Random return host in groups and balanced accross the cluster

[Nicolas G]

Hi,

I have an inventory file with the bellow servers :

# servers

[dbservers]
db01
db02
db03
db04

In my template I want to populate the config by choosing randomly one of the servers from the dbservers group .

Unfortunately the bellow will return all servers :  

{% for host in groups['dbservers'] %}
    server_address = {{ hostvars[host]['inventory_hostname'] }}
{% endfor %}

1) I need Ansible to return only 1 server (random) .

2) I will run this on a big cluster of servers so I would like that random is actually balanced across the cluster equally or else a db servers will get more connections than the others and could hit a capacity issue.

Can someone help me ? 

Thanks in advance.

[Paul Markham]

There's a random number filter; see http://docs.ansible.com/ansible/playbooks_filters.html#random-number-filter.

Try something like:

server_address = {{ groups['dbservers'] | random }}

[Nicolas G]

Thank you Paul , that seems to work . However I'm not completely satisfied with how balanced the random filter has worked.

Do you know if there is a better way to have the server_address more equally balanced across a big bluster of servers ? 

[Brian Coca]

you could probably do a mod on the length of the group and assign depending on a sequence.

​random results even out over large numbers but can be very skewed on small samples.​

----------

Brian Coca

[Nicolas G]

Brian Coca can you be more specific ? I'm trying something like the bellow but I"m stuck, it's deploying all the servers in every run instead of only a random one ..

{% for host in groups['dbservers'] %}

  {% if loop.index % (loop.length + 1) %}
server_address = {{ host }}
    {{ loop.index % loop.length + 1 }}
  {% endif %}
{% endfor %}