Blue-Green Deployment

Run two identical environments and switch traffic after the new one passes checks.

deploy

Detailed Description

Blue-green deployment minimizes downtime by preparing the new version before traffic moves.

It is strongest when database migrations are backward compatible.

Use expand-contract migrations: expand schema first, deploy, backfill, then remove old fields in a later release.

Keep Blue warm for a rollback window and use load balancer draining so in-flight requests complete safely.

Store sessions in shared infrastructure (for example Redis) so auth/session continuity survives traffic flips.

Visual Diagram

Blue  [v1.0] ← 100% live traffic
Green [v2.0] ← deploy + test here

Switch:  Blue → standby
         Green → 100% live traffic

Rollback: flip switch back instantly

Tradeoffs

Pros

Fast rollback and clean release boundary

Cons

Requires duplicate capacity

Examples: AWS CodeDeploy, Kubernetes service switching