DevOps, SRE & Cloud Interview · CI/CD & Automation · Lesson 2 of 3
Blue-green, canary & rolling
Deployment strategies exist to release a new version without downtime and with a fast way back. They trade off cost, blast radius, and rollback speed.
- Rolling: replace instances a few at a time behind the load balancer. Cheap (no extra fleet) but rollback is slow and both versions serve traffic during the roll.
- Blue-green: run two full environments; deploy to the idle 'green', then flip all traffic. Instant rollback (flip back), but you pay for double capacity briefly.
- Canary: route a small slice (1 percent, then 5, then 25) of real traffic to the new version, watch metrics, and promote or abort. Smallest blast radius; needs good observability and automated analysis.
Canary in a coal mineA canary release exposes the new version to a tiny fraction of users first — if error rate or latency spikes on that slice, you abort before most users ever see it. The metrics are the canary.
Answer with the trade-offDo not just name a strategy — say why. 'Blue-green for instant rollback when I can afford double capacity; canary when I want the smallest blast radius and have the metrics to auto-analyze it.' That is a senior answer.
◆ Lock it in
- Rolling = cheap, gradual, slow rollback; blue-green = instant flip/rollback, double cost; canary = smallest blast radius, needs metrics.
- All three aim for zero-downtime releases with a fast path back.
- Choose by rollback speed, cost, and blast radius — and say the trade-off out loud.
Feynman drill — say it out loudContrast blue-green and canary deployments, and name one situation where each is the better fit.
Step 1 rate your confidence · Step 2 pick your answer
Which deployment strategy gives the fastest rollback but temporarily needs double the capacity?
Step 1 — how sure are you?