Configure GitLab CI needs: keyword with DAG to eliminate sequential stage bottlenecks and fan out parallel jobs

Steps

1. Replace the default stage-based ordering by adding needs: to jobs that should run as soon as their dependencies complete, regardless of stage: job-b: stage: test; needs: [job-a] — job-b starts immediately after job-a succeeds without waiting for all stage: build jobs
2. To receive artifacts from a dependency, specify needs with artifacts: true: needs: [{job: build-frontend, artifacts: true}] so the artifacts from that specific job are downloaded before the dependent job starts
3. For jobs that should run without any dependency (i.e., immediately at pipeline start), set needs: [] explicitly — without this, a job in a later stage waits for all prior-stage jobs even if they are unrelated
4. Model a fan-out/fan-in pattern: one build job produces an artifact, multiple parallel test jobs declare needs: [build] with artifacts: true, and a final report job declares needs over all test job names
5. Validate the DAG structure with the GitLab CI Lint tool (Repository > CI/CD > CI Lint or API endpoint /api/v4/ci/lint) which visualizes the job dependency graph
6. Set rules: with if: $CI_PIPELINE_SOURCE to conditionally include or exclude branches of the DAG without restructuring the entire pipeline YAML