Working with dynamic inventory

If your Ansible inventory fluctuates over time, with hosts spinning up and shutting down in response to business demands, the static inventory solutions described in How to build your inventory will not serve your needs. You may need to track hosts from multiple sources: cloud providers, LDAP, Cobbler, and/or enterprise CMDB systems.

Ansible integrates all of these options through a dynamic external inventory system. Ansible supports two ways to connect with external inventory: Inventory plugins and inventory scripts.

Inventory plugins take advantage of the most recent updates to the Ansible Core code. We recommend plugins over scripts for dynamic inventory. You can write your own plugin to connect to additional dynamic inventory sources.

You can still use inventory scripts if you choose. When we implemented inventory plugins, we ensured backwards compatibility through the script inventory plugin. The examples below illustrate how to use inventory scripts.

If you prefer a GUI for handling dynamic inventory, the inventory database on AWX or Red Hat Ansible Automation Platform syncs with all your dynamic inventory sources, provides web and REST access to the results, and offers a graphical inventory editor. With a database record of all of your hosts, you can correlate past event history and see which hosts have had failures on their last playbook runs.

Inventory script example: Cobbler 

Ansible integrates seamlessly with Cobbler, a Linux installation server originally written by Michael DeHaan and now led by James Cammarata, who works for Ansible.

While primarily used to kickoff OS installations and manage DHCP and DNS, Cobbler has a generic layer that can represent data for multiple configuration management systems (even at the same time) and serve as a ‘lightweight CMDB’.

To tie your Ansible inventory to Cobbler, copy this script to /etc/ansible and chmod +x the file. Run cobblerd any time you use Ansible and use the -i command line option (for example, -i /etc/ansible/cobbler.py) to communicate with Cobbler using Cobbler’s XMLRPC API.

Add a cobbler.ini file in /etc/ansible so Ansible knows where the Cobbler server is and some cache improvements can be used. For example:

[cobbler]

# Set Cobbler's hostname or IP address
host = http://127.0.0.1/cobbler_api

# API calls to Cobbler can be slow. For this reason, we cache the results of an API
# call. Set this to the path you want cache files to be written to. Two files
# will be written to this directory:
#   - ansible-cobbler.cache
#   - ansible-cobbler.index

cache_path = /tmp

# The number of seconds a cache file is considered valid. After this many
# seconds, a new API call will be made, and the cache file will be updated.

cache_max_age = 900

First test the script by running /etc/ansible/cobbler.py directly. You should see some JSON data output, but it may not have anything in it just yet.

Let’s explore what this does. In Cobbler, assume a scenario somewhat like the following:

cobbler profile add --name=webserver --distro=CentOS6-x86_64
cobbler profile edit --name=webserver --mgmt-classes="webserver" --ksmeta="a=2 b=3"
cobbler system edit --name=foo --dns-name="foo.example.com" --mgmt-classes="atlanta" --ksmeta="c=4"
cobbler system edit --name=bar --dns-name="bar.example.com" --mgmt-classes="atlanta" --ksmeta="c=5"

In the example above, the system ‘foo.example.com’ is addressable by ansible directly, but is also addressable when using the group names ‘webserver’ or ‘atlanta’. Since Ansible uses SSH, it contacts system foo over ‘foo.example.com’, only, never just ‘foo’. Similarly, if you tried “ansible foo”, it would not find the system… but “ansible ‘foo*’” would do, because the system DNS name starts with ‘foo’.

The script provides more than host and group info. In addition, as a bonus, when the ‘setup’ module is run (which happens automatically when using playbooks), the variables ‘a’, ‘b’, and ‘c’ will all be auto-populated in the templates:

# file: /srv/motd.j2
Welcome, I am templated with a value of a={{ a }}, b={{ b }}, and c={{ c }}

Which could be executed just like this:

ansible webserver -m setup
ansible webserver -m template -a "src=/tmp/motd.j2 dest=/etc/motd"

Note

The name ‘webserver’ came from Cobbler, as did the variables for the config file. You can still pass in your own variables like normal in Ansible, but variables from the external inventory script will override any that have the same name.

So, with the template above (motd.j2), this results in the following data being written to /etc/motd for system ‘foo’:

Welcome, I am templated with a value of a=2, b=3, and c=4

And on system ‘bar’ (bar.example.com):

Welcome, I am templated with a value of a=2, b=3, and c=5

And technically, though there is no major good reason to do it, this also works:

ansible webserver -m ansible.builtin.shell -a "echo {{ a }}"

So, in other words, you can use those variables in arguments/actions as well.

Other inventory scripts 

In Ansible 2.10 and later, inventory scripts moved to their associated collections. Many are now in the ansible-community/contrib-scripts repository. We recommend you use Inventory plugins instead.

Using inventory directories and multiple inventory sources 

If the location given to -i in Ansible is a directory (or as so configured in ansible.cfg), Ansible can use multiple inventory sources at the same time. When doing so, it is possible to mix both dynamic and statically managed inventory sources in the same ansible run. Instant hybrid cloud!

In an inventory directory, executable files are treated as dynamic inventory sources and most other files as static sources. Files which end with any of the following are ignored:

~, .orig, .bak, .ini, .cfg, .retry, .pyc, .pyo

You can replace this list with your own selection by configuring an inventory_ignore_extensions list in ansible.cfg, or setting the ANSIBLE_INVENTORY_IGNORE environment variable. The value in either case must be a comma-separated list of patterns, as shown above.

Any group_vars and host_vars subdirectories in an inventory directory are interpreted as expected, making inventory directories a powerful way to organize different sets of configurations. See Passing multiple inventory sources for more information.

Static groups of dynamic groups 

When defining groups of groups in the static inventory file, the child groups must also be defined in the static inventory file, otherwise ansible returns an error. If you want to define a static group of dynamic child groups, define the dynamic groups as empty in the static inventory file. For example:

[tag_Name_staging_foo]

[tag_Name_staging_bar]

[staging:children]
tag_Name_staging_foo
tag_Name_staging_bar

Working with dynamic inventory

Inventory script example: Cobbler

Other inventory scripts

Using inventory directories and multiple inventory sources

Static groups of dynamic groups

Inventory script example: Cobbler 

Other inventory scripts 

Using inventory directories and multiple inventory sources 

Static groups of dynamic groups 