Developers complain that the CI pipeline takes 40 minutes. How would you make it faster?
What the interviewer wants to hear
- Measure first: find the slowest stages
- Cache dependencies and Docker layers
- Run jobs in parallel and shard tests
- Only build or test what changed (affected projects)
- Right-size runners and fix flaky tests
Model answer
I would start by measuring, not guessing. I look at timings per job and step over the last few weeks to find the biggest costs. Usually the quick wins are caching: dependency caches keyed by the lockfile, and Docker layer caching with BuildKit and a registry cache, plus a better Dockerfile order. Next I run independent jobs such as lint, unit tests and security scans in parallel, and shard large test suites across several runners. In a monorepo, tools like Nx, Turborepo or Bazel can build and test only the affected projects. I also check runner size, since a bigger machine can be cheaper than developer waiting time. Finally I find and fix or quarantine flaky tests, because retries waste a lot of time. I track pipeline duration as a metric so it does not slowly grow again.
Can you answer it under time pressure?
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