Lesson 17 — Blue-green switch

Two copies, one switch

Blue-green deployment keeps two identical environments. One (say blue) serves users; you deploy the new version to the other (green), test it, then flip the router so all traffic goes to green. If something is wrong, flip back — the old version is still running.

Note: real DevOps tools (CI servers, load balancers, Docker, Kubernetes) cannot run in your browser, so this exercise is a small plain-JS simulation of the logic those tools use.

Why it matters: deploys and rollbacks become a single, instant switch with zero downtime.