What are GitLab environments and how do you define them in CI/CD?

Environments in GitLab CI/CD represent deployment targets - staging, production, review apps, etc. Defining an environment in a job links that deployment to the environment, enabling GitLab to track which version is deployed where and provide rollback capabilities.

# Basic environment definition
deploy-staging:
  stage: deploy
  script:
    - ./deploy.sh staging
  environment:
    name: staging
    url: https://staging.example.com
  rules:
    - if: $CI_COMMIT_BRANCH == "main"

deploy-production:
  stage: deploy
  script:
    - ./deploy.sh production
  environment:
    name: production
    url: https://example.com
  rules:
    - if: $CI_COMMIT_TAG
      when: manual              # requires manual approval

# Dynamic review environments per merge request
review-app:
  stage: deploy
  script:
    - deploy-to-review.sh $CI_COMMIT_REF_SLUG
  environment:
    name: review/$CI_COMMIT_REF_SLUG
    url: https://$CI_COMMIT_REF_SLUG.review.example.com
    on_stop: stop-review
  rules:
    - if: $CI_PIPELINE_SOURCE == "merge_request_event"

stop-review:
  stage: deploy
  script:
    - teardown-review.sh $CI_COMMIT_REF_SLUG
  environment:
    name: review/$CI_COMMIT_REF_SLUG
    action: stop
  rules:
    - if: $CI_PIPELINE_SOURCE == "merge_request_event"
      when: manual

Environments are visible under Deploy > Environments in the GitLab UI, showing the current deployed version, who deployed it, and when. The on_stop option links a cleanup job that tears down ephemeral environments (like review apps) when no longer needed.

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