Walk through of a YAML template

Template sections

A Heat template is divided into several sections. These are the sections that will typically appear in a Heat template:

• heat_template_version - A field that allows you to specify which version of Heat the template was written for (optional)
• description - A field that allows you to describe the intent of the template to a human audience (optional)
• parameters - The specification of any arguments that the user might be required to provide (optional)
• resources - The specification of the OpenStack resources that are to be created (mandatory)
• outputs - Any expected values that are to be returned once the template has been processed (optional).

The following table contains a guided tour of a template in the NeCTAR sample template repository that will install the Apache web server on a single Ubuntu instance.

Section	Template content
Version indicator

The first line of a YAML template should list a version indicator. If present Heat will parse the file appropriately. If not present then Heat will assume that the file adheres to the most recent specification.

Section	Template content
Description

Description is optional section that allows you to provide a description of the template.

Section	Template content
Parameters

Parameters is an optional section that allows you to specify any input parameters that might be required when the configuration described in the template is built.

Section	Template content
Parameters

Each parameter is in its own nested block, with the name appearing in the first line, and its attributes as further nested elements.

Section	Template content
Parameters

• type is a mandatory element for a parameter that can be one of: string, number, json or comma_delimited_list
• description is an optional attribute that provided guidance to people using the template.

Section	Template content
Parameters

• default is an optional attribute that provides a default value for the parameter.
• Constraints on the user entered value can also be set.

Section	Template content
Resources

Resources is a mandatory section that defines the resources that the application will need.

Section	Template content
Resources

Each resource is in its own nested block, with the name appearing in the first line, and attributes as further nested elements

Type defines the OpenStack resource to be built. The complete list supported can be found in the template guide This resource type will build a security group.

Each resource has its own attribute set. These are documented in the template guide - e.g.: for the AWS::EC2::SecurityGroup

Section	Template content
Resources

This resource type builds an instance with apache installed.

The AWS::CloudFormation::Init section provides instructions as to what should be done on the instance once it has booted.

Fn::Select is a function that will select an item from a list.

get_param is a function that returns the value of the named parameter that was set by the user.

The Heat manual contains a complete list of built in functions Currently NeCTAR supports releases up to Juno.

UserData is a mechanism by which information can be passed to an instance at launch time. Typically this will be either a a shell script or a configuration file.

Note that the chosen image for the instance must have the cloudinit packaged installed for AWS::CloudFormation::Init to work.

Section	Template content
Resources

These resource types will notify the heat engine when the software is fully installed on the instance.

Section	Template content
Outputs

The outputs will show once the heat engine has finished building the template.