Setting host vars programmatically with dynamic inventory

[Ches Martin]

Hi folks,

I've been trying to come up with a solution to what essentially boils down to this:

Say I'm writing, or simply using, a role which depends on a host var being set for all targeted hosts. An example is ZooKeeper, where each member of an ensemble needs a unique ID configured, and needs to enumerate all the other members in its config file (common for host in groups / hostvars pattern). I'd like to achieve this with an EC2 tag on each host, but say that the role already references a specific hostvar name (which naturally isn't ec2_tag_the_key_name_i_want).

Can anyone suggest a programmatic way to map values to a particular host var name during a run? set_fact only appears applicable to the context of a single host as it executes a run (a with_items: groups[foo] loop does not work). I don't particularly want to juggle names in DNS for these hosts in order to use named files in host_vars.

Thanks for your help!

Regards,

Ches

[Michael DeHaan]

I'm somewhat unclear on this one as the playbook parts and the dynamic inventory parts are different things.

Can you elaborate on what "map values to a particular host var name" would mean?

If I may guess at the question, group_vars/<groupname> does in fact work with dynamic groups, if you want everything in ec2_tag_the_key_name_I_want to get a value like x: 42.

(set_fact looping over a with_items doesn't allow you to set vars for other hosts, true, it just makes the value of "item" be different in each iteration of the loop is all)

Let me know more info and I can perhaps get a bit closer, but I *think* group_vars is what you want.

 

Thanks for your help!

Regards,

Ches

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[Ches Martin]

On Friday, September 12, 2014 6:31:37 AM UTC+7, Michael DeHaan wrote:

Can you elaborate on what "map values to a particular host var name" would mean?

If I may guess at the question, group_vars/<groupname> does in fact work with dynamic groups, if you want everything in ec2_tag_the_key_name_I_want to get a value like x: 42.  

<snip>

Let me know more info and I can perhaps get a bit closer, but I *think* group_vars is what you want.

Thanks for your help Michael.

group_vars can actually solve part of the problem in a kludgy way, I'm not sure if this is what you had in mind. Let me elaborate the ZooKeeper example, it's concrete for me, though there are some other cases where I want essentially the same thing. Here's the snippet of a zoo.cfg template for enumerating cluster members, pretty standard stuff:

{% for host in groups[zookeeper_ensemble_group] %}
server.{{ hostvars[host].zookeeper_myid }}={{ hostvars[host].zookeeper_bind_address }}:2888:3888
{% endfor %}

So, what I meant by "map values to a particular host var name" is that I effectively want to assign/alias zookeeper_myid = ec2_tag_zookeeper_id and zookeeper_bind_address = ec2_private_ip_address for each host in the group, before this zookeeper role runs. I was hoping for a way to do this without having to modify an upstream role, or in writing my own shareable roles to not have them coupled to choices of a particular inventory source, so users can simply consistently override a sensibly-named variable.

After you mentioned it, group_vars does partly help, in a very redundant way via the dynamic groups that include tag values along with key:

---
# group_vars/tag_zoo_keeper_id_1
zookeeper_myid: 1

# This of course doesn't work, we can't use dynamic values here
zookeeper_bind_address: '{{ ec2_private_ip_address }}'

That achieves the alias for the zookeeper_myid var, at the expense of maintaining three things: an EC2 tag, the group_vars file name, and the static value in the file. And doesn't help with the dynamic address, so not really workable :-\

Maybe you meant something different, or does the explanation help to spot any alternatives?

Thanks again!

-Ches

[Michael Peters]

You could modify your dynamic inventory. I think in the past it's been
said that the ec2.py, etc are starting points for your own dynamic
inventory scripts. This way you can do what you want with your
ec2_tag_* variables and makes it easier when part of your
infrastructure isn't even on EC2.

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[Michael DeHaan]

"is that I effectively want to assign/alias zookeeper_myid = ec2_tag_zookeeper_id andzookeeper_bind_address = ec2_private_ip_address for each host in the group"

Are you saying you wish to add them to another group?

The add_host module is good for that.

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[Ches Martin]

On Friday, September 12, 2014 9:30:19 PM UTC+7, Michael DeHaan wrote:

"is that I effectively want to assign/alias zookeeper_myid = ec2_tag_zookeeper_id andzookeeper_bind_address = ec2_private_ip_address for each host in the group"

Are you saying you wish to add them to another group?

The add_host module is good for that.

No, these are host variables, since they're both unique to a given host (in a cluster). I just want to be able to assign the values from my inventory (ec2_ hostvar names) to the specific hostvar variable names that a role is looking for. Without changing the role—assuming it's vendor code and should remain general anyway.

Basically, assuming this were possible somehow, it's what I want to do (note the same dict key on both sides of assignment):

hostvars[host].zookeeper_myid = hostvars[host].ec2_private_ip_address

-Ches

[Josh Smift]

CM> Basically, assuming this were possible somehow, it's what I want to do
CM> (note the same dict key on both sides of assignment):
CM>
CM> hostvars[host].zookeeper_myid = hostvars[host].ec2_private_ip_address

This seems like the thing that someone else helped me figure out how to
do, e.g. I now have as part of my aws-launch.yml playbook:

- name: Add new instance to a temporary group
local_action:
module: add_host
name: "{{ item.public_ip }}"
groups: newly_launched
ec2_tag_Name: "{{ instance_hostname }}"
with_items: ec2.instances

Does that help? (instance_hostname is a variable that I passed in to the
playbook on the command line, which gets set for real at AWS with

instance_tags:
Name: "{{ instance_hostname }}"

in the ec2 launch task.)

-Josh (j...@care.com)



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[Ches Martin]

On Saturday, September 13, 2014 1:20:44 AM UTC+7, Josh Smift wrote:

This seems like the thing that someone else helped me figure out how to
do, e.g. I now have as part of my aws-launch.yml playbook:

  - name: Add new instance to a temporary group
    local_action:
      module: add_host
      name: "{{ item.public_ip }}"
      groups: newly_launched
      ec2_tag_Name: "{{ instance_hostname }}"
    with_items: ec2.instances

Does that help?

Thanks for the suggestion Josh, that seems like a promising lead, I didn't realize until now that add_host can add host variables, not just add hosts to groups. Except that, so far, I'm not sure that it works to set new host variables if the referenced host already existed in inventory. I've tried this:

- name: Set hostvar aliases
  add_host: >
    name='{{ item }}'
    zookeeper_myid='{{ hostvars[item].ec2_tag_ZooKeeperID }}'
    zookeeper_bind_address='{{ hostvars[item].ec2_private_ip_address }}'
  with_items: groups[zookeeper_ensemble_group]

which would get me what I'm wanting pretty readably, but it doesn't seem to work.

I tried changing hash_behaviour to "merge" just to see if that could allow this work, no dice. I'll have to dig further in implementation to understand, and this issue might be relevant since the above is a local action:

https://github.com/ansible/ansible/issues/5791

-Ches