Understanding YAML

The default renderer for SLS files is the YAML renderer. YAML is a markup language with many powerful features. However, Salt uses a small subset of YAML that maps over very commonly used data structures, like lists and dictionaries. It is the job of the YAML renderer to take the YAML data structure and compile it into a Python data structure for use by Salt.

Though YAML syntax may seem daunting and terse at first, there are only three very simple rules to remember when writing YAML for SLS files.

Rule One: Indentation

YAML uses a fixed indentation scheme to represent relationships between data layers. Salt requires that the indentation for each level consists of exactly two spaces. Do not use tabs.

Rule Two: Colons

Python dictionaries are, of course, simply key-value pairs. Users from other languages may recognize this data type as hashes or associative arrays.

Dictionary keys are represented in YAML as strings terminated by a trailing colon. Values are represented by either a string following the colon, separated by a space:

my_key: my_value

In Python, the above maps to:

{'my_key': 'my_value'}

Alternatively, a value can be associated with a key through indentation.



The above syntax is valid YAML but is uncommon in SLS files because most often, the value for a key is not singular but instead is a list of values.

In Python, the above maps to:

{'my_key': 'my_value'}

Dictionaries can be nested:

  second_level_dict_key: value_in_second_level_dict

And in Python:

    'first_level_dict_key': {
        'second_level_dict_key': 'value_in_second_level_dict'

Rule Three: Dashes

To represent lists of items, a single dash followed by a space is used. Multiple items are a part of the same list as a function of their having the same level of indentation.

- list_value_one
- list_value_two
- list_value_three

Lists can be the value of a key-value pair. This is quite common in Salt:

  - list_value_one
  - list_value_two
  - list_value_three

In Python, the above maps to:

{'my_dictionary': ['list_value_one', 'list_value_two', 'list_value_three']}

Learning More

One easy way to learn more about how YAML gets rendered into Python data structures is to use an online YAML parser to see the Python output.

One excellent choice for experimenting with YAML parsing is: http://yaml-online-parser.appspot.com/


Jinja statements and expressions are allowed by default in SLS files. See Understanding Jinja.