Interpreting JSON Encoded Data

What it is

JSON is a text format for structured data, and this objective asks you to read a piece of it and say what it contains.

JSON (JavaScript Object Notation) is used by programs to exchange information, and most REST APIs return their answers in it. Despite the name, it is not tied to JavaScript. Nearly every language can read and write it.

It is popular because it is easy for people to read and easy for software to process. In network automation, you meet it when a controller returns a list of devices, when a device reports its interfaces, or when you send a configuration to an API.

How it works

JSON is built from objects and arrays that hold simple values and can be nested inside each other.

The building blocks

JSON has two structures and a few simple value types.

An object is a collection of key-value pairs inside curly braces. The key is always a text string in double quotes. A colon separates it from its value, and commas separate the pairs.

{
  "hostname": "R1",
  "model": "ISR4331",
  "uptime_days": 42,
  "managed": true
}

An array is an ordered list of values inside square brackets, separated by commas. The values can be of any type, including objects.

["GigabitEthernet0/0", "GigabitEthernet0/1", "Loopback0"]

The value types are:

Notice the difference between "42" and 42. The first is a string, and the second is a number. Likewise "true" is a string, and true is a boolean.

Nesting

Objects and arrays can hold other objects and arrays. This is called nesting, and it is how JSON describes complex data.

Here is a device with a list of interfaces, each of which is an object:

{
  "device": {
    "hostname": "SW1",
    "interfaces": [
      {
        "name": "GigabitEthernet0/1",
        "enabled": true,
        "vlan": 10,
        "ip": null
      },
      {
        "name": "GigabitEthernet0/2",
        "enabled": false,
        "vlan": 20,
        "ip": "192.168.20.1"
      }
    ]
  }
}

Finding a value

To read nested data, follow the path from the outside in. Keys are followed by name, and arrays are followed by position. Positions in an array start at zero.

• The hostname is found at device, then hostname. The value is "SW1".
• The interfaces are a list at device, then interfaces. It has two items.
• The first interface is position 0. Its name is "GigabitEthernet0/1".
• The second interface is position 1. Its vlan is 20.
• The first interface has "ip": null, which means it has no address assigned.
• The second interface is not enabled, because "enabled": false.

Written as a path in a program, the VLAN of the second interface is often shown as device.interfaces[1].vlan.

A useful reading habit is to ask three questions of each part: is it an object, an array or a simple value, what is the key, and what is the type of the value.

Syntax rules

JSON is strict, and small mistakes make the whole document invalid.

• Keys and strings use double quotes. Single quotes are not allowed.
• Pairs are separated by commas, but there must be no trailing comma after the last item.
• Braces and brackets must be balanced. Every { needs a } and every [ needs a ].
• JSON does not allow comments.
• Keys inside one object should be unique.

A broken example, with two mistakes:

{
  'hostname': "R1",
  "uptime_days": 42,
}

The first key uses single quotes, and there is a trailing comma after the last pair.

JSON compared with XML and YAML

All three describe structured data, but they look different.

Many tools can convert from one to the other.