Compare rolling, blue-green, and canary deployments. Which would you pick and why?
What they are really testing: Whether you can weigh cost against risk rather than naming a favourite. The strongest answers connect the choice to the specific service and to how fast you can detect a bad release.
A real interview question
Compare rolling, blue-green, and canary deployments. Which would you pick and why?
What most people say
drag me
“Canary is the safest so I would always use canary.”
It ignores that canary needs metrics good enough to judge a small sample, plus routing control and automation. Recommending it for a low-traffic internal service means waiting hours for statistically meaningless data.
The follow-ups they ask next
What has to be true of your database for any of these?
Schema changes must be backward compatible for the overlap window: additive columns, no renames or drops until the old version is fully gone. Expand and contract, in two releases.
How do you decide automatically whether a canary is healthy?
Compare the canary against the baseline on error rate and latency percentiles over a set window, with a minimum request count so the sample is meaningful, and roll back automatically on breach.
What the interviewer is listening for
- Ties the choice to traffic volume and detection
- Raises schema compatibility unprompted
- Knows blue-green does not prevent bad releases
What sinks the answer
- Names one as universally best
- Ignores database compatibility
- Thinks canary works at any traffic level
If you genuinely do not know
Say this instead of freezing. Reasoning out loud from what you do know beats silence every single time, and a good interviewer is listening for exactly that.
“Rolling is [batch replacement, no extra cost, slow rollback]. Blue-green is [two environments, instant switch, double cost]. Canary is [a small slice watched on real metrics]. I pick based on [traffic volume and how fast I would detect a bad release], and either way [the schema must work with both versions].”
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