Software Engineering Principles & Lifecycle Handbook

Clean Code & Refactoring

Clean code communicates intent, keeps responsibilities narrow, and makes change safe. Apply SOLID principles pragmatically: one reason to change, extensions without rewriting stable code, substitutable abstractions, focused interfaces, and dependency inversion.

DRY removes duplicated knowledge; KISS keeps solutions understandable. Refactor in small, behavior-preserving steps protected by tests. Common smells include long methods, primitive obsession, hidden side effects, feature envy, shotgun changes, and unclear names.

// Before: mixed responsibilities and repeated knowledge
function checkout(order) {
  const total = order.items.reduce((sum, item) => sum + item.price * item.qty, 0)
  if (total > 100) order.discount = total * 0.1
  sendEmail(order.customer.email, `Total: ${total - (order.discount || 0)}`)
}

// After: focused, testable operations
const calculateSubtotal = (items) => items.reduce((sum, item) => sum + item.price * item.qty, 0)
const applyDiscount = (subtotal) => subtotal > 100 ? subtotal * 0.9 : subtotal

Software Architecture Patterns

Architecture defines boundaries, dependencies, data ownership, and operational responsibilities. Choose the simplest shape that supports current risk and expected change rather than adopting complexity for prestige.

Presentation → Application → Domain → Infrastructure
      ↓              ↓             ↓
   HTTP/UI       use cases     business rules

// Events decouple time: OrderPlaced → ReserveStock → SendReceipt

Version Control & Git Workflows

Git records a project’s history as a graph of commits. Keep commits small and descriptive, branch from a current base, and use pull requests to make review, automated checks, and release intent visible.

Gitflow separates long-lived release branches, while trunk-based development favors short-lived branches and frequent integration. Interactive rebase is useful for cleaning local history before review; never rewrite a shared branch without coordination.

git switch main
git pull --ff-only
git switch -c feat/receipt-emails
# implement and test
git add .
git commit -m "feat: send enrollment receipt"
git rebase -i origin/main
git push -u origin feat/receipt-emails

# Resolve a conflict: inspect → edit markers → add → continue
git add path/to/file
git rebase --continue

RESTful API Design & Integration

Model APIs around resources and predictable representations. Use GET for safe reads, POST for creation or commands, PUT for replacement, PATCH for partial updates, and DELETE for removal. Document the contract with OpenAPI so consumers can generate clients and validate payloads.

Use status codes consistently: 200/201 for successful results, 204 for an empty success, 400 for invalid input, 401 for missing authentication, 403 for insufficient permission, 404 for missing resources, and 409 for conflicts. Validate every input server-side and keep access tokens short-lived.

POST /api/v1/enrollments
Authorization: Bearer eyJ...
Content-Type: application/json

{ "courseId": "web-101", "studentId": "stu_42" }

HTTP/1.1 201 Created
{ "id": "enr_9", "status": "active" }

// JWT validation: verify signature, issuer, audience, expiry, and scopes.

Testing & Quality Assurance

Unit tests isolate a function or module, integration tests verify boundaries such as a database adapter, and end-to-end tests exercise a user journey in a realistic environment. A balanced test pyramid gives fast feedback without ignoring critical workflows.

Test-driven development follows red, green, refactor: write a failing example, implement the smallest behavior that passes, then improve design while preserving the contract. Tests should be deterministic, focused, and explicit about failure messages.

describe("calculateTotal", () => {
  it("applies a ten percent discount above 100", () => {
    expect(calculateTotal([{ price: 120, qty: 1 }])).toBe(108)
  })
})

// E2E intent: login → open course → submit lesson → verify completion.

CI/CD Pipelines & Deployment

Continuous integration builds and tests every change. Continuous delivery keeps the main branch deployable; continuous deployment automatically releases changes that pass policy checks. Pipelines should be reproducible, observable, secure, and fast enough to encourage frequent integration.

Docker packages an application with its runtime dependencies. Blue-green deployment switches traffic between two environments for a fast rollback, while canary deployment gradually exposes a small percentage of users and expands only when health signals remain good.

FROM node:22-alpine AS build
WORKDIR /app
COPY package*.json ./
RUN npm ci
COPY . .
RUN npm run build

FROM node:22-alpine
WORKDIR /app
COPY --from=build /app .
CMD ["npm", "start"]

# Release gates: tests → security scan → deploy → smoke test → monitor