Blue-green deployment is a release strategy that runs two identical production environments, blue and green, where only one serves live traffic at a time, allowing a new version to be verified before it's exposed to users.
Key Points: • The currently live environment (say, blue) keeps serving all traffic while the new version is deployed to the idle environment (green). • Once the green environment is tested and confirmed healthy, a router or load balancer switches traffic to it, typically in a single, near-instant cutover. • The old environment (blue) stays running and idle, giving an immediate rollback path by simply switching traffic back. • Deployment risk is greatly reduced because the new version is fully validated in a production-identical environment before real users see it. • The main downside is cost: you need double the infrastructure running simultaneously during the switch window.
Example: An e-commerce site deploys a new checkout flow to the green environment, runs smoke tests against it directly, and only then repoints the load balancer from blue to green, with zero downtime for shoppers and an instant rollback available if something looks wrong.
Interview Tip: A concise interview answer is:
"Blue-green deployment keeps two identical environments, with only one live at a time. You deploy and test the new version on the idle one, then switch traffic over once it's verified, which gives zero downtime and an instant rollback if needed, at the cost of running double the infrastructure briefly."