Provide support and enhancements based on user feedback.
Popular Agile Frameworks
🔹 Scrum – Work is divided into Sprints with daily stand-up meetings.
🔹 Kanban – Uses a visual board to track progress (To-Do, In Progress, Done).
🔹 Extreme Programming (XP) – Focuses on high-quality coding with continuous testing.
🔹 Lean Development – Aims to reduce waste and increase efficiency.
Advantages of Agile Model
✅ Highly Flexible – Can adapt to changes anytime.
✅ Early & Continuous Delivery – Working software is released frequently.
✅ Customer Satisfaction – Clients can see progress and give feedback.
✅ Better Risk Management – Bugs and issues are found early.
✅ Improved Team Collaboration – Daily stand-ups improve communication.
Disadvantages of Agile Model
❌ Requires High Customer Involvement – Frequent feedback is needed.
❌ Difficult to Predict Costs & Timeline – Since requirements change frequently.
❌ Not Ideal for Small Projects – Works best for medium-to-large projects with evolving needs.
❌ Needs Experienced Team – Requires self-organized and skilled developers.
When to Use the Agile Model?
✅ When requirements are expected to change frequently.
✅ When fast delivery of working software is needed.
✅ When a customer-centric approach is required.
✅ When working with a collaborative and skilled development team.
Waterfall Model vs. Agile Model
| Feature | Waterfall Model | Agile Model |
|---|---|---|
| Approach | Linear & Sequential | Iterative & Incremental |
| Requirement Changes | Not Allowed | Allowed Anytime |
| Testing Phase | After Development | Continuous Testing |
| Delivery Time | At the End of Development | After Every Sprint (2-4 weeks) |
| Customer Involvement | Low | High |
| Flexibility | Low | High |
Spiral Model in SDLC
The Spiral Model is a risk-driven software development model that combines elements of the Waterfall Model and Iterative Development. It is best suited for large, complex, and high-risk projects. The model focuses on continuous risk analysis and prototyping, ensuring that potential issues are identified early in the development process.
Phases of the Spiral Model
Each cycle in the Spiral Model is called a spiral and consists of four main phases:
1️⃣ Planning (Identification of Requirements)
- Gather system requirements (similar to Waterfall’s requirement phase).
- Identify constraints, objectives, and risks.
- Plan the project timeline and resources.
2️⃣ Risk Analysis (Evaluate Risks & Develop Prototypes)
- Identify possible risks (technical, cost-related, schedule-related).
- Create a prototype or a small version of the system to reduce risks.
- Stakeholders review the prototype and provide feedback.
3️⃣ Engineering (Development & Testing)
- Develop the actual system based on the refined requirements.
- Test the software for errors and ensure functionality.
- The system is refined and improved with each spiral iteration.
4️⃣ Evaluation & Review (Customer Feedback & Planning Next Phase)
- Present the developed part of the system to stakeholders.
- Gather feedback and modify the system accordingly.
- Plan the next iteration based on the updated requirements and risks.
🚀 This process repeats in multiple spirals until the final product is fully developed.
Advantages of the Spiral Model
✅ Risk Management – Best for high-risk projects, as risks are continuously analyzed.
✅ Customer Involvement – Frequent feedback ensures the product meets user needs.
✅ Flexible & Iterative – New changes can be incorporated in each iteration.
✅ Early Detection of Issues – Prototyping helps identify problems early.
✅ Suitable for Large & Complex Projects – Helps in managing big projects efficiently.
Disadvantages of the Spiral Model
❌ Expensive – High cost due to continuous risk analysis and prototyping.
❌ Time-Consuming – Multiple iterations can take longer to complete.
❌ Complex Process – Requires experienced risk assessment and management.
❌ Not Ideal for Small Projects – Best suited for large projects with significant risks.
When to Use the Spiral Model?
✅ When the project is large and complex.
✅ When frequent risk assessment is necessary.
✅ When requirements are expected to change.
✅ When developing high-risk software (e.g., banking, healthcare, aerospace).
Spiral Model vs. Other Models
| Feature | Spiral Model | Waterfall Model | Agile Model |
|---|---|---|---|
| Approach | Iterative with risk analysis | Linear & Sequential | Iterative & Incremental |
| Risk Management | High | Low | Moderate |
| Customer Involvement | High | Low | High |
| Flexibility | Medium | Low | High |
| Best For | Large, High-Risk Projects | Small, Well-Defined Projects | Fast, Evolving Projects |
V-Model (Verification and Validation Model)
The V-Model, also known as the Validation and Verification Model, is an extension of the Waterfall Model where each development phase is associated with a corresponding testing phase. The shape of the model resembles a V, where the left side represents development (Verification), and the right side represents testing (Validation).
Phases of the V-Model
The V-Model consists of two main parts:
1️⃣ Verification (Development Phase – Left Side of V)
2️⃣ Validation (Testing Phase – Right Side of V)
1️⃣ Verification (Development Phases) – LEFT SIDE
✅ Requirement Analysis
- Gather and analyze customer requirements.
- Create a System Requirement Specification (SRS) document.
