Manage pairs of servers with Ansible - ansible

Question

My setup contains "pairs of servers" in the sense that every "instance" of my service consists of two servers (say database and web) on (usually) different hosts. That is, I don't have a single database server that many web servers talk to -- instead, each web server talks to its own database server. I want to manage multiple such pairs, but I don't know the best way to model the pairs using Ansible.
The main host organization tools in Ansible seem to be groups and roles. However, these only group together hosts of the same kind. I can use that to, for example, set up the necessary software on my database and web servers. However, I don't think I can use groups or roles to tell every web server to use exactly one of the database servers.
It also makes little sense to use an Ansible group to model one server pair, since I then cannot distinguish between the two different roles in these groups.
Although my setup consists of server pairs I'd also be interested in modeling multiple instances of n-to-m host relations (say n database servers and m web servers) in general.

Answer 1

You could just use ansible variables to provide the necessary links. E.g., in your inventory:
[webservers]
web0 dbserver=db0
web1 dbserver=db1
[dbservers]
db0
db1
Now, when running tasks on your web servers, you can simply refer to the dbserver variable to find the appropriate server. Assuming a standard naming scheme, you could automate this through the use of the set_fact module.
Update
You don't need to specify the links in both directions; you could use
something like this to find the webservers which should have access to
a given database server:
- name: allow access to webserver
command: >
iptables -A INPUT
-s {{hostvars[item].ansible_default_ipv4.address}}
with_items: "{{groups.webservers}}"
when: "hostvars[item].dbserver == {{inventory_hostname}}"

Answer 2

I ended up using the following approach: Each pair of servers gets their own group, and group variables are used to define which server runs on which host:
[test]
test-host
[test:vars]
web_server = test-host
db_server = test-host
[production]
web
db
[production:vars]
web_server = web
db_server = db
The tasks for web and database servers are then put into separate roles (web_server and db_server). I then switch between those roles based on the group variables:
- hosts: all
roles:
- { role: web_server, when: "web_server == inventory_hostname" }
- { role: db_server, when: "db_server == inventory_hostname" }
This setup allows me to easily define shared variables for roles and pairs, and it also allows me to easily target just one pair (via --limit).