Customize dynamic inventory - ansible

Question

Groups configured in Ansible inventory:
Group_A has 30 servers
Group_B has 40 Servers
Group_C has 15 Servers
I want to take 10 servers from each group and make a new group without editing the inventory manually.
These 10 servers is a variable that can change dynamically. If that works I got another question what if the inventory itself is dynamic
[Group_C]
server-1
server-2
server-3
...
server-10
''' New group created From 3 grouped servers now will be used in a playbook '''

Answer 1

(ansible 2.8.3)
If the inventory is dynamic we don't know the hosts. Let's assume we could choose any of them. Let's create the list of the selected hosts first and then loop add_hosts. With the inventory
[Group_A]
A-0
A-1
..
A-29
[Group_B]
B-0
B-1
..
B-39
[Group_C]
C-0
C-1
..
C-14
the plays below
- name: Create Group_X
hosts: localhost
vars:
no_of_servers: 2
my_groups:
- Group_A
- Group_B
- Group_C
tasks:
- set_fact:
my_list: "{{ my_list|default([]) +
groups[item][0:no_of_servers] }}"
loop: "{{ my_groups }}"
- add_host:
name: "{{ item }}"
groups: Group_X
loop: "{{ my_list }}"
- debug:
msg: "{{ groups['Group_X'] }}"
- name: Use Group_X
hosts: Group_X
gather_facts: false
tasks:
- debug:
msg: "{{ inventory_hostname }} is member of {{ group_names }}"
run_once: true
give
ok: [localhost] => {
"msg": [
"A-0",
"A-1",
"B-0",
"B-1",
"C-0",
"C-1"
]
}
ok: [A-0] => {
"msg": "A-0 is member of [u'Group_A', u'Group_X']"
}
Random choice.
It is possible to make the selection of the hosts random with the simple plugin below
$ cat filter_plugins/list_methods.py
import random
def list_sample(l,n):
return random.sample(l,n)
class FilterModule(object):
def filters(self):
return {
'list_sample' : list_sample
}
With the modification below
- set_fact:
my_list: '{{ my_list|default([]) +
groups[item]|list_sample(no_of_servers) }}'
the plays give for example
ok: [localhost] => {
"msg": [
"A-8",
"A-9",
"B-8",
"B-2",
"C-2",
"C-5"
]
}
ok: [A-8] => {
"msg": "A-8 is member of [u'Group_A', u'Group_X']"
}