Anchors & Aliases — DRY YAML

Define once with &anchor, reuse with *alias

YAML supports anchors — give a node a name with &name — and aliases — reference it later with *name. The parsed result reuses the same node value, so two anchors-of-the-same-thing produce one logical value (or two copies, depending on the parser's deep-copy policy).

Merge keys (`<<:`)

Merge keys are a YAML 1.1 extension: <<: *base merges all keys from an anchored map into the current map, with the current map's keys overriding. This is how docker-compose and CI configs share base configuration without duplication.

Heads-up: merge keys are not in YAML 1.2 spec, but PyYAML, ruamel, js-yaml, and most working tools still support them. Treat as 'works almost everywhere; check your parser before relying.'

When to reach for anchors

Two or more nodes that should stay in sync. Without anchors, you have two places to update; with anchors, you have one source of truth. The cost is one extra layer of indirection at read time — worth it once you have repetition.

Principle: anchors and aliases are the YAML equivalent of variables. They make repeated configuration maintainable. Overuse them and the file becomes hard to read; underuse them and you maintain three copies of the same env-var block. Aim for 'used three times or more = anchor it.'

Code

Anchor + alias for repeated values·yaml

default_timeout: &timeout 30

endpoints:
  - name: search
    timeout: *timeout
  - name: index
    timeout: *timeout
  - name: admin
    timeout: *timeout

Merge keys for shared base configs·yaml

base_service: &base
  image: ghcr.io/cwk/api:1.0
  restart: unless-stopped
  environment:
    LOG_LEVEL: info

services:
  api:
    <<: *base
    ports: ["8000:8000"]
  worker:
    <<: *base
    command: ["python", "worker.py"]
  admin:
    <<: *base
    ports: ["8001:8001"]
    environment:
      LOG_LEVEL: debug   # overrides base

Anchor on a list element·yaml

shared_env: &shared_env
  - name: TZ
    value: Asia/Seoul
  - name: LOG_LEVEL
    value: info

containers:
  - name: api
    env: *shared_env
  - name: worker
    env: *shared_env

Anchors & Aliases — DRY YAML

Define once with &anchor, reuse with *alias

Merge keys (`<<:`)

When to reach for anchors

Code

External links

Exercise

Progress

Comments 0

Define once with &anchor, reuse with *alias

Merge keys (<<:)

When to reach for anchors

Code

External links

Exercise

Progress

Merge keys (`<<:`)