openstack-manuals/doc/hot-guide/source/hello_world.rst

Writing a hello world HOT template

HOT is a new template format meant to replace the CloudFormation-compatible format (CFN) as the native format supported by the Orchestration module over time. This guide is targeted towards template authors and explains how to write HOT templates based on examples. A detailed specification of HOT can be found at hot_spec.

This section gives an introduction on how to write HOT templates, starting from very basic steps and then going into more and more detail by means of examples.

A most basic template

The most basic template you can think of contains only a single resource definition using only predefined properties. For example, the template below could be used to deploy a single compute instance:

heat_template_version: 2013-05-23

description: Simple template to deploy a single compute instance

resources:
  my_instance:
    type: OS::Nova::Server
    properties:
      key_name: my_key
      image: ubuntu-trusty-x86_64
      flavor: m1.small

Each HOT template has to include the heat_template_version key with value 2013-05-23, the current HOT version. While the description key is optional, it is good practice to include some useful text that describes what users can do with the template. In case you want to provide a longer description that does not fit on a single line, you can provide multi-line text in YAML, for example:

description: >
  This is how you can provide a longer description
  of your template that goes over several lines.

The resources section is required and must contain at least one resource definition. In the above example, a compute instance is defined with fixed values for the key_name, image and flavor properties.

Note

All the defined elements (key pair, image, flavor) have to exist in the OpenStack environment where the template is used.

Input parameters

Input parameters defined in the parameters section of a template allow users to customize a template during deployment. For example, this allows for providing custom key pair names or image IDs to be used for a deployment. From a template author's perspective, this helps to make a template more easily reusable by avoiding hardcoded assumptions.

The following example extends the previous template to provide parameters for the key pair, image and flavor properties of the resource:

heat_template_version: 2013-05-23

description: Simple template to deploy a single compute instance

parameters:
  key_name:
    type: string
    label: Key Name
    description: Name of key-pair to be used for compute instance
  image_id:
    type: string
    label: Image ID
    description: Image to be used for compute instance
  flavor:
    type: string
    label: Instance Type
    description: Type of instance (flavor) to be used

resources:
  my_instance:
    type: OS::Nova::Server
    properties:
      key_name: { get_param: key_name }
      image: { get_param: image_id }
      flavor: { get_param: flavor }

Values for the three parameters must be defined by the template user during the deployment of a stack. The get_param intrinsic function retrieves a user-specified value for a given parameter and uses this value for the associated resource property.

For more information about intrinsic functions, see hot_spec_intrinsic_functions.

Providing default values

You can provide default values for parameters. If a user doesn't define a value for a parameter, the default value is used during the stack deployment. The following example defines a default value m1.small for the flavor property:

parameters:
  flavor:
    type: string
    label: Instance Type
    description: Flavor to be used
    default: m1.small

Note

If a template doesn't define a default value for a parameter, then the user must define the value, otherwise the stack creation will fail.

Hidding parameters values

The values that a user provides when deploying a stack are available in the stack details and can be accessed by any user in the same tenant. To hide the value of a parameter, use the hidden boolean attribute of the parameter:

parameters:
  database_password:
    type: string
    label: Database Password
    description: Password to be used for database
    hidden: true

Restricting user input

You can restrict the values of an input parameter to make sure that the user defines valid data for this parameter. The constraints property of an input parameter defines a list of constraints to apply for the parameter. The following example restricts the flavor parameter to a list of three possible values:

parameters:
  flavor:
    type: string
    label: Instance Type
    description: Type of instance (flavor) to be used
    constraints:
      - allowed_values: [ m1.medium, m1.large, m1.xlarge ]
        description: Value must be one of m1.medium, m1.large or m1.xlarge.

The following example defines multiple contraints for a password definition:

parameters:
  database_password:
    type: string
    label: Database Password
    description: Password to be used for database
    hidden: true
    constraints:
      - length: { min: 6, max: 8 }
        description: Password length must be between 6 and 8 characters.
      - allowed_pattern: "[a-zA-Z0-9]+"
        description: Password must consist of characters and numbers only.
      - allowed_pattern: "[A-Z]+[a-zA-Z0-9]*"
        description: Password must start with an uppercase character.

The list of supported constraints is available in the hot_spec_parameters_constraints section.

Note

You can define multiple constraints of the same type. Especially in the case of allowed patterns this not only allows for keeping regular expressions simple and maintainable, but also for keeping error messages to be presented to users precise.

Template outputs

In addition to template customization through input parameters, you can provide information about the resources created during the stack deployment to the users in the outputs section of a template. In the following example the output section provides the IP address of the my_instance resource:

outputs:
  instance_ip:
    description: The IP address of the deployed instance
    value: { get_attr: [my_instance, first_address] }

Note

Output values are typically resolved using intrinsic function such as the get_attr. See hot_spec_intrinsic_functions for more information about intrinsic functions..

See hot_spec_outputs for more information about the outputs section.