How to make yourself to like YAML

2003 when I was employed at straightec GmbH, my boss was one of the most brilliant software engineers I’ve ever met. I’ve learnt a lot from him and I am still applying most of his philosophy and guiding principles in my daily work. To make an impression of him, its enough to know that we have used Smalltalk to develop a real-world, successful commercial product.

One of his principles was, “if you need a special programming language to write your configuration, it means your main development language is crap”. He would usually proceed demostrating that a configuration file written in Smalltalk is at least not worse than a file written in INI format or in XML.

So, naturally, I had preposessions against JSON or YAML, preferring to keep my configurations and my infrastructure scripts in my main programming language, in this case Python.

Alas, life forces you to abandon your principles from time to time, and the need to master and to use Ansible and Kubernetes has forced me to learn YAML.

Here is how you can learn YAML if you in principle against of it, but you have to learn it anyway.

This is YAML for a string

Some string

and this is for a number

42

And this is a one-level dictionary with string as keys and strings or numbers as values

key1: value1
key2: 3.1415
key3: "500" # if you want to force the type to be string, use double quotes

Next is a one-dimensional array of strings

- item 1
- item 2
- third item
- строки могут быть utf-8

Nested dictionaries

data_center:
  rack_8:
    slot_0:
      used: 1
      power_consumption: 150  
    slot_1:
      used: 1
      power_consumption: 2  
    slot_2:
      used: 0

You can glue nested levels together like this:

data_center.rack_8.slot_0:
      used: 1
      power_consumption: 150    

A dictionary having an array as some value

scalar_value: 123
array_value:
  - first
  - second
  - third
another_way_defining_array_value: ["first", "second", "third"]

Now something that I was often doing wrong (and still doing wrong from time to time): an array of dictionaries. Each dictionary has two keys: “name” and “price”

- name: Chair
  price: 124€       # note that price is indented and stays directly under name. 
                    # any other position of price is incorrect.
- name: Table
  price: 800€       

# note that there is nothing special in the "name", you can use any key to be first:

- price: 300€
  name: Another table       

- price: 12€
  name: Plant pot
       

Just to be perfectly clear, the YAML above is equivalent to the following JSON

[
  {
    "name": "Chair",
    "price": "124€"
  },
  {
    "name": "Table",
    "price": "800€"
  },
  {
    "price": "300€",
    "name": "Another table"
  },
  {
    "price": "12€",
    "name": "Plant pot"
  }
]

Finally, you can put several objects into one file by delimiting them with —

---
type: Container
path: some/url/here
replicas: 1
label: my_app
---
type: LoadBalancer
selector: my_app
port: 8080

This should cover about 80% of your needs when writing simple YAML.

If you want to continue learning YAML, I recommend you to read about

• how to write a string spanning over several lines
• how to reference one object from another inside of the same YAML
• And the Jinja2 templating syntax that is used very often together with YAML