[Ansible] Dynamicaly update /etc/hosts files on target servers based on Ansible inventory, respecting idempotency

Ansible is just a great peace of work.

# START Blha Blha Blha

We are using it more and more every day at ${myDayJob}, and it helps us in every aspects of configuration management, deployment, and orchestration

If you still don't know it : Just GO check it out -> http://www.ansibleworks.com/docs/

One important aspect with making good Ansible playbooks is to respect idempotency

Wikipedia Source : Idempotence is the property of certain operations in mathematics and computer science, that can be applied multiple times without changing the result beyond

That means we want to be able to use a playbook to deploy, configure or update our full solution stack, but also make sure the already deployed blocks are conformed to what Ansible knows to be right about the topology and configuration, BUT without modifying any of those configurations if they comply with Ansible inventory/desired configuration.

# END Blha Blha Blha

When dealing with /etc/hosts files, we had a solution based on dynamically creating fragments from a template looking like bellow, and then using the assemble module to apply changes to the target servers :

        {% for k, v in hostvars.iteritems() -%}
        {{ hostvars[k]['ansible_default_ipv4']['address'] }}  {{ k }} {{ v['ansible_hostname'] }}
        {% endfor %}

The drawback with this method was that the already existing file on the target was deleted and copied over by the newly created one, (discarding any changes made locally on the target, but this is not the point here, and local changes are a bad habit any way), marking the task step as "changed" and the Ansible "PLAY RECAP" displaying this change.

We wanted to respect a true idempotence and get rid of those undesired/unnecessary modification.

After a few miss-steps we came up with the following solution :

 - lineinfile:  dest=/etc/hosts regexp='.*{{item}}$'
                line='${hostvars.${item}.ansible_default_ipv4.address} ${item}'
                state=present
 only_if: is_set('${hostvars.${item}.ansible_default_ipv4.address}')
 with_items: '{{groups.all}}'     

What it does is pretty straight forward :

  for all target hosts defined by the playbook
    for all hosts defined in the groups, set {{item}} value to its corresponding hostname
      find any line matching the regexp  '.*{{item}}$'  in /etc/hosts -> eg : '192.168.0.1 appServer1'
      if found check it matches the desired couple "hostname" "ip" (with "ip" being '${hostvars.${item}.ansible_default_ipv4.address})
          if it matches, don't change any thing
          else change it to the desired "hostname ip"
        else (if not found), add the desired "hostname ip" at the end of file

The "only_if" statement is used to skip hosts we couldn't get facts from (host couldn't be reached, etc ...), thus preventing from adding garbage info into the target /etc/hosts

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Juts writing this quick post make me realize (once again) how power full is Ansible :
  - 3 lines of statements to described the desired configuration state and apply eventual changes
  - 7 lines to explain it in plain english (and not even sure I am cristal clear ...)

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[EDIT] : Thanks to @rothgar

As  Justin mentioned in his comment, this syntax is now deprecated

Please use this one :

# Idempotent way to build a /etc/hosts file with Ansible using your Ansible hosts inventory for a source.
# Will include all hosts the playbook is run on.
# Inspired from http://xmeblog.blogspot.com/2013/06/ansible-dynamicaly-update-etchosts.html

- name: "Build hosts file"
  lineinfile:   dest=/etc/hosts 
                regexp='.*{{ item }}$
                line="{{ hostvars[item].ansible_default_ipv4.address }} {{item}}"
                state=present
  when: hostvars[item].ansible_default_ipv4.address is defined
  with_items: groups['all']