Blue green runs two complete environments and switches all traffic at once, so rollback is instant but you pay for double capacity and every user sees the change simultaneously. A canary sends a small slice of traffic to the new version, compares its metrics against the old, and ramps up gradually, which limits blast radius but needs traffic shaping and good telemetry. Pick canary for high traffic services and blue green for infrequent, tightly coupled releases.
Why interviewers ask this
This checks whether you understand risk management rather than deployment mechanics. The interviewer wants the trade offs stated honestly, including the cost of blue green and the telemetry prerequisite for canaries. The strongest answers point out that neither strategy saves you from a bad database migration, which is where most real deploy incidents actually come from.
How to structure your answer
- Define both in one sentence each, focused on traffic.
- Give the trade offs: cost, blast radius, rollback speed.
- State which you pick under which conditions.
- Note what neither one solves, especially schema changes.
Example answer
Blue green means two full environments. Green is live, blue gets the new version, you run your checks against blue, then you flip the router and everyone moves at once. The upside is that rollback is just flipping back, which is seconds. The downsides are that you are paying for two production environments and that if something is wrong, one hundred percent of users get it at the same moment. A canary sends maybe one or five percent of traffic to the new version, and you compare error rate, latency and business metrics against the control before ramping to twenty five, fifty, then all. Blast radius is tiny and the ramp is automatic if the comparison stays clean. What canary needs is real telemetry and a way to split traffic, so a service mesh or a load balancer that supports weighting. In practice I default to canary for anything user facing with steady traffic, and blue green where traffic is too low for a canary to be statistically meaningful. Neither of them rescues you from a destructive migration, which is a separate discipline.
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See how it worksFollow-up questions to expect
- How do you make a canary decision when traffic is low or very spiky?
- How do sticky sessions or stateful connections complicate either approach?
- What metrics would you automate the canary rollback on?
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