YAML Basics for DevNet

Taking a look at the Cisco DevNet Associate exam topics (DEVASC 200-901), one of the first objectives is to compare different data formats. Those data formats you should be able to compare are XML, JSON, and YAML.

To start, I decided to take a look at YAML and what it has to offer from a very basic overview. I’m very new when it comes to DevOps or NetDevOps but this will become my notes of sorts as I look into the DevNet Associate certification.

YAML is a human-readable structured data format which stands for Stands for Yet Another Markup Language or now YAML Ain’t Markup Language, depending on where you look ;) The official documentation page states the latter.

Goals of YAML

• Data serialization standard for programming languages.
• Human-readable structured data format.
• Portable between programming languages.
• Consistent model.
• Expressive and extensible.
• Easy to implement and use.

A quick overview of YAML

• Writable and readable.
• Case sensitive
• Files end in .yaml or .yml
• Spaces are used instead of tabs (because various tools treat tabs differently)
• Indentation is used for structure
• Colons separate key-value pairs
• Dashes create bullet lists
• The start of a YAML file begins with three dashes

Data structures:

• Mappings (hashes/dictionaries)
• Sequences (arrays/lists)
• Scalars (strings/numbers)

Working with Blocks
Block collections use indentation for scope and begin each entry on its new line. Block sequences use a dash and space to indicate each entry. Mappings use a colon and space to mark a key-value pair.

To create a comment, you must use a hash tag.

Sequence
A sequence entry will be denoted by a dash -.

 - netsw-idf-01
 - netsw-idf-02
 - netsw-idf-03 

Scalar
A scalar is just a single value. They can be strings, numbers, Boolean, or null values.

# scalars can be quoted
“GigabitEthernet1/0/1”

# scalars can also be unquoted
GigabitEthernet1/0/2 

Mapping Scalars to Sequences
Also creates an array.

routers:
    - rtr-idf-01
    - rtr-idf-02
switches:
    - netsw-idf-01
    - netsw-idf-02

Sample Inventory in YAML

—--
sites:
    - name: san-jose # This is a comment. Begin block 1.
        hosts:
            - hostname: r1
              host: 192.168.1.1
              os: cisco_ios
            - hostname: sw1
              host: 192.168.1.5
              os: cisco_ios
            - hostname: sw2
              host: 192.168.1.6
              os: junos

    - name: san-diego # This is a comment. Begin block 2.
        hosts:
            - hostname: r2
              host: 192.168.2.1
              os: cisco_ios
            - hostname: sw1
              host: 192.168.2.5
              os: cisco_ios

Have we created a list or a dictionary? List items which is a dictionary.

Here’s a sample YAML script used to configure a VLAN on a Cisco switch.

---
- hosts: cisco
  connection: network_cli
  tasks:
    - nxos_vlan:
      vlan_id: 10
      name: STORAGE