Chapter 5. Understanding Heat Templates

Some of the scenarios in this guide use custom Heat templates to define certain aspects of the Overcloud, such as network isolation and network interface configuration. This section provides a basic introduction on Heat templates so that you can understand the structure and format of these templates in the context of the Red Hat Enterprise Linux OpenStack Platform director.

5.1. Heat Templates

The director uses Heat Orchestration Templates (HOT) as a template format for its Overcloud deployment plan. Templates in HOT format are mostly expressed in YAML format. The purpose of a template is to define and create a stack, which is a collection of resources that Heat creates and the configuration per resources. Resources are objects in OpenStack and can include compute resources, network configuration, security groups, scaling rules, and custom resources.
The structure of a Heat template has three main sections:
  • Parameters - These are settings passed to Heat, which provides a way to customize a stack, and any default values for parameters without passed values. These are defined in the parameters section of a template.
  • Resources - These are the specific objects to create and configure as part of a stack. OpenStack contains a set of core resources that span across all components. These are defined in the resources section of a template.
  • Output - These are values passed from Heat after the stack's creation. You can access these values either through the Heat API or client tools. These are defined in the output section of a template.
Here is an example of a basic Heat template: 
heat_template_version: 2013-05-23

description: > A very basic Heat template.

parameters:
  key_name:
    type: string
    default: lars
    description: Name of an existing key pair to use for the instance
  flavor:
    type: string
    description: Instance type for the instance to be created
    default: m1.small
  image:
    type: string
    default: cirros
    description: ID or name of the image to use for the instance

resources:
  my_instance:
    type: OS::Nova::Server
    properties:
      name: My Cirros Instance
      image: { get_param: image }
      flavor: { get_param: flavor }
      key_name: { get_param: key_name }

output:
  instance_name:
    description: Get the instance's name
    value: { get_attr: [ my_instance, name ] }
This template uses the resource type type: OS::Nova::Server to create an instance called my_instance with a particular flavor, image, and key. The stack can return the value of instance_name, which is My Cirros Instance.