Blue-Green Deployment Strategy

What is a blue-green deployment, and what are its advantages and disadvantages compared to other deployment strategies?

CI/CDintermediate

// interview question

What is a blue-green deployment, and what are its advantages and disadvantages compared to other deployment strategies?

Sample answer

Blue-green deployment maintains two identical production environments (blue and green). One serves live traffic while the other receives the new version. After testing, traffic switches to the updated environment. Advantages: instant rollback, zero downtime, and safe testing. Disadvantages: requires double resources and can be complex with databases.

Why this matters

Blue-green deployments provide a safety net for releases by keeping the previous version running until the new one is verified. This strategy is particularly valuable for critical applications where downtime or failed deployments have significant business impact.

Code examples

Kubernetes blue-green with services

yaml

# Blue deployment (current production)
apiVersion: apps/v1
kind: Deployment
metadata:
  name: app-blue
spec:
  replicas: 3
  selector:
    matchLabels:
      app: myapp
      version: blue
  template:
    metadata:
      labels:
        app: myapp
        version: blue
---
# Service pointing to blue
apiVersion: v1
kind: Service
metadata:
  name: app-service
spec:
  selector:
    app: myapp
    version: blue  # Switch to 'green' after verification
  ports:
  - port: 80

Switch traffic to green

bash

# Update service selector to point to green
kubectl patch service app-service \
  -p '{"spec":{"selector":{"version":"green"}}}'

# Rollback to blue if needed
kubectl patch service app-service \
  -p '{"spec":{"selector":{"version":"blue"}}}'

Common mistakes to avoid

Not accounting for database schema changes that may break the old version
Forgetting to warm up the new environment before switching traffic
Not having proper health checks to verify the green environment before cutover

Likely follow-ups

How do you handle database migrations in a blue-green deployment?
What is the difference between blue-green and canary deployments?
How would you implement blue-green deployments with AWS or other cloud providers?

Answer out loud first, then check yourself against the model answer.

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