Build template based on variables of other hosts

I want to build a template which differs for each host.
Assume these four hosts with the following variables:

# host_1 host_vars
my:
  info:
    - name: one
    - name: two
    
# host_2 host_vars
my:
  info:
    - name: two
    - name: three

# host_3 host_vars
my:
  info:
    - name: one
    - name: three

# host_4 host_vars
my:
  info:
    - name: whatever

The result of the template should be:

# for host_1
one host_1 host_3
two host_1 host_2

# for host_2
two host_2 host_1
three host_2 host_3

# for host_3
one host_3 host_1
three host_3 host_2

# for host_4
whatever host_4

My question is: I want to keep the configuration simple.
Where should I build this logic?
Should I share facts among hosts during Ansible runtime and then template it? Or is there a solution to build this easily during templating already?

Your description is a bit abstract and I'm not totally sure I get your full requirement. But to me it looks like you are reinventing the wheel.

If I rephrase the above as I get it: you have hosts belonging to groups and you want a template that will, for each group the host belongs to, write a line with the name of the group followed by the current host and then all other members.

Here is a minimal example which achieves that requirement

File structure:

$ tree
.
├── inventories
│   └── default
│       └── hosts.yml
├── playbook.yml
└── templates
    └── my_template.j2

3 directories, 3 files

Our test inventories/default/hosts.yml inventory:

---
one:
  hosts:
    host_1:
    host_3:

two:
  hosts:
    host_1:
    host_2:

three:
  hosts:
    host_2:
    host_3:

whatever:
  hosts:
    host_4:

The content of templates/my_template.j2 is:

{% for current_group in group_names %}
{% set other_hosts = groups[current_group] | difference([inventory_hostname]) %}
{{ current_group }} {{ inventory_hostname }} {{ other_hosts | join(' ') }}
{% endfor %}

And the playbook.yml to test that template:

---
- hosts: all
  # This is for test only
  connection: local
  gather_facts: false

  tasks:
    - name: Show result of template
      vars:
        template_result: "{{ lookup('template', 'my_template.j2') }}"
        to_debug_result: "{{ template_result | split('\n') }}"
      ansible.builtin.debug:
        var: to_debug_result

gives as a result:

$ ansible-playbook -i  inventories/default/ playbook.yml 

PLAY [all] ************************************************************************************************************************************************************************************************

TASK [Show result of template] ****************************************************************************************************************************************************************************
ok: [host_1] => {
    "to_debug_result": [
        "one host_1 host_3",
        "two host_1 host_2",
        ""
    ]
}
ok: [host_3] => {
    "to_debug_result": [
        "one host_3 host_1",
        "three host_3 host_2",
        ""
    ]
}
ok: [host_2] => {
    "to_debug_result": [
        "three host_2 host_3",
        "two host_2 host_1",
        ""
    ]
}
ok: [host_4] => {
    "to_debug_result": [
        "whatever host_4 ",
        ""
    ]
}

PLAY RECAP ************************************************************************************************************************************************************************************************
host_1                     : ok=1    changed=0    unreachable=0    failed=0    skipped=0    rescued=0    ignored=0   
host_2                     : ok=1    changed=0    unreachable=0    failed=0    skipped=0    rescued=0    ignored=0   
host_3                     : ok=1    changed=0    unreachable=0    failed=0    skipped=0    rescued=0    ignored=0   
host_4                     : ok=1    changed=0    unreachable=0    failed=0    skipped=0    rescued=0    ignored=0

Since you can access the variables of other hosts with the help of the special variable hostvars, you can definitely template the fact that some variables are also present in another host variables.

