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Better explain magic variables

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Michael DeHaan 2013-02-16 15:49:42 -05:00
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docsite/rst/playbooks2.rst
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@ -88,22 +88,28 @@ that is preferred::
{{ ansible_eth0["ipv4"]["address"] }}
Accessing Information About Other Hosts
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Magic Variables, and How To Access Information About Other Hosts
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If your database server wants to check the value of a 'fact' from another node, or an inventory variable
Even if you didn't define them yourself, ansible provides a few variables for you, automatically.
The most important of these are 'hostvars', 'group_names', and 'groups'.
Hostvars lets you ask about the variables of another host, including facts that have been gathered
about that host. If you haven't yet talked to that host in any play yet at this point in the playbook
or set of playbooks, you can get at the variables, but you will not be able o see the facts.
If your database server wants to use the value of a 'fact' from another node, or an inventory variable
assigned to another node, it's easy to do so within a template or even an action line::
${hostvars.hostname.factname}
.. note::
No database or other complex system is required to exchange data
between hosts. The hosts that you want to reference data from must
be included in either the current play or any previous play if you
are using a version prior to 0.8. If you are using 0.8, and you have
not yet contacted the host, you'll be able to read inventory variables
but not fact variables. Speak to the host by including it in a play
to make fact information available.
Note in playbooks if your hostname contains a dash or periods in it, escape it like so::
${hostvars.{test.example.com}.ansible_distribution}
In Jinja2 templates, this can also be expressed as::
{{ hostvars['test.example.com']['ansible_distribution'] }}
Additionally, *group_names* is a list (array) of all the groups the current host is in. This can be used in templates using Jinja2 syntax to make template source files that vary based on the group membership (or role) of the host::
@ -118,12 +124,20 @@ For example::
# something that applies to all app servers.
{% endfor %}
Use cases include pointing a frontend proxy server to all of the app servers, setting up the correct firewall rules between servers, etc.
A frequently used idiom is walking a group to find all IP addresses in that group::
*inventory_hostname* is the name of the hostname as configured in Ansible's inventory host file. This can
{% for host in groups['app_servers'] %}
{{ hostvars[host]['ansible_eth0']['ipv4']['address'] }}
{% endfor %}
An example of this could include pointing a frontend proxy server to all of the app servers, setting up the correct firewall rules between servers, etc.
Just a few other 'magic' variables are available... There aren't many.
Additionally, *inventory_hostname* is the name of the hostname as configured in Ansible's inventory host file. This can
be useful for when you don't want to rely on the discovered hostname `ansible_hostname` or for other mysterious
reasons. If you have a long FQDN, *inventory_hostname_short* (in Ansible 0.6) also contains the part up to the first
period.
reasons. If you have a long FQDN, *inventory_hostname_short* also contains the part up to the first
period, without the rest of the domain.
Don't worry about any of this unless you think you need it. You'll know when you do.