YAML Cheatsheet

What is YAML

Data serializaion language (like XML and JSON)
Standard format to transfer data
Extensions : .yaml and .yml
YAML is a superset of JSON: any valid JSON file is also a valid YAML file
Data structures defined in line separation and indentation

YAML Use Cases

Docker-compose, Ansible, Kubernetes and many more

Key value pairs

app: user-authentication
port: 9000
# A comment
version: 1.7
# A second comment

For strings, you can use either double quotes, single quotes or no quotes at all. If you use \n, you have to use double quotes or YAML don't recognize it.

Objects

microservice:
  app: user-authentication
  port: 9000
  version: 1.7

The space has to be the exact same for each attribute between objects. You can use an online YAML validator because it is sensitive about those spaces.

Lists & Boolean

microservice:
  - app: user-authentication
    port: 9000
    version: 1.7
    deployed: false # yes and no, on and off works too
    versions:
    - 1.9
    - 2.0
    - 2.1 # You can use lists inside of list items, always align them.
  - app: shopping-cart
    port: 9002
    versions: [2.4, 2.5, "hello"]
    # You can use arrays instead, and have a mix of numbers and strings. 

microservices: 
  - user-authentication
  - shopping-cart

Boolean pitfalls:

yes: true  # Interpreted as boolean true
no: false  # Interpreted as boolean false
on: true   # Also interpreted as true
off: false # Also interpreted as false

If you actually want "yes", "no", "on" and "off" as strings, quote them:

user-input: "yes"  # String, not a boolean

Use !!str, !!int and !!bool for Explicit Types

Sometimes YAML thinks it knows what you mean. Force it to behave.

bad-example: 00123   # YAML assumes this is an octal number (!!!)
good-example: !!str "00123"  # Now it's a string, not octal

Real Kubernetes YAML Configuration Example

Key-value pairs
metadata: object
labels: object
spec: object
containers: list of objects
ports: list
volumeMounts: list of objects

apiVersion: v1
kind: Pod
metadata:
  name: nginx
  labels:
    app: nginx
  spec:
    containers:
    - name: nginx-container
      image: nginx
      ports:
      - containerPort: 80
      volumeMounts:
      - name: nginx-vol
        mountPath: /usr/nginx/html
    - name: sidecar-container
      image: curlimages/curl
      command: ["/bin/sh"]
      args: ["-c", "echo Hello from the sidecar container; sleep 300"]

Multi Line Strings

multilineString: |
  this is a multiline String
  and this is the next line.
  next line

singlelineString: >
  this is a single line String
  that should be all on one line.
  some other stuff

Use | pipes if you want YAML to interpret this as multi line text. The line breaks will stay.
Greater than sign > will be interpreted as a single line.

Real Kubernetes examples

apiVersion: v1
kind: ConfigMap
metadata:
  name: mosquito-config-file
data:
  mosquito.conf: |
    log_dest stdout
    log_type all
    log_timestamp true
    listener 9001

You can put a whole shell script inside a YAML file.

command:
  - sh
  - -c
  - |
    http () {
        local path="${1}"
        set -- -XGET -s --fail
        curl -k "$@" "http://localhost:5601${path}"
    }
    http "/app/kibana"

Environment Variables

You can access them using a dollar sign inside your YAML configuration.

command:
  - /bin/sh
  - -ec
  - >-
    mysql -h 127.0.0.1 -u root -p$MYSQL_ROOT_PASSWORD -e 'SELECT 1'

Placeholders

Instead of directly defining values, you can put placeholders with double brackets. It gets replaced using a template generator.

apiVersion: v1
kind: Service
metadata:
  name: {{ .Values.service.name }}
spec:
  selector:
    app: {{ .Values.service.app }}
  ports:
    - protocol: TCP
      port: {{ .Values.service.port }}
      targetPort: {{ .Values.service.targetport }}

YAML Anchors & Aliases (DRY Principle)

YAML lets you reuse values using anchors (&) and aliases (*)

default-config: &default
  app: user-authentication
  port: 9000
  version: 1.7

microservice:
  - <<: *default  # Reuses the default config
    deployed: false
  - app: shopping-cart
    port: 9002
    version: 2.4

Merge Keys (Combine Multiple Defaults)

Anchors can also be merged into objects:

common-config: &common
  logging: true
  retries: 3

extra-config:
  <<: *common  # Merges the common config
  retries: 5  # Overrides specific values

Multiple YAML documents

This is especially useful when you have multiple components for one service. Separate them with three dashes.

apiVersion: v1
kind: ConfigMap
metadata:
  name: mosquito-config-file
data:
  mosquito.conf: |
    log_dest stdout
    log_type all
    log_timestamp true
    listener 9001

---
apiVersion: v1
kind: Secret
metadata:
  name: mosquito-secret-file
type: Opaque
data:
  secret.file: |
    cbdfdfg654fgdfg6f5sb132v1f6sg854g6s8g66IYUHGFKJHGVfd21=

In Kubernetes, you can use both YAML or JSON, but YAML is cleaner and more readable.

YAML Linting Tools

A CLI tool: yamllint

kubectl apply --dry-run=client -f file.yaml (Validates YAML syntax for Kubernetes)

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