Running a playbook on multiple host groups one at a time - ansible

Question

I want to run a playbook containing some roles on multiple host groups I create dynamically with the group_by module.
I'm able to do it like the example below (ping replacing my actual role).
I was wondering if there is a way to run each group separately in a loop instead of listing all instance ids. I don't want to create a duplicate line with every instance id.
The purpose here is to deploy to one instance in every data center at a time instead of running all with a low serial which takes a long time.
There might be a different way of doing it, I don't want to create static groups in the inventory for each instance_id as well.
---
- hosts: tag_type_edgenode
tasks:
- group_by: key=instance_id_{{instance_id}}
register: dyn_groups
- hosts: instance_id_1
tasks:
- ping:
- hosts: instance_id_2
tasks:
- ping:
- hosts: instance_id_3
tasks:
- ping:
- hosts: instance_id_4
tasks:
- ping:

Answer 1

If you have equal count of hosts in each group, you can use pattern + serial.
Ansible forms host list by pattern moving through groups sequentially. So if you have equal count of hosts, then batches formed by serial will be equal to groups.
In your example, if you have exactly 3 hosts in each group, you can use:
- hosts: instance_id_*
serial: 3
tasks:
- ping:
If you don't mind a bit of Ansible patching, you can modify _get_serialized_batches method.
Add this code just before while len(all_hosts) > 0::
if 'serialize_by_var' in play.get_vars():
param = play.get_vars()['serialize_by_var']
sb = []
def by_param(x):
vrs = x.get_vars()
if param in vrs:
return vrs[param]
else:
return None
s_hosts = sorted(all_hosts,key=by_param)
for k, g in itertools.groupby(s_hosts, by_param):
sb.append(list(g))
display.vv('Serializing by host var "{}": {}'.format(param,sb))
return sb
And you can serialize hosts by any variable like this:
- hosts: tag_type_edgenode
vars:
serialize_by_var: instance_id
tasks:
- ping

Answer 2

There is a simpler method using the size (length) of each group. However, this runs on all members of a group, flowing through the groups sequentially. I think OP was asking for how to act on the first member of each group, which I am also working on figuring out.
- name: "Restore selected devices in model_dc"
hosts: group_1, group_2, group_3, group_4, group_5, group_6, group_7, !excluded
any_errors_fatal: true # Stop entire play if one host fails
gather_facts: no
order: inventory
serial:
- "{{ groups['group_1'] | length }}" # The number of hosts for first batch
- "{{ groups['group_2'] | length }}" # The number of hosts for second batch
- "{{ groups['group_3'] | length }}"
- "{{ groups['group_4'] | length }}"
- "{{ groups['group_5'] | length }}"
- "{{ groups['group_6'] | length }}"
- "{{ groups['group_7'] | length }}" # The number of hosts for every batch until the end.

Answer 3

Building off of Konstantin's idea, you can do something like this using aliases and a list of patterns:
---
- hosts: "*-server-batch-1,*-servers-batch-2,*-server-batch-3"
serial: 3
...
...
[london]
london-server-batch-1 ansible_host=server1.london.com
london-server-batch-2 ansible_host=server2.london.com
london-server-batch-3 ansible_host=server3.london.com
[tokyo]
tokyo-server-batch-1 ansible_host=server1.tokyo.com
tokyo-server-batch-2 ansible_host=server2.tokyo.com
tokyo-server-batch-3 ansible_host=server3.tokyo.com