Simple definition
Blue-green deployment maintains two application environments and switches traffic to the new version after validation.
Purpose and use
A blue-green deployment prepares a new version in a parallel environment before transferring traffic to it.
How does it work?
Routing or an entry point selects the active environment. The previous version can remain available during validation.
Example and verification
Before switching, test dependencies, sessions and data migrations, then verify service through its public entry point.
Points to watch
Switching traffic back does not automatically restore a modified database. Writes and schema changes can prevent simple rollback.
In short
The approach supports application switching when state compatibility and rollback are prepared.