CI/CD Interview Questions and Answers

Blue/green is a deployment strategy with two identical environments. Blue runs the current version, and green runs the new version.

Process:

1. Deploy the new release to the idle environment and run smoke tests.
2. Shift traffic to it, usually by switching a router, load balancer, or DNS.
3. If something breaks, switch traffic back to the old environment almost instantly.
4. Keep the old environment until the new one is proven, then reuse it for the next release.

aws elbv2 modify-listener --listener-arn arn:... \
  --default-actions Type=forward,TargetGroupArn=arn:...green

Benefits: near-zero downtime, fast rollback, and the ability to test in production-like conditions. Drawbacks: double the infrastructure cost and tight data or schema compatibility between versions. It works best with stateless services and backward-compatible database migrations.

Stages and jobs structure a pipeline into ordered, parallelisable work.

• Stage: a logical phase such as build, test, or deploy. Stages run in order, and a later stage starts only when earlier stages succeed. Stages express the release lifecycle.
• Job: the actual unit of work inside a stage, running on a runner or agent. Jobs within the same stage can run in parallel. Each job has steps, scripts, and its own environment.

stages: [build, test, deploy]
test:
  stage: test
  script:
    - npm ci
    - npm test

For example, a test stage may run unit, integration, and lint jobs concurrently, and then a deploy stage runs only if all pass. Jobs can produce artefacts that later jobs consume. Some tools call these stages and jobs, while others use workflows and jobs, but the concept is the same: ordered phases and parallel tasks.