What is blue-green deployment?
Short answer
Blue-green deployment is a release technique that runs two identical production environments. One (blue) serves live traffic while the new version is deployed and tested on the other (green); traffic is then switched to green in one step. If something goes wrong, switching back to blue is an instant rollback.
How it works
- Blue runs the current version and receives all traffic.
- Deploy the new version to green, run smoke tests and warm caches while users see nothing.
- Switch the load balancer, DNS or router to send traffic to green.
- Keep blue running for a while. If errors rise, switch back in seconds.
The hard part: the database
Both environments usually share one database, so schema changes must work with the old and the new code at the same time. Use the expand and contract approach: add new columns or tables first (expand), deploy code that uses them, and remove the old ones in a later release (contract). Never rename or drop a column in the same release that stops using it.
Blue-green or canary?
Blue-green switches all users at once. A canary release sends a small share of traffic, say 5%, to the new version first and increases it gradually, which limits the impact of a bad release but needs more routing and monitoring. Many platforms, including Kubernetes, managed container services and edge platforms with versioned deployments, support both.
