How to assign a dns name to a variable in Ansible?

Question

I am porting some automation scripts that used ansible with static inventories to use dynamic inventory (we have a AWS Cloudformation stack per environment and it seems silly to update static files every time we re-create a stack).

There is a single master node per stack and a bunch of worker nodes. To discriminate between master or workers in the playbooks I am using a custom tag (e.g. tag_node_type_master or tag_node_type_worker) as the host group (and using --limit tag_env_XXX to restrict to one specific environment).

The problem comes when I need to let the worker nodes know which is their master node. Currently there is a template that references a statically defined variable with the private DNS name of the master node, and that's not an acceptable option anymore.

I am trying to do something in the vars section like master_node: {{tag_node_type_master[0]}} (pick the first, and only, element of that hosts group), but that does not work (it says that the variable is undefined, which makes sense because it's not a variable but a hosts group).

Answer 1

I'm not an Ansible expert but I'm pretty familiar with AWS and other configuration management systems. I would suggest to do this:

1) You can pass master node private IP address or DNS name to EC2 instance by instance UserData property in CloudFormation template (http://is.gd/6cmsRt). If you want to make it more generic I would put a private ELB in front of your master node so you don't have to tackle with IP addresses or DNS names changes when the instance is e.g. relaunched. CloudFormation has available intrinsic function Fn::GetAtt which you can use to get such parameters (http://is.gd/slXkIE).

2) I would save such parameters by UserData as Ansible local facts (Ansible >= 1.3, http://is.gd/QdZrUm). A snippet for UserData:

FACTS=/etc/ansible/facts.d/preferences.fact
echo [MasterNode] > $FACTS
echo Elb={ "Fn::GetAtt" : [ "MasterELB", "DNSName" ]} >> $FACTS
echo Dns={ "Fn::GetAtt" : [ "MasterInstance", "PrivateDnsName" ]} >> $FACTS
echo Ip={ "Fn::GetAtt" : [ "MasterInstance", "PrivateIp" ]} >> $FACTS

3) I would use such custom local facts in your playbooks

{{ ansible_local.preferences.MasterNode.Elb }}
{{ ansible_local.preferences.MasterNode.Dns }}
{{ ansible_local.preferences.MasterNode.Ip }}

This is not a complete solution. You still need to have a procedure to bootstrap EC2 instances, to create facts directory etc. I use a similar procedure with SaltStack and it works well. Any comments are welcomed.