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How to build your inventory

Ansible works against multiple managed nodes or 'hosts' in your infrastructure at the same time, using a list or group of lists known as inventory. Once your inventory is defined, you use patterns to select the hosts or groups you want Ansible to run against.

The default location for inventory is a file called /etc/ansible/hosts. You can specify a different inventory file at the command line using the -i <path> option. You can also use multiple inventory files at the same time, and/or pull inventory from dynamic or cloud sources or different formats (YAML, ini, and so on), as described in Working with dynamic inventory. Introduced in version 2.4, Ansible has Inventory Plugins to make this flexible and customizable.


Inventory basics: formats, hosts, and groups

The inventory file can be in one of many formats, depending on the inventory plugins you have. The most common formats are INI and YAML. A basic INI /etc/ansible/hosts might look like this:

The headings in brackets are group names, which are used in classifying hosts and deciding what hosts you are controlling at what times and for what purpose. Group names should follow the same guidelines as Creating valid variable names.

Here's that same basic inventory file in YAML format:


Default groups

There are two default groups: all and ungrouped. The all group contains every host. The ungrouped group contains all hosts that don't have another group aside from all. Every host will always belong to at least 2 groups (all and ungrouped or all and some other group). Though all and ungrouped are always present, they can be implicit and not appear in group listings like group_names.


Hosts in multiple groups

You can (and probably will) put each host in more than one group. For example a production webserver in a datacenter in Atlanta might be included in groups called [prod] and [atlanta] and [webservers]. You can create groups that track:

Extending the previous YAML inventory to include what, when, and where would look like:

You can see that one.example.com exists in the dbservers, east, and prod groups.

You can also use nested groups to simplify prod and test in this inventory, for the same result:

You can find more examples on how to organize your inventories and group your hosts in Inventory setup examples.


Adding ranges of hosts

If you have a lot of hosts with a similar pattern, you can add them as a range rather than listing each hostname separately:

In INI:

In YAML:

You can specify a stride (increments between sequence numbers) when defining a numeric range of hosts:

In INI:

In YAML:

For numeric patterns, leading zeros can be included or removed, as desired. Ranges are inclusive. You can also define alphabetic ranges:


Adding variables to inventory

You can store variable values that relate to a specific host or group in inventory. To start with, you may add variables directly to the hosts and groups in your main inventory file. As you add more and more managed nodes to your Ansible inventory, however, you will likely want to store variables in separate host and group variable files. See Defining variables in inventory for details.


Assigning a variable to one machine: host variables

You can assign a variable to a single host, then use it later in playbooks. In INI:

In YAML:

Unique values like non-standard SSH ports work well as host variables. You can add them to your Ansible inventory by adding the port number after the hostname with a colon:

Connection variables also work well as host variables:

If you list non-standard SSH ports in your SSH config file, the openssh connection will find and use them, but the paramiko connection will not.


Inventory aliases

You can also define aliases in your inventory:

In INI:

In YAML:

In the above example, running Ansible against the host alias 'jumper' will connect to 192.0.2.50 on port 5555. See behavioral inventory parameters to further customize the connection to hosts.

Values passed in the INI format using the key=value syntax are interpreted differently depending on where they are declared:

Generally speaking, this is not the best way to define variables that describe your system policy. Setting variables in the main inventory file is only a shorthand. See Organizing host and group variables for guidelines on storing variable values in individual files in the 'host_vars' directory.


Assigning a variable to many machines: group variables

If all hosts in a group share a variable value, you can apply that variable to an entire group at once. In INI:

In YAML:

Group variables are a convenient way to apply variables to multiple hosts at once. Before executing, however, Ansible always flattens variables, including inventory variables, to the host level. If a host is a member of multiple groups, Ansible reads variable values from all of those groups. If you assign different values to the same variable in different groups, Ansible chooses which value to use based on internal rules for merging.


Inheriting variable values: group variables for groups of groups

You can make groups of groups using the :children suffix in INI or the children: entry in YAML. You can apply variables to these groups of groups using :vars or vars::

In INI:

In YAML:

If you need to store lists or hash data, or prefer to keep host and group specific variables separate from the inventory file, see Organizing host and group variables.

Child groups have a couple of properties to note:

  • Any host that is member of a child group is automatically a member of the parent group.

  • A child group's variables will have higher precedence (override) a parent group's variables.

  • Groups can have multiple parents and children, but not circular relationships.

  • Hosts can also be in multiple groups, but there will only be one instance of a host, merging the data from the multiple groups.


Organizing host and group variables

Although you can store variables in the main inventory file, storing separate host and group variables files may help you organize your variable values more easily. Host and group variable files must use YAML syntax. Valid file extensions include '.yml', '.yaml', '.json', or no file extension. See YAML Syntax if you are new to YAML.

Ansible loads host and group variable files by searching paths relative to the inventory file or the playbook file. If your inventory file at /etc/ansible/hosts contains a host named 'foosball' that belongs to two groups, 'raleigh' and 'webservers', that host will use variables in YAML files at the following locations:

For example, if you group hosts in your inventory by datacenter, and each datacenter uses its own NTP server and database server, you can create a file called /etc/ansible/group_vars/raleigh to store the variables for the raleigh group:

You can also create directories named after your groups or hosts. Ansible will read all the files in these directories in lexicographical order. An example with the 'raleigh' group:

All hosts in the 'raleigh' group will have the variables defined in these files available to them. This can be very useful to keep your variables organized when a single file gets too big, or when you want to use Ansible Vault on some group variables.

