How to dynamically generate a list of MongoDB replica set members with Ansible

Question

I'm using Ansible to create a MongoDB replica set. I'm following the mongodb_replicaset module, example:

# Create a replicaset called 'rs0' with the 3 provided members
- name: Ensure replicaset rs0 exists
  mongodb_replicaset:
    login_host: localhost
    login_user: admin
    login_password: ?????
    replica_set: rs0
    members:
    - mongodb1:27017
    - mongodb2:27017
    - mongodb3:27017
  when: groups.mongod.index(inventory_hostname) == 0

(From the docs https://docs.ansible.com/ansible/latest/modules/mongodb_replicaset_module.html )

I've got my replication servers defined in the hosts inventory file

[replication_servers]
mongodb1 ansible_host=192.168.50.4
mongodb2 ansible_host=192.168.50.5
mongodb3 ansible_host=192.168.50.6

This works ok, but I'd like to avoid hardcoding (and duplicating) the host names in the members attribute, and somehow use the already defined [replication_servers] group. Is this possible with Ansible?

Answer 1

Quoting members : "... Supply as a simple csv string, ie mongodb1:27017,mongodb2:27017,mongodb3:27017. If a port number is not provided then 27017 is assumed."

Try

    members: "{{ groups.replication_servers|join(',') }}"

This will create simple csv string (port 27017 is default)

    members: "mongodb1,mongodb2,mongodb3"

---

Create replicaset with ports

It's possible to create the string with ports. For example, with the inventory below

[replication_servers]
mongodb1 ansible_host=192.168.50.4 mongodb_port=27017
mongodb2 ansible_host=192.168.50.5 mongodb_port=27017
mongodb3 ansible_host=192.168.50.6 mongodb_port=27017

this expression

    members: "{{ groups.replication_servers|
                 zip(groups.replication_servers|
                     map('extract', hostvars, 'mongodb_port')|
                     list)|
                 map('join', ':')|
                 join(',') }}"

gives

    members: "mongodb1:27017,mongodb2:27017,mongodb3:27017"