YAML Basics Every DevOps Engineer Must Know

YAML has gained a lot of popularity over the last few years as it became part of crucial DevOps tools, technologies and processes such as Ansible, Kubernetes, CI/CD pipelines and so on.

We have already covered lots of tutorials on Ansible and Kubernetes. I thought of covering YAML essentials so that you must be aware for a smoother working with your DevOps tools configuration.

What is YAML?

YAML stands for "YAML Ain't Markup Language" originally was an acronym for 'Yet Another Markup Language'. YAML is a “data serialization” language and basically a human-readable structured data format.

It is designed to be read and write friendly. The object serialization feature of YAML presents itself as a practicable alternative to JSON. YAML is a superset of JSON with the use of indentation-based scoping to denote the structure like Python.

Here's a sample YAML example:

--- 
 Student-ID: 314159
 First-Name: Linus
 Last-Name: Torvalds

Phone-numbers:
    - 281.555.7777
    - 832.676.8888
    - 937.996.9999

Addresses:
    - Street: 123 Main St.
    - City: Houston
    - State: Tx
---

YAML basic rules you should always remember

If you don't want to see repeated errors while parsing your YAML file, you must always keep the following in your mind while working on YAML:

• Tabs are NOT allowed in YAML. You should use space for indention.
• Though the amount of space doesn't matter as long as the child node indentation is more than the parent, it is a good practice to keep the same number of spaces.
• There must be space between different elements of YAML (explained later).
• YAML is case-sensitive.
• YAML file should end with extensions like .yaml or .yml.
• YAML allows UTF-8, UTF-16 and UTF-32 encoding.

Let's understand the YAML syntax now.

Elements of a YAML file: Basic syntax

A YAML file is used to describe data. In a YAML file the content is all about a collection of key-value pairs where the value can be anything ranging from a string to a tree.

Let us understand it by an example. This is a Kubernetes service manifest file.

kind: Service
metadata:  
  name: web-app-svc
spec:  
  type: NodePort  
  ports:  
  - port: 8080         #service port    
    targetPort: 8080   #Pod Port    
    nodePort: 30012  #Node Port from the range - 30000-32767  
  selector:    
    app: web-app

It's self explainable that it's a set of key value pair elements: Name: Value.

As you can see from the file above, a YAML file is constructed of a number of different elements. Together, they can be used to describe a wide variety of structures.

1. Spaces or indentation

In YAML, you indent with whitespace, not tabs. And there MUST be a space between elements.

Correct specification:

Kind: Service

Incorrect specification:

Kind:Service

Because there is no space after the colon in the above statement!

2. Comments in YAML

Comments in YAML can be defined by placing a hash in front of an item '#'. Comments can be made at the start of a line of anywhere in the line.

Here's an example.

#Configuring the port  
  ports:  
  - port: 8080         #service port    
    targetPort: 8080   #Pod Port    
    nodePort: 30012  #Node Port from the range - 30000-32767 

As you can see, it has one single line comment and three inline comments.

How to Add Comments in YAML

New to YAML? See how to add single-line, inline comments in YAML. Also learn about commenting out multiple lines quickly.

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3. Scalar (key-value)

Scalars are the strings and numbers that make up the data on the page. In simple terms they are the key value pairs.

kind: Service
metadata:  
  name: web-app-svc

4. Collections & Lists

List and collection elements or members are the lines that begin at the same indentation level, starting with a dash followed by a space.

- web-app-prod 
- prod-deployments 
- prom-monitored

It is a basic list with each item in the list placed in its own line with an opening dash.

5. Nested collections

If you want to create a nested sequence with items and sub-items, you can do so by placing a single space before each dash in the sub-items.

- 
 - web-app-prod 
 - prod-deployments 
 - prom-monitored
-  
 - web-app-test 
 - staging-deployments 
 - not-monitored

6. Dictionaries

Dictionaries comprise a key: value format with contents indented.

ports:    
- port: 8080         #service port    
  targetPort: 8080   #Pod Port    
  nodePort: 30012  #Node Port from the range - 30000-32767

You can merge and mix-up collections of lists and dictionaries like this:

ports:    
- port: 8080         #service port    
  targetPort: 8080   #Pod Port    
  nodePort:       
  - 30012       
  - 30013       
  - 30014

These are very basic concepts of YAML but essential for a DevOps engineer.

You don't need special editor for YAML. Your favorite text editor should already be supporting YAML or use a plugin if required.

There are many things you can dig deeper and learn. For that, you may always refer to YAML's official documentation.

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