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Chapter 6. Configuring Advanced Customizations for the Overcloud

This chapter follows on from Chapter 5, Configuring Basic Overcloud Requirements. At this point, the director has registered the nodes and configured the necessary services for Overcloud creation. Now you can customize your Overcloud using the methods in this chapter.

Note

The examples in this chapter are optional steps for configuring the Overcloud. These steps are only required to provide the Overcloud with additional functionality. Use only the steps that apply to the needs of your environment.

6.1. Understanding Heat Templates

The custom configurations in this chapter use Heat templates and environment files to define certain aspects of the Overcloud, such as network isolation and network interface configuration. This section provides a basic introduction to heat templates so that you can understand the structure and format of these templates in the context of the Red Hat OpenStack Platform director.

6.1.1. 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 of the 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 called My Cirros Instance.
When Heat processes a template it creates a stack for the template and a set of child stacks for resource templates. This creates a hierarchy of stacks that descend from the main stack you define with your template. You can view the stack hierarchy using this following command: 
$ heat stack-list --show-nested

6.1.2. Environment Files

An environment file is a special type of template that provides customization for your Heat templates. This includes three key parts:
  • Resource Registry - This section defines custom resource names, linked to other heat templates. This essentially provides a method to create custom resources that do not exist within the core resource collection. These are defined in the resource_registry section of an environment file.
  • Parameters - These are common settings you apply to the top-level template's parameters. For example, if you have a template that deploys nested stacks, such as resource registry mappings, the parameters only apply to the top-level template and not templates for the nested resources. Parameters are defined in the parameters section of an environment file.
  • Parameter Defaults - These parameters modify the default values for parameters in all templates. For example, if you have a Heat template that deploys nested stacks, such as resource registry mappings,the parameter defaults apply to all templates. In other words, the top-level template and those defining all nested resources. The parameter defaults are defined in the parameter_defaults section of an environment file.

Important

It is recommended to use parameter_defaults instead of parameters When creating custom environment files for your Overcloud. This is so the parameters apply to all stack templates for the Overcloud.
An example of a basic environment file: 
resource_registry:
  OS::Nova::Server::MyServer: myserver.yaml

parameter_defaults:
  NetworkName: my_network

parameters:
  MyIP: 192.168.0.1
For example, this environment file (my_env.yaml) might be included when creating a stack from a certain Heat template (my_template.yaml). The my_env.yaml files creates a new resource type called OS::Nova::Server::MyServer. The myserver.yaml file is a Heat template file that provides an implementation for this resource type that overrides any built-in ones. You can include the OS::Nova::Server::MyServer resource in your my_template.yaml file.
The MyIP applies a parameter only to the main Heat template that deploys along with this environment file. In this example, it only applies to the parameters in my_template.yaml.
The NetworkName applies to both the main Heat template (in this example, my_template.yaml) and the templates associated with resources included the main template, such as the OS::Nova::Server::MyServer resource and its myserver.yaml template in this example.

6.1.3. Core Overcloud Heat Templates

The director contains a core heat template collection for the Overcloud. This collection is stored in /usr/share/openstack-tripleo-heat-templates.
There are many heat templates and environment files in this collection. However, the main files and directories to note in this template collection are:
  • overcloud.yaml - This is the main template file used to create the Overcloud environment.
  • overcloud-resource-registry-puppet.yaml - This is the main environment file used to create the Overcloud environment. It provides a set of configurations for Puppet modules stored on the Overcloud image. After the director writes the Overcloud image to each node, heat starts the Puppet configuration for each node using the resources registered in this environment file.
  • environments - A directory that contains example environment files to apply to your Overcloud deployment.