0 inventory.ini

[appservers:children]
datacenter-1
datacenter-2

[datacenter-1]
appservers00[1:3].datacenter-1.example.com

[datacenter-2]
appservers00[1:3].datacenter-2.example.com

# or, should I group the servers how I want them updated instead? 

[group1]
appservers001.datacenter-1.example.com
appservers001.datacenter-2.example.com

[group2]
appservers002.datacenter-1.example.com
appservers002.datacenter-2.example.com

[group3]
appservers003.datacenter-1.example.com
appservers003.datacenter-2.example.com

1 playbook.yml

- hosts:  appservers
  serial: 2

3 groupings...

$ ansible-playbook -i inventory.ini --list-hosts playbook.yml 

playbook: playbook.yml

  play #1 (appservers): host count=6
    appservers001.datacenter-1.example.com
    appservers002.datacenter-1.example.com
    appservers003.datacenter-1.example.com
    appservers001.datacenter-2.example.com
    appservers002.datacenter-2.example.com
    appservers003.datacenter-2.example.com

# which is going to update the hosts serially through that list?
# I want them to be updated in groups by their hostnames: 001, 002, 003, etc..

# I can achieve this by successively calling the playbook with a limit:
$ ansible-playbook -i inventory.ini --list-hosts --limit *001* playbook.yml 

playbook: playbook.yml

  play #1 (appservers): host count=2
    appservers001.datacenter-1.example.com
    appservers001.datacenter-2.example.com

# is there another way to organize the inventory, to make it work like this by default?