An Ansible inventory plugin for DigitalOcean does not yet exist. Fortunately, with some bash scripting and the help of jq, we can write a script which generates an inventory for us.

If we had the following set of servers:

A static Ansible inventory file could look like

[appservers]
100.65.89.4     internal_ip=10.100.101.90
100.65.89.10    internal_ip=10.100.101.23
100.65.89.24    internal_ip=10.100.101.99

[databases]
100.202.73.9    internal_ip=10.200.4.187

[load-balancer]
100.200.33.196  internal_ip=10.200.4.136

Servers come and go, and keeping track of them in this manner is no fun.

The script

#!/bin/bash

json=$(
  curl -s -X GET \
  https://api.\digitalocean.com/v2/droplets \
  -H "Authorization: Bearer ${DO_TOKEN}"
)

filter_droplets () {
  result=$(
    echo "${1}" \
    | jq --arg role $2 '
        .droplets
        | .[]
        | select(.tags[] == $role)
        | .networks.v4
        | [
          (.[] | select(.type == "public") | .ip_address),
          (.[] | select(.type == "private") | .ip_address)
        ]'
  )
}

filter_droplets "${json}" role-appserver
appservers="${result}"

filter_droplets "${json}" role-database
databases="${result}"

filter_droplets "${json}" role-load-balancer
load_balancer="${result}"

meta_hostvars=$(
  echo "${appservers} ${load_balancer} ${databases}" \
  | jq -s '
    .
    |= [
      (
        .[]
        | {
            (.[0]): { "internal_ip": .[1] }
          }
      )
    ]
    | add'
)

jq '.' << EOH
{
  "appservers": {
    "hosts": $(echo "${appservers}" | jq -s '. | [(.[] | first)]')
  },
  "load_balancer": {
    "hosts": $(echo "${load_balancer}" | jq -s '. | [(.[] | first)]')
  },
  "databases": {
    "hosts": $(echo "${databases}" | jq -s '. | [(.[] | first)]')
  },
  "_meta": {
    "hostvars": ${meta_hostvars}
  }
}
EOH

To execute the script, first make sure the DO_TOKEN variable is set with your personal access token:

export DO_TOKEN=<your digitalocean personal access token>
./dynamic-inventory.sh

The output should look similar to this:

{
  "appservers": {
    "hosts": [
      "100.65.89.4",
      "100.65.89.10",
      "100.65.89.24",
    ]
  },
  "load_balancer": {
    "hosts": [
      "100.200.33.196"
    ]
  },
  "databases": {
    "hosts": [
      "100.202.73.9"
    ]
  },
  "_meta": {
    "hostvars": {
      "100.65.89.4": {
        "internal_ip": "10.100.101.90"
      },
      "100.65.89.10": {
        "internal_ip": "10.100.101.23"
      },
      "100.65.89.24": {
        "internal_ip": "10.100.101.99"
      },
      "100.202.73.9": {
        "internal_ip": "10.200.4.187"
      },
      "100.200.33.196": {
        "internal_ip": "10.200.4.136"
      }
    }
  }
}

To use it for a playbook, supplement the static inventory with a path to the script file:

ansible-playbook --inventory=dynamic-inventory.sh my-playbook.yml

How it works

json=$(
  curl -s -X GET \
  https://api.\digitalocean.com/v2/droplets \
  -H "Authorization: Bearer ${DO_TOKEN}"
)

Fetch a list of all available droplets.

filter_droplets () {
  result=$(
    echo "${1}" \
    | jq --arg role $2 '
        .droplets
        | .[]
        | select(.tags[] == $role)
        | .networks.v4
        | [
          (.[] | select(.type == "public") | .ip_address),
          (.[] | select(.type == "private") | .ip_address)
        ]'
  )
}

Pipe JSON data into a jq filter which filters droplets based on a role (role-appserver, role-database, role-load-balancer) and returns a list of arrays in the format [ <public ip>, <private ip> ].

[
  "100.202.73.9",
  "10.200.4.187"
]
[
  "100.200.33.196",
  "10.200.4.136"
]

filter_droplets "${json}" role-appserver
appservers="${result}"

filter_droplets "${json}" role-database
databases="${result}"

filter_droplets "${json}" role-load-balancer
load_balancer="${result}"

Set server group data from results of the filter_droplets function call.

meta_hostvars=$(
  echo "${appservers} ${load_balancer} ${databases}" \
  | jq -s '
    .
    |= [
      (
        .[]
        | {
            (.[0]): { "internal_ip": .[1] }
          }
      )
    ]
    | add'
)

Setup the _meta property by piping the joined lists of all the server groups into jq and using the slurp (-s) option to generate valid JSON. The jq filter will create key/value pairs where the key is the public ip (.[0]) and the value is an object containing the private ip ({"internal_ip": .[1]}). add merges the the array of key/value pairs into a single object.

Finally we generate the JSON output.

  "appservers": {
    "hosts": $(echo "${appservers}" | jq -s '. | [(.[] | first)]')
  },

Once again slurp the list of IP arrays to create a valid JSON array of arrays, and for each sub-array, use the first item.