The needs: keyword allows you to define explicit dependencies between jobs, creating a Directed Acyclic Graph (DAG) of your pipeline. This means a job can start as soon as the specific jobs it needs have completed successfully, rather than waiting for the entire previous stage to finish. This can significantly speed up your pipeline execution by running jobs earlier than they would in a strict stage-based order.

Example .gitlab-ci.yml with needs:

YAML

# .gitlab-ci.yml

stages:
  - build
  - test
  - deploy

default:
  image: alpine:latest
  # For 'needs' to automatically download artifacts from prerequisite jobs,
  # those jobs must define 'artifacts'. We'll add simple artifacts for demonstration.

# --- Build Stage Jobs ---
build_backend:
  stage: build
  script:
    - echo "--- Building Backend ---"
    - sleep 10 # Simulate build time
    - mkdir -p backend_build/
    - echo "backend_app_v1" > backend_build/version.txt
    - echo "Backend built successfully."
  artifacts:
    paths:
      - backend_build/
    expire_in: 1 hour

build_frontend:
  stage: build
  script:
    - echo "--- Building Frontend ---"
    - sleep 15 # Simulate build time
    - mkdir -p frontend_build/
    - echo "frontend_app_v1" > frontend_build/ui_bundle.js
    - echo "Frontend built successfully."
  artifacts:
    paths:
      - frontend_build/
    expire_in: 1 hour

# --- Test Stage Jobs ---
# This job will start as soon as 'build_backend' finishes, without waiting for 'build_frontend'.
test_backend_api:
  stage: test
  needs: ["build_backend"] # Explicitly depends on 'build_backend'
  script:
    - echo "--- Testing Backend API ---"
    - echo "Checking backend build artifacts..."
    - if [ -f "backend_build/version.txt" ]; then cat backend_build/version.txt; else echo "Backend version file not found!"; exit 1; fi
    - sleep 8  # Simulate test time
    - echo "Backend API tests PASSED."

# This job will start as soon as 'build_frontend' finishes, without waiting for 'build_backend'.
test_frontend_ui:
  stage: test
  needs: ["build_frontend"] # Explicitly depends on 'build_frontend'
  script:
    - echo "--- Testing Frontend UI ---"
    - echo "Checking frontend build artifacts..."
    - if [ -f "frontend_build/ui_bundle.js" ]; then echo "Frontend bundle found."; else echo "Frontend bundle not found!"; exit 1; fi
    - sleep 10 # Simulate test time
    - echo "Frontend UI tests PASSED."

# This job needs both build jobs to complete before it can start.
# It will start as soon as BOTH 'build_backend' AND 'build_frontend' are done.
integration_tests:
  stage: test
  needs: ["build_backend", "build_frontend"] # Depends on multiple jobs
  script:
    - echo "--- Running Integration Tests ---"
    - echo "Verifying backend and frontend integration..."
    - if [ -f "backend_build/version.txt" ] && [ -f "frontend_build/ui_bundle.js" ]; then echo "All necessary artifacts found."; else echo "Missing artifacts for integration!"; exit 1; fi
    - sleep 12 # Simulate integration test time
    - echo "Integration tests PASSED."

# --- Deploy Stage Job ---
# This job will start as soon as 'integration_tests' finishes.
# It also implicitly benefits from artifacts if integration_tests produced any relevant for deployment.
deploy_application:
  stage: deploy
  needs: ["integration_tests"]
  script:
    - echo "--- Deploying Application ---"
    - echo "Deploying based on successful integration tests."
    - sleep 10 # Simulate deployment time
    - echo "Application deployed successfully!"
  when: on_success # Default, but good for deploy jobs

Code language: PHP (php)

Explanation:

1. needs: Keyword:
   • Used within a job definition to specify an array of other jobs that must complete successfully before this job can start.
   • It allows you to create a more granular dependency chain than what stages provide.
2. stages: Still Define the Overall Flow (and Visualization):
   • Jobs still belong to stages. Stages are used for:
     • Visual grouping in the GitLab CI/CD pipeline graph.
     • A fallback execution order if needs: is not used.
     • Determining the order if needs: creates parallel paths that eventually converge on a stage.
   • However, needs: can make a job start earlier than its defined stage if its needs dependencies are met.
3. test_backend_api Job:
   • stage: test
   • needs: ["build_backend"]: This is the key. test_backend_api will not wait for build_frontend (which is also in the build stage) to complete. As soon as build_backend finishes successfully, test_backend_api can start, even if build_frontend is still running.
4. test_frontend_ui Job:
   • stage: test
   • needs: ["build_frontend"]: Similar to the backend test, this job starts as soon as build_frontend is done, independently of build_backend‘s completion or test_backend_api‘s status (unless test_backend_api was also in its needs list).
5. integration_tests Job:
   • stage: test
   • needs: ["build_backend", "build_frontend"]: This job depends on both build_backend and build_frontend. It will only start after both of these prerequisite jobs have completed successfully.
6. deploy_application Job:
   • stage: deploy
   • needs: ["integration_tests"]: The deployment will only start after integration_tests is successful.
7. Artifacts and needs:
   • When you use needs:, by default, GitLab will attempt to download artifacts from all the jobs listed in the needs array. This is equivalent to needs: [{ job: "job_name", artifacts: true }].
   • In the example, build_backend produces an artifact in backend_build/. test_backend_api (which needs: ["build_backend"]) will have access to this backend_build/ directory and its contents.
   • If you don’t want artifacts from a needed job, you can specify needs: [{ job: "job_name", artifacts: false }].
8. Impact of Failure:
   • If any job listed in a needs array fails (or is skipped, or canceled), the dependent job will be skipped by default and will not run.

How needs: Speeds Up Pipelines:

Imagine the build stage without needs:

• build_backend (10s) and build_frontend (15s) run in parallel.
• The build stage finishes after the longest job, so after 15s.
• Only then would the test stage begin.

With needs::

• build_backend finishes in 10s. test_backend_api can start immediately.
• build_frontend finishes in 15s. test_frontend_ui can start immediately.
• integration_tests starts when both build_backend and build_frontend are done (i.e., after 15s in this example).
• The overall pipeline can proceed much faster because testing for completed build components starts earlier.

Key Concepts and Benefits:

• DAG (Directed Acyclic Graph): needs: allows you to define your pipeline as a graph of dependencies, rather than a strict linear sequence of stages.
• Reduced Latency: Jobs start as soon as their true dependencies are met, leading to shorter overall pipeline durations.
• Increased Efficiency: Better utilization of runners as jobs don’t wait unnecessarily.
• Implicit Artifact Download: Simplifies passing artifacts between dependent jobs.
• Optional Dependencies: You can also use needs: [{ job: "optional_job", optional: true }]. If optional_job fails or is skipped, the current job will still run (but it won’t download artifacts from the optional job if it didn’t produce them).

Using needs: is a powerful way to optimize your GitLab CI/CD pipelines, especially for projects with multiple independent build components or test suites that can run as soon as their specific dependencies are ready.