An example of template doing so would be:

{% for info in my.info %}
  {{- info.name -}}
  {% for host in hostvars
        | dict2items
        | selectattr('value.my.info', 'defined')
     if host.value.my.info
        | selectattr('name', '==', info.name)
  %} {{ host.key }}{% endfor %}

{% endfor %}

In this example we are using:

• the filter dict2items, to make a list of hosts dictionaries out of the dictionary of hosts that is hostvars
• the filter selectattr, twice, to filter out any host(s) where my.info would not be defined, then, filter out other hosts that do not share the same info.name as the current host
• a for ... if ... construct to reject any host not having the same info[].name as the current host

For example, with the inventory:

all:
  children:
    demo:
      hosts:
        host_1:
          ansible_host: ansible-node-1
          my:
            info:
              - name: one
              - name: two
        host_2:
          ansible_host: ansible-node-2
          my:
            info:
              - name: two
              - name: three
        host_3:
          ansible_host: ansible-node-3
          my:
            info:
              - name: one
              - name: three
        host_4:
          ansible_host: ansible-node-4
          my:
            info:
              - name: whatever

And the debug task:

- debug:
    msg: >-
      {% for info in my.info %}
        {{- info.name -}}
        {% for host in hostvars
              | dict2items
              | selectattr('value.my.info', 'defined')
           if host.value.my.info
              | selectattr('name', '==', info.name)
        %} {{ host.key }}{% endfor %}

      {% endfor %}

We get a result of:

ok: [host_4] => 
  msg: |-
        whatever host_4
ok: [host_1] => 
  msg: |-
    one host_1 host_3
    two host_1 host_2    
ok: [host_2] => 
  msg: |-
    two host_1 host_2
    three host_2 host_3    
ok: [host_3] => 
  msg: |-
    one host_1 host_3
    three host_2 host_3    

<sup>

Given the inventory

shell> cat hosts
host_1
host_2
host_3
host_4

and the host_vars

shell> cat host_vars/host_1/my_info.yml 
my:
  info:
    - name: one
    - name: two

shell> cat host_vars/host_2/my_info.yml 
my:
  info:
    - name: two
    - name: three

shell> cat host_vars/host_3/my_info.yml 
my:
  info:
    - name: one
    - name: three

shell> cat host_vars/host_4/my_info.yml 
my:
  info:
    - name: whatever

</sup>

<hr>

Short answer: Create the dictionary

name_hosts:
  one: [host_1, host_3]
  three: [host_2, host_3]
  two: [host_1, host_2]
  whatever: [host_4]

Then, the template is trivial

{% for i in my.info %}
{{ i.name }} {{ name_hosts[i.name]|join(' ') }}
{% endfor %}

<hr>

Details: There are more options. Use Community.General in the first example. If you can't or don't want to use Community.General the second example provides solution with Ansible.Builtin functions only.

Example 1.

Instantiate lists of hosts and names my_info

  - set_fact:
      my_info: "{{ my.info|product([{'hosts': [inventory_hostname]}])|
                           map('combine') }}"
  - debug:
      var: my_info|to_yaml

gives

TASK [debug] ************************************************
ok: [host_2] => 
  my_info:
    - hosts: [host_2]
      name: two
    - hosts: [host_2]
      name: three
ok: [host_1] => 
  my_info:
    - hosts: [host_1]
      name: one
    - hosts: [host_1]
      name: two
ok: [host_4] => 
  my_info:
    - hosts: [host_4]
      name: whatever
ok: [host_3] => 
  my_info:
    - hosts: [host_3]
      name: one
    - hosts: [host_3]
      name: three

Extract all lists my_info and merge the items by name

  name_hosts: "{{ ansible_play_hosts_all|
                  map('extract', hostvars, 'my_info')|
                  community.general.lists_mergeby('name', list_merge='append')|
                  items2dict(key_name='name', value_name='hosts') }}"

gives

  name_hosts:
    one: [host_1, host_3]
    three: [host_2, host_3]
    two: [host_1, host_2]
    whatever: [host_4]

Create the template

  - debug:
      msg: |
        {% for i in my.info %}
        {{ i.name }} {{ name_hosts[i.name]|join(' ') }}
        {% endfor %}        

gives

TASK [debug] ************************************************
ok: [host_1] => 
  msg: |-
    one host_1 host_3
    two host_1 host_2    
ok: [host_3] => 
  msg: |-
    one host_1 host_3
    three host_2 host_3    
ok: [host_2] => 
  msg: |-
    two host_1 host_2
    three host_2 host_3    
ok: [host_4] => 
  msg: |-
        whatever host_4

