heat/doc/source/template_guide/hot_guide.rst

Heat Orchestration Template (HOT) Guide

HOT is a template format supported by the heat, along with the other template format, i.e. the Heat CloudFormation-compatible format (CFN). 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.

Status

HOT is in the process of surpassing the functionality of the CFN. This guide will be updated periodically whenever new features get implemented for HOT.

Writing a hello world HOT template

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 may contain only a single resource definition using only predefined properties (along with the mandatory Heat template version tag). For example, the template below could be used to simply deploy a single compute instance.

heat_template_version: 2015-04-30

description: Simple template to deploy a single compute instance

resources:
  my_instance:
    type: OS::Nova::Server
    properties:
      key_name: my_key
      image: F18-x86_64-cfntools
      flavor: m1.small

Each HOT template has to include the heat_template_version key with a valid version of HOT, e.g. 2015-10-15 (see hot_spec_template_version for a list of all versions). While the description 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 example above, a compute instance is defined with fixed values for the 'key_name', 'image' and 'flavor' parameters.

Note that all those elements, i.e. a key-pair with the given name, the image and the flavor have to exist in the OpenStack environment where the template is used. Typically a template is made more easily reusable, though, by defining a set of input parameters instead of hard-coding such values.

Template input parameters

Input parameters defined in the parameters section of a HOT template (see also hot_spec_parameters) 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.

Sticking to the example used above, it makes sense to allow users to provide their custom key-pairs, provide their own image, and to select a flavor for the compute instance. This can be achieved by extending the initial template as follows:

heat_template_version: 2015-04-30

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
  instance_type:
    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: instance_type }

In the example above, three input parameters have been defined that have to be provided by the user upon deployment. The fixed values for the respective resource properties have been replaced by references to the corresponding input parameters by means of the get_param function (see also hot_spec_intrinsic_functions).

You can also define default values for input parameters which will be used in case the user does not provide the respective parameter during deployment. For example, the following definition for the instance_type parameter would select the 'm1.small' flavor unless specified otherwise by the user.

parameters:
  instance_type:
    type: string
    label: Instance Type
    description: Type of instance (flavor) to be used
    default: m1.small

Another option that can be specified for a parameter is to hide its value when users request information about a stack deployed from a template. This is achieved by the hidden attribute and useful, for example when requesting passwords as user input:

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

Restricting user input

In some cases you might want to restrict the values of input parameters that users can supply. For example, you might know that the software running in a compute instance needs a certain amount of resources so you might want to restrict the instance_type parameter introduced above. Parameters in HOT templates can be restricted by adding a constraints section (see also hot_spec_parameters_constraints). For example, the following would allow only three values to be provided as input for the instance_type parameter:

parameters:
  instance_type:
    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 constraints section allows for defining a list of constraints that must all be fulfilled by user input. For example, the following list of constraints could be used to clearly specify format requirements on a password to be provided by users:

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.

Note that 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.

Providing template outputs

In addition to template customization through input parameters, you will typically want to provide outputs to users, which can be done in the outputs section of a template (see also hot_spec_outputs). For example, the IP address by which the instance defined in the example above can be accessed should be provided to users. Otherwise, users would have to look it up themselves. The definition for providing the IP address of the compute instance as an output is shown in the following snippet:

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

Output values are typically resolved using intrinsic function such as the get_attr function in the example above (see also hot_spec_intrinsic_functions).