Dynamically assign master/slave variables in Ansible role - linux

Question

I have a simple mariadb role which permits to setup master/slave replication on two servers. In order to do this, I have to define in my inventory my 2 nodes like this:
node1 master=true
node2 slave=true
This way, I can setup one role to setup master/slave replication using Ansible when statement playing with this vars.
- name: Setup master conf
template: >-
src="templates/master.conf.j2"
dest="{{ master_config_file }}"
when:
- master is defined
Now, I would like to get something more automatic that could dynamically and randomly assign a master variable to one node, and slave variable to all other nodes.
I have seen some Ansible doc about variables and filters, but none of them seems to be adapted to that. I guess that I have to develop my own Ansible variable plugin to do that.

Answer 1

You can utilise facts.d. Something like this:
- hosts: all
become: yes
tasks:
- file:
path: /etc/ansible/facts.d
state: directory
- shell: echo '{{ my_facts | to_json }}' > /etc/ansible/facts.d/role.fact
args:
creates: /etc/ansible/facts.d/role.fact
vars:
my_facts:
is_master: "{{ true if play_hosts.index(inventory_hostname) == 0 else false }}"
register: role_fact
# refresh facts if fact has been just added
- setup:
when: role_fact | changed
- set_fact:
is_master: "{{ ansible_local.role.is_master }}"
- debug:
var: is_master
This will create role.fact on remote nodes if it is not there and use is_master fact from it. During subsequent runs ansible_local.role.is_master is fetched automatically.

Answer 2

You can use a dynamic group to do that. Another use case : You don't know which node is the master because it is elected, and you need to performs actions only on master.
To use a dynamic group, you need to define two pays in your playbook :
First one in order to determine which node is the master and add it in a dynamic group, you need to use a command
Then execute tasks on master, slaves
Following playbook determine which nodes are masters and slaves and execute a play on each types :
- hosts: all
tasks:
- shell: <command on node to retrieve node type>
register: result__node_type
- name: If node is a master, add it in masters group
add_host:
name: "{{ inventory_hostname }}"
groups: temp_master
when: result__node_type.stdout == "MASTER"
- name: If node is a slave, add it in slaves group
add_host:
name: "{{ inventory_hostname }}"
groups: temp_slave
when: result__node_type.stdout == "SLAVE"
- name: No master found, then assign first one (or random if you want) to masters group
add_host:
name: "groups['all'][0]"
groups: temp_master
run_once: yes
when: groups['temp_master'] | length == 0
- name: No slave found, then assign others to slaves group
add_host:
name: "groups['all'][0]"
groups: temp_slave
run_once: yes
with_items: "{{ groups['all'][1:] }}"
when: groups['temp_slave'] | length == 0
- hosts: temp_master
gather_facts: false
tasks:
- debug:
msg: "Action on master {{ ansible_host }}"
- hosts: temp_slave
gather_facts: false
tasks:
- debug:
msg: "Action on slave {{ ansible_host }}"