What is YAML? #

YAML is a very popular data serialization language like JSON or XML for writing configuration files. YAML is used with many DevOps tools and applications such as Docker, Kubernetes etc.

YAML stands for YAML Ain’t Markup Language and YAML files have either .yaml or .yml as their extension.

What does serialization languages do? #

Serialization languages facilitate different applications written in a variety of technologies and programming langauges to transfer data using a common, agreed on and standard format such as YAML.

YAML, JSON and XML are the most popular serialization languages

Why learn YAML? How does it compare to JSON and XML? #

YAML is the most human readable and intuitive format when compared to alternatives and hence, is a great fit for writing configurations of almost all the mordern DevOps tools such as Docker compose files, Ansible, Prometheus, Kubernetes and many more.

Let us look at the differences between YAML, JSON and XML:

YAML #

A simple config in YAML would look something like this:

microservices:
  - app: user-authentication
    port: 8000
    version: 1.0

• Note that there are no special characters to define data structures.
• YAML uses line seperation and space identation for defining the structure of data.

XML #

The same configuration in XML would look like this:

<microservices>
  <microservices>
    <app>user-authentication</app>
    <port>8000</port>
    <version>1.0</version>
  </microservices>
</microservices>

Note that here we are using tags (<> </>) to define data structure.

JSON #

The same configuration in JSOn would look like this:

{
  microservices: [
    {
      app: "user-authentication"
      port: 8000
      version: "1.0"
    }
  ]
}

Note that here, we are using curly braces ({}) to define data structure.

NOTE: YAML is a superset of JSON i.e any valid JSON file is also a valid YAML file.

YAML Syntax #

YAML consists of a number of components such as key-value pairs, objects, lists etc. which helps us structure the data as needed.

Let’s go through them one by one:

Key-value pairs #

app: user-authentication
port: 8000
version: 1.7

• In the above example app: user-authentication or port: 8000 are called key-value pairs.
• Note that user-authentication in the above example is a String, we don’t need to to include Strings in " unless we’re using special characters such as a newline in which case we’ll have to write this key-value pair as app: "user-authentication \n"

Comments #

# This is a comment
app: user-authentication
port: 8000
# This is an another comment
version: 1.7

• We can write comments in YAML using #
• Adding comments is always considered a good practice to make the config more easily readable and human understandable.

Objects #

microservice:
  app: user-authentication
  port: 8000
  version: 1.7

• We can group multiple key-value pairs inside of an object.
• In the above example we have the object microservice

NOTE:
Always take care of the spaces and identation while writing YAML. There are many code editor extensions and websites which can help with YAML validation.

Lists #

microservice:
  - app: user-authentication
    port: 8000
    version: 1.7
  - app: user-data
    port: 9000
    version: 2.0
  - app: user-name
    port: 7000
    version: ["1.0", 2.0, 3.0, "latest"]    
    
microservices:
  - user-authentication
  - user-data
  - user-name

• As shown above, we can create lists using -
• Lists helps us organize data when we have multiple key-value pairs under one object or multiple values under one key.
• We can also create nested lists as shown in the example below:

microservice:
  - app: user-authentication
    port: 8000
    version: 1.7
  - app: user-data
    port: 9000
    version:                # nested list
      - 1.0
      - 2.0
      - 3.0
  - app: user-name
    port: 7000
    version: 1.8

• Lists can also be created using this syntax:

microservice:
  - app: user-authentication
    port: 8000
    version: [1.7, 1.8, 1.9, "version"]     # another syntax for lists
  - app: user-data
    port: 9000
    version: 2.0

Booleans #

YAML accepts true/false, yes/no and on/off as boolean data.

microservice:
  app: user-authentication
  port: 8000
  version: 1.7
  deployed1: true # boolean true/false
  deployed2: yes  # boolean yes/no
  deployed3: on   # boolean on/off

Multi-line Strings #

multilineString: |
  this is a multiline string
  and this is the next line
  next line
  etc
    
singlelineString: >
  this is a single line string,
  All three lines
  should be in the same line.  

• As shown in above example, We can use | to display each line of string data type as a newline
• Similarly, we can use > if we want a single line string but don;t want to write it as a single line in our config file to make it more human readable.

Some things to note #

Accessing Environment Variables: #

We can access environment variable in YAML using $<env varibale>

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

In the above example, we are using the env variable MYSQL_ROOT_PASSWORD

Placeholders: #

• We can use placeholders in YAML using double curly braces {{value}}
• For eg: We need this in Helm and Ansible where placeholder value gets replaced using template generator.

Multiple YAML Documents: #

• We can separate different YAML configs using ---
• This comes very handy in Kubernetes where we have multiple components in one service and we want to group them in single YAML file.

apiVersion: v1
kind: configMap
metadata
  name: config-file
  
---

apiVersion: v1
kind: configMap
metadata
  name: config-file

Note that we can write Kubernetes configuration files in both YAML and JSON format but YAML is more commonly used as it is cleaner and more readable

Thank you so much for reading 💖 #

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