What are anchors and aliases in YAML? How to use them to avoid duplicate configuration?

Anchors and Aliases in YAML are powerful reuse mechanisms that can avoid defining the same content repeatedly, improving configuration file maintainability.

Basic Concepts of Anchors and Aliases

Anchor

• Use & symbol to define an anchor
• Create a reference identifier for a value or structure
• Can reference this anchor elsewhere in the document

Alias

• Use * symbol to reference an anchor
• Points to a previously defined anchor
• Copies the anchor's content when referenced

Basic Syntax

yaml
# Define anchor
defaults: &default_config
  timeout: 30
  retry: 3
  log_level: info

# Use alias to reference
service1:
  <<: *default_config
  name: service1
  port: 8000

service2:
  <<: *default_config
  name: service2
  port: 8001

Use Cases

1. Reusing Configuration Values

yaml
# Define common configuration
database: &db_config
  host: db.example.com
  port: 5432
  ssl: true

# Reuse in multiple services
service_a:
  database: *db_config
  name: Service A

service_b:
  database: *db_config
  name: Service B

2. Merging Maps

Use <<: operator to merge anchor content into current map.

yaml
# Define base configuration
base: &base_config
  version: "1.0"
  environment: production
  debug: false

# Inherit and extend
api:
  <<: *base_config
  name: API Service
  port: 8080

worker:
  <<: *base_config
  name: Worker Service
  port: 8081
  concurrency: 10

3. Reusing Lists

yaml
# Define common tags
common_tags: &tags
  - monitoring
  - logging
  - backup

# Use in multiple resources
server1:
  name: web-server-1
  tags: *tags

server2:
  name: web-server-2
  tags: *tags

4. Reusing Complex Structures

yaml
# Define complex service configuration
service_template: &service
  health_check:
    enabled: true
    path: /health
    interval: 30s
  resources:
    cpu: "500m"
    memory: "512Mi"
  replicas: 2

# Create multiple service instances
frontend:
  <<: *service
  name: frontend
  image: nginx:latest
  port: 80

backend:
  <<: *service
  name: backend
  image: node:18
  port: 3000
  resources:
    cpu: "1000m"
    memory: "1Gi"

Advanced Usage

1. Multiple Inheritance

yaml
# Define multiple base configurations
base1: &config1
  timeout: 30
  retry: 3

base2: &config2
  log_level: debug
  verbose: true

# Merge multiple configurations
service:
  <<: [*config1, *config2]
  name: combined-service

2. Overriding Inherited Values

yaml
# Base configuration
defaults: &defaults
  timeout: 30
  retry: 3
  log_level: info

# Override specific values
service:
  <<: *defaults
  name: critical-service
  timeout: 60  # Override default value
  retry: 5     # Override default value

3. Nested Anchors

yaml
# Nested anchor definition
database: &db
  connection: &conn
    host: localhost
    port: 5432
  pool:
    min: 5
    max: 20

# Reference nested anchor
service:
  database: *db
  cache:
    connection: *conn  # Reference nested connection config

Practical Application Examples

Kubernetes Configuration Reuse

yaml
apiVersion: v1
kind: ConfigMap
metadata:
  name: common-config
data:
  app: &app_config
    name: myapp
    version: "1.0.0"
    environment: production
---
apiVersion: apps/v1
kind: Deployment
metadata:
  name: frontend
spec:
  template:
    spec:
      containers:
        - name: frontend
          env:
            - name: APP_NAME
              value: *app_config.name
            - name: APP_VERSION
              value: *app_config.version
---
apiVersion: apps/v1
kind: Deployment
metadata:
  name: backend
spec:
  template:
    spec:
      containers:
        - name: backend
          env:
            - name: APP_NAME
              value: *app_config.name
            - name: APP_VERSION
              value: *app_config.version

Docker Compose Configuration Reuse

yaml
version: '3.8'

x-common-variables: &common-env
  NODE_ENV: production
  LOG_LEVEL: info
  API_TIMEOUT: 30000

services:
  web:
    image: nginx:latest
    environment:
      <<: *common-env
      SERVICE_NAME: web
    ports:
      - "80:80"

  api:
    image: node:18
    environment:
      <<: *common-env
      SERVICE_NAME: api
    ports:
      - "3000:3000"

  worker:
    image: node:18
    environment:
      <<: *common-env
      SERVICE_NAME: worker

Important Notes

1. Anchor Scope: Anchors must be defined before aliases
2. Circular References: Avoid creating circular references, which cause parsing errors
3. Readability: Overusing anchors may reduce readability
4. Version Compatibility: Some YAML parsers may not support advanced features

Best Practices

1. Naming Conventions: Use descriptive anchor names
2. Documentation: Add comments to explain complex anchors
3. Moderate Use: Only use anchors when truly needed for reuse
4. Test Validation: Use YAML validation tools to ensure configuration correctness
5. Team Standards: Establish anchor usage standards within the team

Common Errors

yaml
# ❌ Error: Undefined anchor
service:
  config: *undefined_anchor

# ❌ Error: Circular reference
a: &ref
  b: *ref

# ❌ Error: Anchor defined after alias
service:
  config: *config
defaults: &config
  timeout: 30