Managing Primary/Secondary relationship in cluster

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1

I have a pair of servers (server1..domain, server2..domain) in two environments (staging and production). Server1 is primary, server2 is secondary.

When I originally built the playbook, I had aliases in the inventory file

[staging-cluster]
primary server1.staging.domain
secondary server2.staging.domain

And I’d target the IP of the other host with {{ hostvars[‘primary’][‘ansible_eth0’][‘ipv4’][‘address’] }} to get the of the “other” host. This worked fine, until I needed the playbook to target another set of hosts in production:

[production-cluster]
primary server1.prod.domain
secondary server2.prod.domain

The problem with the above, aliases are global and you can’t have more than one in an inventory. So…this breaks the above lookup {{ hostvars[‘secondary’][‘ansible_eth0’][‘ipv4’][‘address’] }}

As much as I disliked the idea, I then thought to leverage hostvars/serverX.environment.domain with a var like:

primary: true
partner: server2.environment.domain

The above var would exist on server1 since it is the primary and it’s partner is server2

The problem with the above is, I can’t expand {{ partner }} in the above fact (may not have explained that properly, I can’t do this:

{{ hostvars[‘{{ partner }}’][‘ansible_eth0’][‘ipv4’][‘address’] }} or this {{ hostavars[{{partner}}][‘ansible_eth0’][‘ipv4’][‘address’] }}

Anyone have any ideas on how I can manage a primary/secondary role/membership in a cluster? Actually defining if the host is primary/secondary is the easy part…but defining it’s partner and getting facts from its partner, I’m not certain of.

Thanks!!

2

If you wanted to get the ip address of the first node in the production-cluster, you could do something like this. Think of the group as a list and 0 being the first element in the list.

"{{ hostvars[groups['production-cluster'][0]]['ansible_eth0']['ipv4']['address'] }}"

as far as your second example goes, you have some syntax errors in defining your variable. You don’t need to encapsulate “partner” in curly brackets or quotes. The following should work:

{{ hostvars[partner]['ansible_eth0']['ipv4']['address'] }}

  • James
3

{{ hostvars[partner][‘ansible_eth0’][‘ipv4’][‘address’] }} << doesn’t work

“{‘msg’: “AnsibleUndefinedVariable: One or more undefined variables: ‘partner’ is undefined”, ‘failed’: True}”

in inventory/host_vars/server1.domain there is ‘partner: server2.domain’

4

That indicates you haven’t defined “partner” properly. Make sure it’s listed in the right pace in your inventory host_vars.

5

Ah, I didn’t see that last line. Do a:

  • debug: var=partner

Just to make sure you have your variables loading properly.

6

I had the remnants of the aliases so host_vars/server1 didn’t match ‘primary’

I’m not getting the var.

Thanks!

7

Another way is 'differenct'.
(see http://docs.ansible.com/playbooks_variables.html#set-theory-filters)

    - debug: msg="{{ (groups['pair'] | difference(inventory_hostname))[0] }}"

The result is list, not a value. So it requires [0] to get actual hostname.
It works if there are more hosts.