How to deterministic and reproducibly assign increasing port numbers in Ansible?

Question

I am new to Ansible, but I have to maintain a set of playbooks, which correspond to services to be setup in a given environment. They need to be assigned a port, certificates, etc. This results in many files with lists of essentially always the same names and an assignment to them.

In many cases I think I can easily reuse the service_name as variable, but when mapping to IPs, ports or other numeric identifiers I have not yet figured out a way to deterministically assign them different numbers in a way that is reproducible, and preferably remains the same even when new services are added. I have considered using a SQLite database to store the services from and to generate the values from their ids, but I have no idea how to integrate that with Ansible.

I assume that assigning increasing port numbers is not something entirely new; it is something a lot of sysadmins to on a day-to-day basis so there has to be some way to do that.

Edit: We directly add the port numbers etc. in group_vars/all.yml like this:

ports:
    service1:1024
    service2:1025
    service3:1026

The inventory is generated automatically, since we create additional jails (BSD) and depends on the roles that will be executed.

Answer 1

Disclaimer: I'm not using Ansible.

What I would do is use a random "predictable" number. According to Ansible doc you can seed the random number generator:

As of Ansible version 2.3, it’s also possible to initialize the random number generator from a seed. This way, you can create random-but-idempotent numbers:

"{{ 59 |random(seed=inventory_hostname) }} * * * * root /script/from/cron"

So in your case for a port number (I assume unpriviledged) I would go for a variable with something like:

port="{{ 32767 |random(start=1024,seed=service_name) }}"

Max at 32767 to avoid clashing with any client initiated port (See Ephemeral port for the reason).