SDLC — Software Development Life Cycle
The SDLC is the journey a piece of software takes from idea to retirement. Whatever the methodology (waterfall, agile, DevOps), the same phases appear:
Requirement analysis
Understand what the software must do. Output: requirements the whole team agrees on.
Design
Architects decide how the system will be built — components, data, interfaces.
Implementation (coding)
Developers write the actual software against the design.
Testing
The build is verified against requirements — this is where STLC runs in full.
Deployment
The software is released to real users.
Maintenance
Bug fixes, patches and improvements for as long as the product lives.
Requirements: pick date/time, min 1h ahead. Design: new
scheduler service. Code: 2 sprints. Testing: does a 55-min
slot get rejected? Deploy: 10% rollout. Maintain: fix the
timezone bug users found in week one.
STLC — Software Testing Life Cycle
The STLC is the tester's own pipeline. It doesn't wait for the SDLC "testing" phase — good teams start it as soon as requirements exist:
Requirement analysis
Testers read requirements looking for the testable — and the ambiguous. "Fast loading" isn't testable; "loads under 2s on 4G" is.
Test planning
Decide scope, approach, tools, environments, schedule and who tests what. Output: the test plan.
Test case development
Write concrete test cases: steps, data, expected results. "Enter 7-char password → expect rejection message."
Environment setup
Prepare the test bed: servers, devices, test data, builds — so results are reproducible.
Test execution
Run the cases, log results, report defects with clear reproduction steps, retest fixes.
Test closure
Summarize coverage, defects found, lessons learned. Sign off — or explain why not.
SDLC vs STLC at a glance
| Aspect | SDLC | STLC |
|---|---|---|
| Goal | Build and deliver working software | Verify the software meets requirements |
| Scope | The whole product journey | The testing slice of that journey |
| Owner | Entire team (PM, dev, QA, ops) | QA / testing team |
| Starts | At the product idea | As soon as requirements are readable |
| Key outputs | Requirements, design, code, releases | Test plan, test cases, defect reports, sign-off |
| Relationship | The container | Runs inside SDLC, in parallel with development |
The takeaway: STLC is not a phase that starts when coding ends. The earlier testing thinking enters the SDLC, the cheaper every defect becomes.