Master YAML Anchors to Eliminate Repetitive Config in CI/CD Pipelines

Introduction

YAML provides built‑in mechanisms—anchors, aliases, and merge keys—to avoid duplication in CI/CD pipelines, Kubernetes manifests, Docker Compose files, and similar configurations.

Core Elements

Anchor (&) : tags a node for later reference. Syntax: prepend & and a name to a mapping.

Alias (*) : references a previously defined anchor. Syntax: * followed by the anchor name.

Merge key (<<) : special key that merges one or more mappings into the current mapping, typically used together with an alias.

Simple Example

# Define an anchor
base_user_template: &base_user
  age: 30
  city: "San Francisco"

# Use alias
user_a: *base_user

# Merge with overrides
user_b:
  <<: *base_user
  name: "Bob"
  city: "New York"

Result: user_a resolves to {age: 30, city: "San Francisco"}. user_b resolves to {age: 30, city: "New York", name: "Bob"}.

Practical Exercise: Docker Compose → GitLab CI

Scenario 1 – Simplify Docker Compose

# Common configuration anchor
x-common-config: &common-config
  restart: always
  environment:
    - REDIS_HOST=redis
    - POSTGRES_HOST=postgres

services:
  api_service:
    <<: *common-config
    build: ./api
    ports:
      - "8000:8000"

  worker_service:
    <<: *common-config
    build: ./worker
    environment:
      - QUEUE_NAME=high_priority
      - REDIS_HOST=redis
      - POSTGRES_HOST=postgres

Both services inherit restart and environment from the anchor; updating the anchor updates all services.

Scenario 2 – GitLab CI

# Anchor for unstable‑branch jobs
.unstable_template: &unstable_template
  tags:
    - app-web
  only:
    - unstable

build_unstable:
  <<: *unstable_template
  stage: build
  script:
    - echo "Building for unstable..."

deploy_unstable:
  <<: *unstable_template
  stage: deploy
  script:
    - echo "Deploying to unstable..."

The &unstable_template anchor encapsulates common settings; each job merges it with <<: *unstable_template, keeping the CI file DRY.

UML Modeling of Definition‑Reference Relationship

A component diagram can visualize an anchor as a reusable blueprint and aliases as instances that either merge or directly reference the blueprint.

Conclusion

Reduce duplication : define common configuration once.

Improve readability : separate shared logic from service‑specific details.

Simplify maintenance : changing the anchor propagates to all references automatically.