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P Burke
P Burke

Reputation: 1772

Ansible How to Identify Related Hosts?

I use Ansible to create a multi-server environment, within which are two sets of database servers in a master-slave configuration, so four hosts in all.

My inventory file is currently set-up as follows: 

[dbmaster]
set1-db-master
set2-db-master

[dbslave]
set1-db-slave
set2-db-slave

In the playbooks I generally make use of when clauses to distinguish between masters and slaves as a part of the overall installation. For example:

  - name: Stop the Slave Database. 
    command: systemctl stop mysql
    when: inventory_hostname in groups['dbslave']

Which works fine for most tasks. However I now have one particular problem and I can't think of a neat way of solving it. When setting up the master/slave configurations I have to be able to tie the relevant master and slaves together, but there are more than one of each!

That is set1-db-slave has to know which master it needs to point to: set1-db-master, or set2-db-master.

I'm guessing that the process would be to:

  1. firstly find out what position the current node has in groups['dbslave'] array, and
  2. then use that to reference groups.dbslave[i]

But is this a safe assumption to make about the array positions? And even then I'm not sure of the syntax for achieving this.

Upvotes: 1

Views: 244

Answers (1)

Vladimir Botka
Vladimir Botka

Reputation: 68294

Q: "How to identify related hosts?"

A: It's possible to create a dictionary and use it later. For example

- hosts: all
  tasks:
    - set_fact:
        dbsrv: "{{ dict(groups.dbmaster|sort|zip(groups.dbslave|sort)) }}"
      run_once: true
    - debug:
        var: dbsrv
      run_once: true

- hosts: dbmaster
  tasks:
    - debug:
        msg: "Related slave: {{ dbsrv[inventory_hostname] }}"

gives

ok: [set1-db-master] => {
    "dbsrv": {
        "set1-db-master": "set1-db-slave", 
        "set2-db-master": "set2-db-slave"
    }
}

ok: [set1-db-master] => {
    "msg": "Related slave: set1-db-slave"
}
ok: [set2-db-master] => {
    "msg": "Related slave: set2-db-slave"
}

Q: "But is this a safe assumption to make about the array positions?"

A: It's possible to create alphabetical relations (e.g. set1-db-master: set1-db-slave) and use the sort filter

dbsrv: "{{ dict(groups.dbmaster|sort|zip(groups.dbslave|sort)) }}"

Upvotes: 1

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