✅ System Design
- Design the system architecture and high-level modules.
✅ Architectural Design
- Break down the system into smaller modules.
- Define data flow and module interactions.
✅ Module Design
- Detailed design of each module (data structures, algorithms, etc.).
✅ Coding Phase
- Write the actual code based on module design.
2️⃣ Validation (Testing Phases) – RIGHT SIDE
✅ Unit Testing (Corresponds to Module Design)
- Tests individual modules for functionality and correctness.
✅ Integration Testing (Corresponds to Architectural Design)
- Tests how different modules interact with each other.
✅ System Testing (Corresponds to System Design)
- Tests the entire system against requirements.
- Checks performance, security, and usability.
✅ User Acceptance Testing (Corresponds to Requirement Analysis)
- Ensures the software meets customer needs before deployment.
Advantages of V-Model
✅ Highly Structured & Disciplined – Each development phase has a corresponding testing phase.
✅ Early Bug Detection – Errors are caught in earlier stages, reducing costs.
✅ Easy to Manage – Clearly defined steps make tracking progress simple.
✅ Best for Safety-Critical Systems – Used in medical, aviation, and automotive software.
Disadvantages of V-Model
❌ Less Flexible – Changes in requirements are difficult to handle once development starts.
❌ High Cost for Mid-Stage Fixes – Errors found later require major rework.
❌ Not Suitable for Evolving Requirements – Works best for projects with clear and fixed requirements.
When to Use the V-Model?
✅ When requirements are well-defined and stable.
✅ When quality is a top priority (e.g., banking, medical, automotive software).
✅ When early testing is required to prevent major defects.
V-Model vs. Other SDLC Models
| Feature | V-Model | Waterfall Model | Agile Model | Spiral Model |
|---|---|---|---|---|
| Approach | Sequential with Testing at Every Phase | Linear & Sequential | Iterative & Incremental | Iterative with Risk Analysis |
| Testing | Done at every phase (Verification & Validation) | Done at the end | Continuous Testing | Continuous Risk Analysis & Prototyping |
| Flexibility | Low | Low | High | Medium |
| Best For | High-Reliability Systems | Small, Simple Projects | Fast-Changing Projects | High-Risk, Large Projects |
Rapid Application Development (RAD) Model
What is the RAD Model?
The Rapid Application Development (RAD) Model is an iterative and adaptive software development approach that emphasizes quick prototyping and user feedback over long planning phases. It is best suited for projects where requirements change frequently and speed is a priority.
Instead of following a strict step-by-step process like the Waterfall Model, RAD focuses on building functional prototypes quickly, getting user feedback, and refining the product through multiple iterations.
Phases of the RAD Model
The RAD Model consists of four main phases:
1️⃣ Requirements Planning
- Identify business needs and project scope.
- Gather high-level requirements in a short time.
- No extensive documentation—focus is on understanding the core features.
2️⃣ User Design (Prototyping Phase)
- Build interactive prototypes to visualize the system.
- Get immediate user feedback and refine designs.
- Developers and users collaborate closely.
3️⃣ Rapid Construction (Development & Testing)
- Convert prototypes into actual functional components.
- Code is quickly developed and tested in small cycles.
- Continuous feedback from users ensures that the product aligns with expectations.
4️⃣ Deployment & Cutover
- Final system is tested, deployed, and delivered.
- Minimal final testing required since testing is done in every iteration.
- Users receive training (if necessary) and the system goes live.
Advantages of the RAD Model
✅ Faster Development – Prototypes and iterations allow quick completion.
✅ User Involvement – Continuous feedback ensures the software meets expectations.
✅ Flexibility – Easy to accommodate changes in requirements.
✅ Better Quality – Regular testing and user feedback improve software quality.
✅ Reduced Risk – Issues are identified early through prototyping.
Disadvantages of the RAD Model
❌ Not Suitable for Large, Complex Projects – Works best for small to medium-sized applications.
❌ Requires Continuous User Involvement – Users must actively participate in testing and feedback.
❌ High Dependence on Skilled Developers – Requires experienced developers for rapid prototyping.
❌ Difficult to Manage for Large Teams – Requires close collaboration and quick decision-making.
When to Use the RAD Model?
✅ When requirements are not fixed and may change frequently.
✅ When speed is a priority (e.g., startups, time-sensitive projects).
✅ When developing small to medium-sized projects.
✅ When users can provide continuous feedback throughout development.
RAD Model vs. Other SDLC Models
| Feature | RAD Model | Waterfall Model | Agile Model | Spiral Model | V-Model |
|---|---|---|---|---|---|
| Approach | Iterative & Prototype-Based | Sequential | Iterative & Incremental | Iterative with Risk Analysis | Sequential with Testing at Every Phase |
| Speed | Fast | Slow | Fast | Medium | Medium |
| Flexibility | High | Low | High | Medium | Low |
| User Involvement | High | Low | High | Medium | Low |
| Best For | Small-Medium Fast Projects | Simple, Stable Projects | Rapidly Changing Needs | Large, Risky Projects | High-Reliability Systems |