Ansible works against multiple systems in your infrastructure at the same time. It does this by selecting portions of systems listed in Ansible’s inventory, which defaults to being saved in the location /etc/ansible/hosts. You can specify a different inventory file using the -i <path> option on the command line.
Not only is this inventory configurable, but you can also use multiple inventory files at the same time and pull inventory from dynamic or cloud sources or different formats (YAML, ini, etc). Ansible has inventory plugins to make this flexible and customizable.
Hosts and Groups
The inventory file can be in one of many formats, depending on the inventory plugins you have. For this example, the format for /etc/ansible/hosts is an INI-like (one of Ansible’s defaults) and looks like this:
mail.example.com
[webservers]
foo.example.com
bar.example.com
[dbservers]
one.example.com
two.example.com
three.example.com
The headings in brackets are group names, which are used in classifying systems and deciding what systems you are controlling at what times and for what purpose.
A YAML version would look like:
all:
hosts:
mail.example.com:
children:
webservers:
hosts:
foo.example.com:
bar.example.com:
dbservers:
hosts:
one.example.com:
two.example.com:
three.example.com:
It is ok to put systems in more than one group, for instance a server could be both a webserver and a dbserver. If you do, note that variables will come from all of the groups they are a member of.
If you have hosts that run on non-standard SSH ports you can put the port number after the hostname with a colon. Ports listed in your SSH config file won’t be used with the paramiko connection but will be used with the openssh connection.
To make things explicit, it is suggested that you set them if things are not running on the default port:
badwolf.example.com:5309
Suppose you have just static IPs and want to set up some aliases that live in your host file, or you are connecting through tunnels. You can also describe hosts via variables:
In INI:
jumper ansible_port=5555 ansible_host=192.0.2.50
In YAML:
...
hosts:
jumper:
ansible_port: 5555
ansible_host: 192.0.2.50
In the above example, trying to ansible against the host alias “jumper” (which may not even be a real hostname) will contact 192.0.2.50 on port 5555. Note that this is using a feature of the inventory file to define some special variables. Generally speaking, this is not the best way to define variables that describe your system policy.
If you are adding a lot of hosts following similar patterns, you can do this rather than listing each hostname:
[webservers]
www[01:50].example.com
For numeric patterns, leading zeros can be included or removed, as desired. Ranges are inclusive. You can also define alphabetic ranges:
[databases]
db-[a:f].example.com
You can also select the connection type and user on a per host basis:
[targets]
localhost ansible_connection=local
other1.example.com ansible_connection=ssh ansible_user=mpdehaan
other2.example.com ansible_connection=ssh ansible_user=mdehaan
Host Variables
It is easy to assign variables to hosts that will be used later in playbooks:
[atlanta]
host1 http_port=80 maxRequestsPerChild=808
host2 http_port=303 maxRequestsPerChild=909
Group Variables
Variables can also be applied to an entire group at once:
The INI way:
[atlanta]
host1
host2
[atlanta:vars]
ntp_server=ntp.atlanta.example.com
proxy=proxy.atlanta.example.com
The YAML version:
atlanta:
hosts:
host1:
host2:
vars:
ntp_server: ntp.atlanta.example.com
proxy: proxy.atlanta.example.com
Be aware that this is only a convenient way to apply variables to multiple hosts at once; even though you can target hosts by group, variables are always flattened to the host level before a play is executed.
Groups of Groups, and Group Variables
It is also possible to make groups of groups using the :children suffix in INI or the children: entry in YAML. You can apply variables using :vars or vars::
INI
[atlanta]
host1
host2
[raleigh]
host2
host3
[southeast:children]
atlanta
raleigh
[southeast:vars]
some_server=foo.southeast.example.com
halon_system_timeout=30
self_destruct_countdown=60
escape_pods=2
[usa:children]
southeast
northeast
southwest
northwest
YAML
all:
children:
usa:
children:
southeast:
children:
atlanta:
hosts:
host1:
host2:
raleigh:
hosts:
host2:
host3:
vars:
some_server: foo.southeast.example.com
halon_system_timeout: 30
self_destruct_countdown: 60
escape_pods: 2
northeast:
northwest:
southwest:
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.
