How to spread number of instances across availability zones in heat?

Question

I have an instance resource along these lines:

 masters:
type: OS::Heat::ResourceGroup
properties:
  count: { get_param: num_masters }
  resource_def:
    type: heat_stack_server.yaml
    properties:
      name:
        str_replace:
          template: cluster_id-k8s_type-%index%
          params:
            cluster_id: { get_param: cluster_id }
            k8s_type: master
      cluster_env: { get_param: cluster_env }
      cluster_id:  { get_param: cluster_id }
      type:        master
      image:       { get_param: master_image }
      flavor:      { get_param: master_flavor }
      key_name:    { get_resource: keypair }
      net:         { get_resource: net }
      subnet:      { get_resource: subnet }
      secgrp:
        - { get_resource: master-secgrp }
        - { get_resource: node-secgrp }
      floating_network: { get_param: external_net }
      net_name:
        str_replace:
          template: openshift-ansible-cluster_id-net
          params:
            cluster_id: { get_param: cluster_id }
depends_on:
  - interface

It creates num_masters. Now, I want to guarantee these masters will be created in different availability zones (so that when one of them fails, the other will continue to work).

Say, I have 3 AZ and num_masters == 5. How to spread them, so that zone1 contains nodes 1 and 4, zone2 - 2 and 5, and so on?

Ansible has that loop.cycle thing where you could pass over a list of options over and over. Any ideas how to do it in OS?

Undespairable · Accepted Answer · 2016-06-20T16:05:03.980

OK, I found a solution. I see that someone ticked my question up, so I understand that there's someone else searching for a solution, so I'd better share mine.

You rarely use (and I certainly don't) Heat in isolation from other Configuration Management frameworks. I use it alongside Ansible. So in order to spread nodes between availability zones (AZ) you may consider to prepare this spread yourself. First, I have in my Ansible vars file a list of all AZ available (sorry for the pun):

zones:
 - 'zone1'
 - 'zone2'

Alternatively, you can query Openstack for that list. When you have it, you fill it into the environment file of your stack like this:

{% set zone_cycler = cycler( *zones ) %}
master_availability_zones: [{% for n in range(1,master_number+1) %}"{{ zone_cycler.next() }}"{% if not loop.last %}{{','}} {% endif %}{% endfor %}]

So for five hosts and two zones you'll get this:

master_availability_zones: ["zone1","zone2","zone1","zone2","zone1"]

Then you pass this list into your host resource group like this:

  master_availability_zones:
    type: comma_delimited_list
    label: Master Availability zones
    description: Availability zone mapping for masters

  master_nodes:
    type: OS::Heat::ResourceGroup
    properties:
      count: { get_param: master_number }
      resource_def:
        type: master_template.yaml
        properties:
          ...
          availability_zones: { get_param: master_availability_zones }
          index: "%index%"
          ...

Don't forget to pass along index variable as well, you'll need it on the other side, in master_template.yaml:

  master_node:
    type: OS::Nova::Server
    properties:
      ...
      availability_zone: { get_param: [ availability_zones, { get_param: index } ] }
      ...

Voila, you now have scalable procedure accomodating for arbitrary host and zone numbers.

If heat supported some basic math functions, like the mod function, it wouldn't be necessary to repeat the AZs. Instead the index could be the current index mod number_of_azs. — Daniel Watrous, Jun 20 '16 at 15:07
It could be easier still in that case: you just make your zone name a template and fill zone number as the ```index mod az_number``` — Undespairable, Jun 20 '16 at 16:08

How to spread number of instances across availability zones in heat?

1 Answers1