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The Six Phases of the SDLC—Explained for Practitioners

November 7, 2025 • 4 min read • Gerardo Perrucci

The Six Phases of the SDLC—Explained for Practitioners

Table of Contents

Introduction

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"Programs must be written for people to read, and only incidentally for machines to execute." — Harold Abelson & Gerald Jay Sussman

The Software Development Life Cycle (SDLC) is the backbone of disciplined software delivery. It gives teams a shared vocabulary and a predictable path from idea to reliable software. International standards such as ISO/IEC/IEEE 12207 and bodies of knowledge like SWEBOK formalize these processes so organizations can scale quality and repeatability.

1) Planning

Goal: Align on business outcomes, users, scope, risks, and success metrics.

What to produce

  • Problem statement and measurable goals (e.g., "reduce checkout drop-off by 15%").
  • Stakeholder map, constraints, and high-level scope.
  • Risk register and go/no-go criteria.

Tips

  • Write one page that explains value, not features.
  • Choose a delivery model (Waterfall, Iterative, Agile) based on risk and uncertainty (see "Which SDLC model when?" below).

2) Design

Goal: Create the blueprint: system architecture, contracts, UX flows, and test strategy.

What to produce

  • Architecture diagram, module boundaries, data contracts.
  • UX flows and accessibility criteria.
  • Non-functional requirements (reliability, latency, security, cost).

Tips

  • Design for observability: logs, metrics, and traces are first-class design artifacts.
  • Decide how the system will be tested and deployed before you code.

3) Development

Goal: Turn the blueprint into working, well-tested code.

Practices that pay off

  • Short-lived branches, small PRs, continuous integration (CI).
  • Pairing or lightweight reviews to keep feedback fast.
  • Keep tests close to the code; automate everything you can.
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"Any fool can write code that a computer can understand. Good programmers write code that humans can understand." — Martin Fowler

4) Testing

Goal: Prove correctness, protect quality, and manage risk before release.

What to cover

  • Unit, integration, and contract tests for critical paths.
  • Non-functional tests (performance, security, accessibility).
  • Test data strategy and deterministic pipelines.

5) Deployment

Goal: Ship safely and repeatedly.

What to implement

  • Automated build → test → release pipelines.
  • Environment parity (dev/stage/prod) and progressive delivery (canary, feature flags).
  • Rollback plans and release notes.

Modern DevOps research consistently links strong CI/CD and platform-engineering practices with better delivery and team outcomes.

6) Maintenance

Goal: Keep the product healthy and improving after release.

What to do

  • Monitor SLIs/SLOs, error budgets, and incident reviews.
  • Patch vulnerabilities, pay down tech debt, and revisit assumptions with new data.
  • Feed learnings back into Planning (close the loop).

Phase-by-Phase Deliverables Checklist (Copy/Paste)

  • Planning: problem statement · success metric · risks assessed · stakeholders aligned
  • Design: architecture diagram · API contracts · test strategy · non-functional requirements
  • Development: unit tests · code review · feature flags · observability hooks
  • Testing: integration tests · performance/security/accessibility checks · rollback plan
  • Deployment: infrastructure as code · automated release · release notes
  • Maintenance: monitoring dashboards · alerting · incident review · backlog of improvements

Agile teams often express these as Definition of Ready/Done per phase and deliver small, frequent increments to maximize learning and reduce risk.

Final Advice

  • Keep phases short and observable; feedback is your risk reducer.
  • Encode process as code (like the gate scripts) so quality is enforced, not just documented.
  • Layer Agile + DevOps practices on top of SDLC to ship smaller, safer, and more often.

References & Further Reading

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