You can also add group_vars/ and host_vars/ directories to your playbook directory. The ansible-playbook command looks for these directories in the current working directory by default. Other Ansible commands (for example, ansible, ansible-console, and so on) will only look for group_vars/ and host_vars/ in the inventory directory. If you want other commands to load group and host variables from a playbook directory, you must provide the --playbook-dir option on the command line. If you load inventory files from both the playbook directory and the inventory directory, variables in the playbook directory will override variables set in the inventory directory.

Keeping your inventory file and variables in a git repo (or other version control) is an excellent way to track changes to your inventory and host variables.


How variables are merged

By default variables are merged/flattened to the specific host before a play is run. This keeps Ansible focused on the Host and Task, so groups don't really survive outside of inventory and host matching. By default, Ansible overwrites variables including the ones defined for a group and/or host (see DEFAULT_HASH_BEHAVIOUR). The order/precedence is (from lowest to highest):

By default Ansible merges groups at the same parent/child level in ASCII order, and the last group loaded overwrites the previous groups. For example, an a_group will be merged with b_group and b_group vars that match will overwrite the ones in a_group.

You can change this behavior by setting the group variable ansible_group_priority to change the merge order for groups of the same level (after the parent/child order is resolved). The larger the number, the later it will be merged, giving it higher priority. This variable defaults to 1 if not set. For example:

In this example, if both groups have the same priority, the result would normally have been testvar == b, but since we are giving the a_group a higher priority the result will be testvar == a.

ansible_group_priority can only be set in the inventory source and not in group_vars/, as the variable is used in the loading of group_vars.


Using multiple inventory sources

You can target multiple inventory sources (directories, dynamic inventory scripts or files supported by inventory plugins) at the same time by giving multiple inventory parameters from the command line or by configuring ANSIBLE_INVENTORY. This can be useful when you want to target normally separate environments, like staging and production, at the same time for a specific action.

Target two sources from the command line like this:

Variable conflicts in the inventories are resolved according to the rules described in How variables are merged and Variable precedence: Where should I put a variable?. The merging order is controlled by the order of the inventory source parameters. If [all:vars] in staging inventory defines myvar = 1, but production inventory defines myvar = 2, the playbook will be run with myvar = 2. The result would be reversed if the playbook was run with -i production -i staging.

Aggregating inventory sources with a directory

You can also create an inventory by combining multiple inventory sources and source types under a directory. This can be useful for combining static and dynamic hosts and managing them as one inventory. The following inventory combines an inventory plugin source, a dynamic inventory script, and a file with static hosts:

You can target this inventory directory simply like this:

It can be useful to control the merging order of the inventory sources if there's variable conflicts or group of groups dependencies to the other inventory sources. The inventories are merged in ASCII order according to the filenames so the result can be controlled by adding prefixes to the files:

If 01-openstack.yml defines myvar = 1 for the group all, 02-dynamic-inventory.py defines myvar = 2, and 03-static-inventory defines myvar = 3, the playbook will be run with myvar = 3.

For more details on inventory plugins and dynamic inventory scripts see Inventory Plugins and Working with dynamic inventory.


Connecting to hosts: behavioral inventory parameters

As described above, setting the following variables control how Ansible interacts with remote hosts.

Host connection

Ansible does not expose a channel to allow communication between the user and the ssh process to accept a password manually to decrypt an ssh key when using the ssh connection plugin (which is the default). The use of ssh-agent is highly recommended.

General for all connections

Specific to the SSH connection

Privilege escalation

See Ansible Privilege Escalation

Remote host environment parameters

Examples from an Ansible-INI host file:


Non-SSH connection types

As stated in the previous section, Ansible executes playbooks over SSH but it is not limited to this connection type. With the host specific parameter ansible_connection=<connector>, the connection type can be changed.

The following non-SSH based connectors are available:

...where...

Here is an example of how to instantly deploy to created containers:

For a full list with available plugins and examples, see Plugin List.

If you're reading the docs from the beginning, this may be the first example you've seen of an Ansible playbook. This is not an inventory file. Playbooks will be covered in great detail later in the docs.


Inventory setup examples

See also Sample Ansible setup, which shows inventory along with playbooks and other Ansible artifacts.


Example: One inventory per environment

If you need to manage multiple environments it's sometimes prudent to have only hosts of a single environment defined per inventory. This way, it is harder to, for instance, accidentally change the state of nodes inside the 'test' environment when you actually wanted to update some 'staging' servers.

For the example mentioned above you could have an inventory_test file:

That file only includes hosts that are part of the 'test' environment. Define the 'staging' machines in another file called inventory_staging:

To apply a playbook called site.yml to all the app servers in the test environment, use the following command:


Example: Group by function

In the previous section you already saw an example for using groups in order to cluster hosts that have the same function. This allows you, for instance, to define firewall rules inside a playbook or role without affecting database servers:


Example: Group by location

Other tasks might be focused on where a certain host is located. Let's say that db01.test.example.com and app01.test.example.com are located in DC1 while db02.test.example.com is in DC2:

In practice, you might even end up mixing all these setups as you might need to, on one day, update all nodes in a specific data center while, on another day, update all the application servers no matter their location.


See also

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