.. highlight: yaml :linenothreshold: 5 .. Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at http://www.apache.org/licenses/LICENSE-2.0 Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License. .. _hello_world: ================================== 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 :ref:`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: .. code-block:: yaml 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: ubuntu-trusty-x86_64 flavor: m1.small Each HOT template must include the ``heat_template_version`` key with the HOT version value, for example, ``2013-05-23``. Consult the :ref:`Heat template version list ` for allowed values and their features. The ``description`` key is optional, however 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: .. code-block:: yaml 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: .. code-block:: yaml 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 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 :ref:`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: .. code-block:: yaml 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. Hiding 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: .. code-block:: yaml 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: .. code-block:: yaml 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 constraints for a password definition: .. code-block:: yaml 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 :ref:`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: .. code-block:: yaml 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 :ref:`hot_spec_intrinsic_functions` for more information about intrinsic functions.. See :ref:`hot_spec_outputs` for more information about the ``outputs`` section.