Achieving Zero‑Downtime Rollouts with Blue‑Green Deployment in Flutter

Introduction

Zero-downtime rollouts are essential for production-grade Flutter applications, ensuring users experience uninterrupted service during major updates. Blue-green deployment, a strategy that maintains two identical environments (Blue and Green), enables seamless traffic switching with minimal risk. In this tutorial, we’ll cover how to implement Flutter blue green deployment on Firebase Hosting, automate your pipeline, and validate each release before full traffic cutover.

Preparing Blue and Green Environments

To get started with Flutter blue green, you need two parallel hosting environments:

• Firebase Hosting Sites

• Separate app configurations

• Distinct backend endpoints (if using Cloud Functions or Cloud Run)

1. Create two sites in firebase.json:

{
  "hosting": [
    {
      "target": "blue",
      "public": "build/web",
      "ignore": ["firebase.json", "**/.*", "**/node_modules/**"]
    },
    {
      "target": "green",
      "public": "build/web",
      "ignore": ["firebase.json", "**/.*", "**/node_modules/**"]
    }
  ]
}

2. Assign each target to its own domain or subdomain:

firebase hosting:sites:create blue
firebase hosting:sites:create green
firebase target:apply hosting blue blue
firebase target:apply hosting green green

3. Configure your Flutter web app’s main.dart to read a compile‐time flavor or environment variable:

const environment = String.fromEnvironment('ENV', defaultValue: 'blue');

void main() {
  runApp(MyApp(environment: environment));
}

Build with:

flutter build web --dart-define=ENV=blue
flutter build web --dart-define=ENV=green

Now you have two separate deployable artifacts: build/web for Blue and Green.

Automating Your CI/CD Pipeline

Automation ensures consistency and reduces human error. Here’s a GitHub Actions example snippet for Flutter blue green deployment:

name: CI/CD

on:
  push:
    branches: [ main ]

jobs:
  build-and-deploy:
    runs-on: ubuntu-latest
    strategy:
      matrix:
        env: [blue, green]
    steps:
      - uses: actions/checkout@v3
      - uses: subosito/flutter-action@v2
      - name: Install dependencies
        run: flutter pub get
      - name: Build Flutter web
        run: flutter build web --dart-define=ENV=${{ matrix.env }}
      - name: Deploy to Firebase
        uses: w9jds/firebase-action@v1
        with:
          args: deploy --only hosting:${{ matrix.env }}
        env:
          FIREBASE_TOKEN

This matrix job builds and deploys both environments in parallel. The next step is switching traffic.

Traffic Switching and Verification

After deploying, you need to route users to the new environment only after smoke tests pass:

1. Smoke test endpoint health checks (e.g., https://green.example.com/health).

2. Use Firebase Hosting rewrite rules or Cloudflare Workers to toggle routing:

"rewrites": [
  {
    "source": "**",
    "destination": "/index.html",
    "hosting": {
      "site": "${activeEnv}"
    }
  }
]

In practice, you’d update activeEnv from blue to green via CLI or API once tests succeed:

# Pseudocode for toggling active environment
firebase functions:call toggleHosting \
  --data '{"site":"green"}'

Alternatively, use Firebase Remote Config to hold an active_env parameter, and in your hosting config read from an HTTPS endpoint that returns the live site.

Safely Rolling Back

If errors emerge post-cutover, rollback is as simple as switching activeEnv back to the previous environment:

• Validate previous site is still up (health check).

• Flip routing back to Blue.

• Investigate logs, fix issues, redeploy Green, then repeat cutover.

Key tips:

• Keep both environments identical except for the new code.

• Automate health checks to avoid manual misconfigurations.

• Retain audit logs of traffic switches for compliance.

Vibe Studio

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Conclusion

Implementing a blue-green deployment for Flutter web on Firebase Hosting drastically reduces risk and downtime. With two synchronized environments and an automated CI/CD pipeline, you can safely preview new features, perform smoke tests, and switch user traffic seamlessly. Should any issue arise, rollback is a quick configuration change.

By adopting Flutter blue green deployment, you ensure your users always have access to a stable release, while giving your team the flexibility to innovate rapidly without fear of disruption.