Developing Dynamic Inventory Sources

• Script Conventions
• Tuning the External Inventory Script

As described in Dynamic Inventory, Ansible can pull inventory information from dynamic sources, including cloud sources.

How do we write a new one?

Simple! We just create a script or program that can print JSON in the right format when fed the proper arguments. You can do this in any language.

Script Conventions

When the external node script is called with the single argument --list, the script must output a JSON encoded hash/dictionary of all the groups to be managed to stdout. Each group’s value should be either a hash/dictionary containing a list of each host/IP, potential child groups, and potential group variables, or simply a list of host/IP addresses, like so:

{
    "databases": {
        "hosts": ["host1.example.com", "host2.example.com"],
        "vars": {
            "a": true
        }
    },
    "webservers": ["host2.example.com", "host3.example.com"],
    "atlanta": {
        "hosts": ["host1.example.com", "host4.example.com", "host5.example.com"],
        "vars": {
            "b": false
        },
        "children": ["marietta", "5points"]
    },
    "marietta": ["host6.example.com"],
    "5points": ["host7.example.com"]
}

Before version 1.0, each group could only have a list of hostnames/IP addresses, like the webservers, marietta, and 5points groups above.

When called with the arguments --host <hostname> (where <hostname> is a host from above), the script must print either an empty JSON hash/dictionary, or a hash/dictionary of variables to make available to templates and playbooks. Printing variables is optional, if the script does not wish to do this, printing an empty hash/dictionary is the way to go:

{
    "favcolor": "red",
    "ntpserver": "wolf.example.com",
    "monitoring": "pack.example.com"
}

Tuning the External Inventory Script

The stock inventory script system detailed above works for all versions of Ansible, but calling --host for every host can be rather expensive, especially if it involves expensive API calls to a remote subsystem. In Ansible 1.3 or later, if the inventory script returns a top level element called “_meta”, it is possible to return all of the host variables in one inventory script call. When this meta element contains a value for “hostvars”, the inventory script will not be invoked with --host for each host. This results in a significant performance increase for large numbers of hosts, and also makes client side caching easier to implement for the inventory script.

The data to be added to the top level JSON dictionary looks like this:

{

    # results of inventory script as above go here
    # ...

    "_meta": {
        "hostvars": {
            "moocow.example.com": {
                "asdf" : 1234
            },
            "llama.example.com": {
                "asdf": 5678
            }
        }
    }
}

To satisfy the requirements of using _meta, to prevent ansible from calling your inventory with --host you must at least populate _meta with an empty hostvars dictionary, such as:

{

    # results of inventory script as above go here
    # ...

    "_meta": {
        "hostvars": {}
    }
}