How does parallel job execution work in GitLab CI/CD?

GitLab CI supports running multiple instances of the same job in parallel using the parallel keyword. This is useful for splitting test suites across multiple runners to reduce total execution time.

# Run the same job N times in parallel
test:
  stage: test
  image: ruby:3.2
  script:
    - bundle exec rspec
  parallel: 5
  # GitLab creates 5 jobs: test 1/5, test 2/5, ..., test 5/5
  # Each job gets: CI_NODE_INDEX (1-5) and CI_NODE_TOTAL (5)
  # Your test framework uses these to split the test suite

# Parallel matrix - run job with different variable combinations
build:
  stage: build
  script:
    - echo "Node: $NODE_VERSION on $OS"
  parallel:
    matrix:
      - NODE_VERSION: ["18", "20", "22"]
        OS: ["ubuntu", "alpine"]
  # Creates 6 jobs: all combinations of NODE_VERSION x OS

# Access which shard you are in:
test-shard:
  script:
    - echo "Running shard $CI_NODE_INDEX of $CI_NODE_TOTAL"
    - run-tests --split $CI_NODE_TOTAL --shard $CI_NODE_INDEX

Parallel jobs all run in the same stage - they start at the same time (assuming runners are available) and the stage completes when all parallel jobs finish. All jobs must pass for the stage to pass.

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