<sup>

Example of a complete playbook for testing

- hosts: all

  vars:

    name_hosts: "{{ ansible_play_hosts_all|
                    map('extract', hostvars, 'my_info')|
                    community.general.lists_mergeby('name', list_merge='append')|
                    items2dict(key_name='name', value_name='hosts') }}"

  tasks:

    - set_fact:
        my_info: "{{ my.info|product([{'hosts': [inventory_hostname]}])|
                             map('combine') }}"
    - debug:
        var: my_info|to_yaml

    - debug:
        var: name_hosts|to_yaml
      run_once: true

    - debug:
        msg: |
          {% for i in my.info %}
          {{ i.name }} {{ name_hosts[i.name]|join(' ') }}
          {% endfor %}          

</sup>

<hr>

Example 2.

Create a dictionary of all my.info

  all_info: "{{ dict(ansible_play_hosts_all|
                     zip(ansible_play_hosts_all|
                         map('extract', hostvars, ['my', 'info'])|
                         map('map', attribute='name'))) }}"

gives

  all_info:
    host_1: [one, two]
    host_2: [two, three]
    host_3: [one, three]
    host_4: [whatever]

Create a list of all names

  all_names: "{{ all_info.values()|flatten|unique }}"

gives

  all_names: [one, two, three, whatever]

Create a dictionary of all names and hosts

  name_hosts: |
    {% filter from_yaml %}
    {% for name in all_names %}
    {{ name }}: {{ all_info|dict2items|selectattr('value', 'contains', name)|map(attribute='key') }}
    {% endfor %}
    {% endfilter %}    

gives

  name_hosts:
    one: [host_1, host_3]
    three: [host_2, host_3]
    two: [host_1, host_2]
    whatever: [host_4]

Now you've got all elements to create the template you want

        - debug:
            msg: |
              {% for host,names in all_info.items() %}
              {{ '#' }} for {{ host }}
              {% for name in names %}
              {{ name }} {{ name_hosts[name]|join(' ') }}
              {% endfor %}

              {% endfor %}              

gives

  msg: |-
    # for host_1
    one host_1 host_3
    two host_1 host_2    
  
    # for host_2
    two host_1 host_2
    three host_2 host_3
  
    # for host_3
    one host_1 host_3
    three host_2 host_3
  
    # for host_4
    whatever host_4

Optionally, each host can use the template

    - debug:
        msg: |
          {% for i in my.info %}
          {{ i.name }} {{ name_hosts[i.name]|join(' ') }}
          {% endfor %}          

gives

TASK [debug] *****************************************
ok: [host_4] => 
  msg: |-
        whatever host_4
ok: [host_1] => 
  msg: |-
    one host_1 host_3
    two host_1 host_2    
ok: [host_2] => 
  msg: |-
    two host_1 host_2
    three host_2 host_3    
ok: [host_3] => 
  msg: |-
    one host_1 host_3
    three host_2 host_3    

<hr>

<sup>

Example of a complete playbook for testing

- hosts: all

  vars:

    all_info: "{{ dict(ansible_play_hosts_all|
                       zip(ansible_play_hosts_all|
                           map('extract', hostvars, ['my', 'info'])|
                           map('map', attribute='name'))) }}"
    all_names: "{{ all_info.values()|flatten|unique }}"
    names_hosts: |
      {% filter from_yaml %}
      {% for tag in all_names %}
      {{ tag }}: {{ all_info|dict2items|selectattr('value', 'contains', tag)|map(attribute='key') }}
      {% endfor %}
      {% endfilter %}      

  tasks:

    - block:
        - debug:
            var: all_info|to_yaml
        - debug:
            var: all_names|to_yaml
        - debug:
            var: names_hosts|to_yaml
        - debug:
            msg: |
              {% for host,names in all_info.items() %}
              {{ '#' }} for {{ host }}
              {% for name in names %}
              {{ name }} {{ names_hosts[name]|join(' ') }}
              {% endfor %}

              {% endfor %}              
      run_once: true

    - debug:
        msg: |
          {% for i in my.info %}
          {{ i.name }} {{ names_hosts[i.name]|join(' ') }}
          {% endfor %}          

</sup>