SDLC definition
The software development life cycle (SDLC) is the structured process teams follow to plan, design, build, test, deploy and maintain software. It breaks development into defined phases with clear outputs, helping teams control quality, cost and timelines. Different SDLC models, such as Waterfall, Agile, iterative and spiral, organize these phases in different ways.
What are the phases of the SDLC?
Most descriptions of the SDLC include the same core phases, even when they name or group them differently. In a sequential model each phase finishes before the next begins, while in iterative models the team cycles through all of them repeatedly in small slices. Either way, skipping a phase usually means its work happens later, at higher cost and under more pressure. Good teams also define clear exit criteria for each phase, such as approved designs or a passing test suite.
- Planning: goals, scope, budget, risks and feasibility.
- Requirements analysis: what users and the business need.
- Design: architecture, data models, interfaces and UX.
- Development: writing and reviewing code.
- Testing: verifying functionality, performance and security.
- Deployment: releasing to production.
- Maintenance: fixes, updates and improvements over time.
Common SDLC models
Waterfall runs the phases in strict sequence and suits projects with stable, well-documented requirements. The V-model pairs each development phase with a matching testing phase and is common in safety-critical industries. Iterative and incremental models deliver the system in pieces, refining with each cycle. The spiral model adds explicit risk analysis to every loop, which suits large, high-risk projects.
Agile models, including Scrum and Kanban, compress all phases into short cycles and release working software frequently. DevOps extends the life cycle into operations, using automation to make building, testing and deploying continuous rather than separate stages. Most modern teams combine Agile planning with DevOps automation, so code moves from commit to production through the same tested pipeline every time. AI coding agents now draft code, tests and reviews inside this loop, which speeds development but makes clear requirements, automated checks and human review more important, not less.
What is a secure SDLC?
A secure SDLC builds security into every phase instead of testing for it at the end. Teams define security requirements during planning, run threat modeling during design, use static analysis, dependency scanning and software supply chain controls such as SBOMs and signed builds during development, and run dynamic testing and penetration tests before release. Frameworks such as the NIST Secure Software Development Framework and OWASP SAMM describe these practices in detail.
Shifting security left is cheaper and more effective. A design flaw found in a threat modeling session costs a meeting to fix, while the same flaw found after launch may require a redesign, an incident response and customer notifications. Security champions inside each team help keep these practices alive between formal audits.
How to choose an SDLC model
Consider how stable the requirements are, how much regulation applies, how often stakeholders can give feedback and how costly late changes would be. New digital products with evolving requirements suit Agile. Regulated systems with fixed specifications may need more formal phases and documentation. Many organizations blend models, for example Agile delivery inside a governance framework with formal approvals at major milestones.
Whatever the model, the life cycle does not end at launch. Maintenance often accounts for a large share of a system's total cost, so plan for monitoring, updates and support from the start. Nexzem tailors its delivery process to each client's compliance and feedback needs rather than applying one fixed model.