Trunk-Based Development: Continuous Integration Done Right

Trunk-based development (TBD) means integrating code into main frequently to avoid merge hell.

Core Principles

1. Short-lived branches: 1-2 days maximum, then merge
2. Small commits: Each commit is a complete, working unit
3. Always green: Main branch is always deployable
4. Feature flags: Hide incomplete features in production

Feature Flags for Incomplete Work

// Feature flag service
class FeatureFlags {
  private flags: Map<string, boolean>;

  constructor(private config: FlagConfig) {
    this.flags = new Map(Object.entries(config));
  }

  isEnabled(flag: string, userId?: string): boolean {
    return this.flags.get(flag) ?? false;
  }
}

// Usage: safely commit incomplete feature
async function processOrder(order: Order, flags: FeatureFlags) {
  if (flags.isEnabled('new-payment-flow')) {
    return await newPaymentService.process(order);
  }
  return await legacyPaymentService.process(order);
}

Branch Strategy

# Trunk-based (recommended)
main <-- single source of truth
  |
  +-- feature/user-auth (1-2 days max)
  +-- fix/login-bug (hours max)

# NOT trunk-based (avoid)
main
develop
release/v1.2
feature/big-feature (3 weeks old!)

Pair/Mob Programming Alternative

# With short branches (< 1 day), integrate directly
git checkout -b feat/add-search
# work for a few hours
git add .
git commit -m "feat: add search endpoint with basic filtering"
git push origin feat/add-search
# create PR, get quick review, merge same day

CI Pipeline for TBD

# .github/workflows/ci.yml
on:
  push:
    branches: [main]
  pull_request:
    branches: [main]

jobs:
  test:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - run: npm ci
      - run: npm run typecheck
      - run: npm run lint
      - run: npm test -- --coverage
      - run: npm run build

  deploy:
    needs: test
    if: github.ref == 'refs/heads/main'
    runs-on: ubuntu-latest
    steps:
      - name: Deploy to production
        run: ./deploy.sh

Preventing Big Bang Merges

// Instead of branching for 2 weeks, commit behind a flag
// Day 1: Add flag and stub
async function getRecommendations(userId: string) {
  if (featureFlags.isEnabled('ml-recommendations')) {
    // TODO: implement ML recommendations
    return [];
  }
  return legacyRecommendations(userId);
}

// Day 2: Implement core logic behind flag
// Day 3: Enable flag for 10% of users
// Day 4: Enable for all users
// Day 5: Remove flag and old code

Branch Protection Rules

main branch protection:
- Require status checks (CI must pass)
- Require 1+ approvals
- Dismiss stale reviews on new commits
- No direct pushes (PR required)
- Linear history required (rebase/squash)

TBD with feature flags enables true continuous delivery—deploying multiple times per day safely.