OpenStack Heat, Specify Boot Order

Another post inspired by ask.openstack.org

The question was:

If there are two instances(os:nova:server), how can I specify the boot order?

The Heat Orchestration Template, or HOT, specification provides a ‘depends_on’ attribute, that works generically on Heat resources. That is, depends_on a network, depends_on another instance, database, Cinder volume, etc.

In the OpenStack docs they provide the following example:

resources:
  server1:
    type: OS::Nova::Server
    depends_on: server2

  server2:
    type: OS::Nova::Server

Or for multiple servers:

resources:
  server1:
    type: OS::Nova::Server
    depends_on: [ server2, server3 ]

  server2:
    type: OS::Nova::Server

  server3:
    type: OS::Nova::Server