Default groups
There are two default groups: all and ungrouped. all contains every host. ungrouped contains all hosts that don’t have another group aside from all. Every host will always belong to at least 2 groups. Though all and ungrouped are always present, they can be implicit and not appear in group listings like group_names.
Splitting Out Host and Group Specific Data
The preferred practice in Ansible is to not store variables in the main inventory file.
In addition to storing variables directly in the inventory file, host and group variables can be stored in individual files relative to the inventory file (not directory, it is always the file).
These variable files are in YAML format. Valid file extensions include ‘.yml’, ‘.yaml’, ‘.json’, or no file extension.
Assuming the inventory file path is: /etc/ansible/hosts
If the host is named ‘foosball’, and in groups ‘raleigh’ and ‘webservers’, variables in YAML files at the following locations will be made available to the host:
/etc/ansible/group_vars/raleigh # can optionally end in '.yml', '.yaml', or '.json'
/etc/ansible/group_vars/webservers
/etc/ansible/host_vars/foosball
For instance, suppose you have hosts grouped by datacenter, and each datacenter uses some different servers. The data in the groupfile ‘/etc/ansible/group_vars/raleigh’ for the ‘raleigh’ group might look like:
---
ntp_server: acme.example.org
database_server: storage.example.org
It is okay if these files do not exist, as this is an optional feature.
As an advanced use case, you can create directories named after your groups or hosts, and Ansible will read all the files in these directories in lexicographical order. An example with the ‘raleigh’ group:
/etc/ansible/group_vars/raleigh/db_settings
/etc/ansible/group_vars/raleigh/cluster_settings
All hosts that are 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 starts to be too big, or when you want to use Ansible Vault on a part of a group’s variables.
Tip: The group_vars/ and host_vars/ directories can exist in the playbook directory OR the inventory directory. If both paths exist, variables in the playbook directory will override variables set in the inventory directory.
Tip: 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. The order/precedence is (from lowest to highest):
- all group (because it is the ‘parent’ of all other groups)
- parent group
- child group
- host
When groups of the same parent/child level are merged, it is done alphabetically, 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.
New in version 2.4.
Starting in Ansible version 2.4, users can use 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:
ansible_group_priority: 10
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.
List of Behavioral Inventory Parameters
Following variables control how Ansible interacts with remote hosts.
- ansible_connection
- ansible_host
- ansible_port
- ansible_user
- ansible_ssh_pass
- ansible_ssh_private_key_file
- ansible_ssh_common_args
- ansible_sftp_extra_args
- ansible_scp_extra_args
- ansible_ssh_extra_args
- ansible_ssh_pipelining
- ansible_ssh_executable (added in version 2.2)
Privilege escalation
- ansible_become
- ansible_become_method
- ansible_become_user
- ansible_become_pass
- ansible_become_exe
- ansible_become_flags
Remote host environment parameters:
- ansible_shell_type
- ansible_python_interpreter
- ansible_*_interpreter
- ansible_shell_executable
Examples from an Ansible-INI host file:
some_host ansible_port=2222 ansible_user=manager
aws_host ansible_ssh_private_key_file=/home/example/.ssh/aws.pem
freebsd_host ansible_python_interpreter=/usr/local/bin/python
ruby_module_host ansible_ruby_interpreter=/usr/bin/ruby.1.9.3
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 Working with 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 via a dynamic external inventory system. Ansible supports two ways to connect with external inventory: Inventory Plugins and inventory scripts <https://github.com/ansible/ansible/tree/stable-2.7/contrib/inventory>.
Inventory plugins take advantage of the most recent updates to Ansible’s core code. 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 via the script inventory plugin.
If you’d like a GUI for handling dynamic inventory, the Ansible Tower inventory database 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.
Following are the different ways to use dynamic inventory Scripts as per your needs:
Cobbler Inventory Scripts
EC2 External Inventory Scripts
OpenStack Inventory Scripts
Write Your own Inventory Scripts
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, or ansible will return 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
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