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This trick helps you execute actions only when a host is in another hostgroup in Ansible. For example, you might deploy munin-node via ansible, but you only want your apache and NGINX plugins deployed on your webservers, not on your database servers. I found this hard to find in the Ansible documentation, only in the mailing list was more information.

This example playbook is rolled out to all the hosts (hosts: all), because all the hosts are monitored via Munin. In my ansible-hosts file I also have two groups, webservers and databaseservers. I only want my http-status plugin on the webservers, and the postgres-status plugin only on the datase servers. This is how you do that.

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Sample from the playbook:

      - name: Deploy munin http status code plugin
        copy: 
          src: files/http-status 
          dest: /usr/share/munin/plugins/http-status.sh 
          owner: root 
          group: root 
          mode: 0755

      - name: Deploy postgres status plugin
        copy: 
          src: files/postgres-status 
          dest: /usr/share/munin/plugins/posgres-status.py 
          owner: root 
          group: root 
          mode: 0755

      - name: Link munin apache plugin
        command: ln -sf /usr/share/munin/plugins/http-status /etc/munin/plugins/http-status
        when: "'webservers' in {{ group_names }}" 

      - name: Link munin postgres plugin
        command: ln -sf /usr/share/munin/plugins/postgres-status.py /etc/munin/plugins/postgres-status
        when: "'databaseservers' in {{ group_names }}"    

In newer versions of Ansible you would replace the symlink command with the file module, that creates a symlink as well.

By using the when: "'GROUP_NAME' in {{ group_names }}" statement in an action you get more control over the nodes specific actions are run on.

Tags: ansible , configuration-management , deployment , devops , munin , python , sudo , sudoers , tutorials , visudo