Ansible: How to inherit variables?

Question

I want to achieve variable inheritance in Ansible. Lets say I have:

group_vars/all

---
ifaces:
   -   name: eth0
       adress: 10.107.13.236
       netmask: 255.255.255.192
       routes:
           - {from: 10.108.100.34/31, via: 10.107.13.193}
           - {from: 10.108.105.128/31, via: 10.107.13.193}
   -   name: eth1
       adress: 10.112.13.236
       netmask: 255.255.255.192
       gateway: 10.112.13.193
       routes:
           - {from: 10.2.1.0/26, via: 10.112.13.254}

Now I want to extend the routes of eth0, like this:

group_vars/webserver

--- ifaces:
   -   name: eth0
       routes:
           - {from: 1.2.3.34, via: 5.6.7.8}
           - {from: 9.9.9.9/9, via: 5.6.7.8}

My desired result is:

---
ifaces:
   -   name: eth0
       adress: 10.107.13.236
       netmask: 255.255.255.192
       routes:
           - {from: 10.108.100.34/31, via: 10.107.13.193}
           - {from: 10.108.105.128/31, via: 10.107.13.193}
           - {from: 1.2.3.34, via: 5.6.7.8}
           - {from: 9.9.9.9/9, via: 5.6.7.8}
   -   name: eth1
       adress: 10.112.13.236
       netmask: 255.255.255.192
       gateway: 10.112.13.193
       routes:
           - {from: 10.2.1.0/26, via: 10.112.13.254}

So the routes should be extendend and not overwritten. I know about setting hash_behaviour: merge in ansible.cfg but that does not satisfy my needs, because I want to append values to the list stored in routes.

The background is, that I need to be able to define some standard routes (note: this is not limited to routes, it is just an example) and enhance these standards for specific groups instead of overriding them.

Is this possible in Ansible?

Answer 1

I would suggest creating lookup_plugin that will do all the heavy-ligting of the merging/appending variables.

for example:

lookup_plugins/myvars.py

import yaml
class LookupModule(object):
    def __init__(self, basedir=None, **kwargs):
        self.basedir = basedir
        self.plugin_name = 'myvars'
    def run(self, vars_file, variable=None, **kwargs):
        all_routes = yaml.load(file('group_vars/all'))['ifaces'][0]['routes']
        all_routes.extend(
            yaml.load(file('group_vars/%s' % vars_file[0]))['ifaces'][0]['routes'])
        return [all_routes]

playbook.yml

---
- hosts: webservers
  gather_facts: no
  connection: local
  tasks:
    - debug: msg=" Hey {{ item }}"
      with_myvars:
        - webservers

Of cource it doesn't fully solve your plroblem and you still need to tweak it to make it work. But you should get an idea on how to achive this from this example.

An output of this should look like this:

PLAY [webservers] ************************************************************* 

TASK: [debug msg=" Hey {{ item }}"] ******************************************* 
ok: [localhost] => (item=[{'via': '10.107.13.193', 'from': '10.108.100.34/31'}, {'via': '10.107.13.193', 'from': '10.108.105.128/31'}, {'via': '5.6.7.8', 'from': '1.2.3.34'}, {'via': '5.6.7.8', 'from': '9.9.9.9/9'}]) => {
    "item": [
        {
            "from": "10.108.100.34/31",
            "via": "10.107.13.193"
        },
        {
            "from": "10.108.105.128/31",
            "via": "10.107.13.193"
        },
        {
            "from": "1.2.3.34",
            "via": "5.6.7.8"
        },
        {
            "from": "9.9.9.9/9",
            "via": "5.6.7.8"
        }
    ],
    "msg": " Hey [{'via': '10.107.13.193', 'from': '10.108.100.34/31'}, {'via': '10.107.13.193', 'from': '10.108.105.128/31'}, {'via': '5.6.7.8', 'from': '1.2.3.34'}, {'via': '5.6.7.8', 'from': '9.9.9.9/9'}]"
}

PLAY RECAP ******************************************************************** 
localhost                  : ok=1    changed=0    unreachable=0    failed=0   

So as you can see it's returning a list of routes, so you can easily place {{ item }} in what ever spot you need.