YAML
CHEAT SHEET.

YAML Basics

Basic Structure

# This is a comment
key: value
number: 42
float: 3.14
boolean: true
null_value: null

# Nested structure (2 spaces)
person:
  name: Alice
  age: 25
  address:
    street: 123 Main St
    city: New York

# List (array)
fruits:
  - apple
  - banana
  - orange

# List of objects
users:
  - name: Alice
    age: 25
  - name: Bob
    age: 30

Data Types

Type	Example	Description
String	name: "Alice"	Quotes optional (unless special chars)
Integer	age: 25	Whole numbers
Float	pi: 3.14159	Decimal numbers
Boolean	active: true	true/false, yes/no, on/off
Null	value: null	null, ~, or empty
Date	date: 2024-01-15	ISO 8601 format
Timestamp	timestamp: 2024-01-15T10:30:00Z	ISO 8601 with time

String Variations

# Unquoted (simple strings)
name: Alice
city: New York

# Double quotes (escape sequences)
message: "Hello, World!\nWelcome to YAML"

# Single quotes (literal)
message: 'Hello, World!\nThis is literal'

# Multi‑line strings (preserve newlines)
description: |
  This is a multi‑line string.
  It preserves newlines and indentation.
  Each line is separated by a newline.

# Multi‑line strings (fold newlines → spaces)
description: >
  This is a folded string.
  Newlines are converted to spaces.
  This becomes a single line.

Collections

Mappings (Dictionaries / Objects)

# Block style (preferred)
person:
  name: Alice
  age: 25
  address:
    street: 123 Main St
    city: New York

# Inline style (flow style)
person: { name: Alice, age: 25, address: { street: 123 Main St, city: New York } }

Sequences (Lists / Arrays)

# Block style (preferred)
fruits:
  - apple
  - banana
  - orange

# Inline style (flow style)
fruits: [apple, banana, orange]

# List of objects
people:
  - name: Alice
    age: 25
  - name: Bob
    age: 30

# Nested lists
matrix:
  - [1, 2, 3]
  - [4, 5, 6]
  - [7, 8, 9]

Anchors & Aliases (Reuse)

# Define anchor (&)
defaults: &defaults
  timeout: 30
  retries: 3

# Reference anchor (*)
job1:
  <<: *defaults
  name: Job 1

job2:
  <<: *defaults
  name: Job 2

# Override specific fields
job3:
  <<: *defaults
  name: Job 3
  timeout: 60    # override default

# Multi‑level anchors
base: &base
  name: base
  config:
    log_level: info

extended: &extended
  <<: *base
  version: 2.0

Explicit Data Types

# Use !! to specify type
int_value: !!int 42
float_value: !!float 3.14
bool_value: !!bool true
str_value: !!str 12345
null_value: !!null ~

# Binary data (Base64)
data: !!binary c29tZSBkYXRhIGhlcmU=

# Timestamp
timestamp: !!timestamp 2024-01-15T10:30:00Z

# Set (unique values)
set: !!set
  ? apple
  ? banana
  ? orange

Multi‑Document YAML

# Multiple documents in one file
# Use --- to separate documents
--- # first document
name: Alice
age: 25

--- # second document
name: Bob
age: 30

--- # third document
fruits:
  - apple
  - banana

# ... (--- at the end is optional)

Common Use Cases

Kubernetes Deployment

apiVersion: apps/v1
kind: Deployment
metadata:
  name: my-app
  namespace: default
  labels:
    app: my-app
spec:
  replicas: 3
  selector:
    matchLabels:
      app: my-app
  template:
    metadata:
      labels:
        app: my-app
    spec:
      containers:
        - name: app
          image: nginx:latest
          ports:
            - containerPort: 80
          env:
            - name: ENV
              value: production
          resources:
            requests:
              memory: 64Mi
              cpu: 250m
            limits:
              memory: 128Mi
              cpu: 500m

Docker Compose

version: '3.8'
services:
  web:
    build: .
    ports:
      - "3000:3000"
    environment:
      - NODE_ENV=production
    volumes:
      - ./data:/app/data
    depends_on:
      - db
      - redis

  db:
    image: postgres:15
    environment:
      POSTGRES_USER: user
      POSTGRES_PASSWORD: password
      POSTGRES_DB: myapp
    volumes:
      - postgres_data:/var/lib/postgresql/data

  redis:
    image: redis:7-alpine

volumes:
  postgres_data:

Helm values.yaml

replicaCount: 3
image:
  repository: my-app
  tag: latest
  pullPolicy: IfNotPresent

service:
  type: ClusterIP
  port: 80

ingress:
  enabled: true
  hostname: myapp.example.com
  annotations:
    nginx.ingress.kubernetes.io/rewrite-target: /

resources:
  limits:
    cpu: 500m
    memory: 512Mi
  requests:
    cpu: 250m
    memory: 256Mi

config:
  logLevel: info
  database:
    host: postgres
    port: 5432
    name: myapp

Ansible Playbook

---
- name: Deploy web application
  hosts: webservers
  become: yes
  vars:
    app_name: myapp
    app_port: 3000
  tasks:
    - name: Install dependencies
      apt:
        name: "{{ item }}"
        state: present
      loop:
        - git
        - python3
        - nginx

    - name: Clone repository
      git:
        repo: https://github.com/user/myapp.git
        dest: /var/www/{{ app_name }}
        version: main

    - name: Start application
      systemd:
        name: "{{ app_name }}"
        state: started
        enabled: yes

GitLab CI/CD

stages:
  - build
  - test
  - deploy

variables:
  DOCKER_IMAGE: registry.gitlab.com/my-user/my-app
  DEPLOY_ENV: production

build:
  stage: build
  script:
    - docker build -t $DOCKER_IMAGE:$CI_COMMIT_SHA .
    - docker push $DOCKER_IMAGE:$CI_COMMIT_SHA

test:
  stage: test
  script:
    - docker run $DOCKER_IMAGE:$CI_COMMIT_SHA npm test

deploy:
  stage: deploy
  script:
    - kubectl set image deployment/my-app app=$DOCKER_IMAGE:$CI_COMMIT_SHA
    - kubectl rollout status deployment/my-app
  only:
    - main

Terraform Variables

variable "region" {
  description = "AWS region"
  type        = string
  default     = "us-east-1"
}

variable "instance_type" {
  description = "EC2 instance type"
  type        = string
  default     = "t3.micro"
}

variable "tags" {
  description = "Resource tags"
  type        = map(string)
  default = {
    Environment = "production"
    Terraform   = "true"
  }
}

variable "enabled" {
  type    = bool
  default = true
}

Prometheus Alert Rules

groups:
  - name: instance
    rules:
      - alert: InstanceDown
        expr: up == 0
        for: 5m
        labels:
          severity: critical
        annotations:
          summary: "Instance {{ $labels.instance }} down"
          description: "{{ $labels.instance }} of job {{ $labels.job }} has been down for more than 5 minutes."

      - alert: HighCPUUsage
        expr: (100 - (avg by(instance) (rate(node_cpu_seconds_total{mode="idle"}[5m])) * 100)) > 80
        for: 10m
        labels:
          severity: warning
        annotations:
          summary: "High CPU usage on {{ $labels.instance }}"
          description: "CPU usage is above 80% for more than 10 minutes."

Common Pitfalls & Tips

Validation Tools

# yamllint – linting
yamllint -s file.yaml

# yq – YAML processor (like jq)
yq eval '.person.name' file.yaml
yq eval -i '.person.age = 30' file.yaml

# Online validator
# https://www.yamllint.com/
# https://jsonformatter.org/yaml-validator