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8.4 KiB
ReStructuredText
231 lines
8.4 KiB
ReStructuredText
..
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Licensed under the Apache License, Version 2.0 (the "License"); you may
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not use this file except in compliance with the License. You may obtain
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a copy of the License at
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http://www.apache.org/licenses/LICENSE-2.0
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Unless required by applicable law or agreed to in writing, software
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distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
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WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
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License for the specific language governing permissions and limitations
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under the License.
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.. _hot_guide:
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=======================================
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Heat Orchestration Template (HOT) Guide
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=======================================
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HOT is a new template format meant to replace the Heat CloudFormation-compatible
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format (CFN) as the native format supported by the Heat over time.
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This guide is targeted towards template authors and explains how to write
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HOT templates based on examples. A detailed specification of HOT can be found
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at :ref:`hot_spec`.
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------
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Status
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------
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HOT support is still under development and needs more work to provide access to
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all functionality currently available via the CFN compatible template interface.
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This guide will be updated periodically whenever new features get implemented
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for HOT.
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----------------------------------
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Writing a hello world HOT template
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----------------------------------
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This section gives an introduction on how to write HOT templates, starting from
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very basic steps and then going into more and more detail by means of examples.
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A most basic template
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---------------------
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The most basic template you can think of may contain only a single resource
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definition using only predefined properties (along with the mandatory Heat
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template version tag). For example, the template below could be used to simply
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deploy a single compute instance.
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::
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heat_template_version: 2013-05-23
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description: Simple template to deploy a single compute instance
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resources:
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my_instance:
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type: OS::Nova::Server
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properties:
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key_name: my_key
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image: F18-x86_64-cfntools
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flavor: m1.small
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Each HOT template has to include the *heat_template_version* key with value
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'2013-05-23' (the current version of HOT). While the *description* is optional,
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it is good practice to include some useful text that describes what users can do
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with the template. In case you want to provide a longer description that does
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not fit on a single line, you can provide multi-line text in YAML, for example:
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::
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description: >
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This is how you can provide a longer description
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of your template that goes over several lines.
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The *resources* section is required and must contain at least one resource
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definition. In the example above, a compute instance is defined with fixed
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values for the 'key_name', 'image' and 'flavor' parameters.
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Note that all those elements, i.e. a key-pair with the given name, the image and
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the flavor have to exist in the OpenStack environment where the template is
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used. Typically a template is made more easily reusable, though, by defining a
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set of *input parameters* instead of hard-coding such values.
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Template input parameters
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-------------------------
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Input parameters defined in the *parameters* section of a HOT template (see also
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:ref:`hot_spec_parameters`) allow users to customize a template during
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deployment. For example, this allows for providing custom key-pair names or
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image IDs to be used for a deployment.
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From a template author's perspective, this helps to make a template more easily
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reusable by avoiding hardcoded assumptions.
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Sticking to the example used above, it makes sense to allow users to provide
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their custom key-pairs, provide their own image, and to select a flavor for the
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compute instance. This can be achieved by extending the initial template as
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follows:
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::
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heat_template_version: 2013-05-23
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description: Simple template to deploy a single compute instance
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parameters:
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key_name:
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type: string
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label: Key Name
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description: Name of key-pair to be used for compute instance
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image_id:
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type: string
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label: Image ID
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description: Image to be used for compute instance
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instance_type:
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type: string
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label: Instance Type
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description: Type of instance (flavor) to be used
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resources:
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my_instance:
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type: OS::Nova::Server
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properties:
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key_name: { get_param: key_name }
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image: { get_param: image_id }
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flavor: { get_param: instance_type }
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In the example above, three input parameters have been defined that have to be
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provided by the user upon deployment. The fixed values for the respective
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resource properties have been replaced by references to the corresponding
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input parameters by means of the *get_param* function (see also
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:ref:`hot_spec_intrinsic_functions`).
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You can also define default values for input parameters which will be used in
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case the user does not provide the respective parameter during deployment. For
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example, the following definition for the *instance_type* parameter would select
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the 'm1.small' flavor unless specified otherwise by the user.
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::
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parameters:
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instance_type:
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type: string
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label: Instance Type
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description: Type of instance (flavor) to be used
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default: m1.small
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Another option that can be specified for a parameter is to hide its value when
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users request information about a stack deployed from a template. This is
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achieved by the *hidden* attribute and useful, for example when requesting
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passwords as user input:
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::
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parameters:
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database_password:
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type: string
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label: Database Password
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description: Password to be used for database
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hidden: true
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Restricting user input
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~~~~~~~~~~~~~~~~~~~~~~
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In some cases you might want to restrict the values of input parameters that
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users can supply. For example, you might know that the software running in a
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compute instance needs a certain amount of resources so you might want to
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restrict the *instance_type* parameter introduced above. Parameters in HOT
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templates can be restricted by adding a *constraints* section (see also
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:ref:`hot_spec_parameters_constraints`).
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For example, the following would allow only three values to be provided as input
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for the *instance_type* parameter:
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::
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parameters:
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instance_type:
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type: string
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label: Instance Type
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description: Type of instance (flavor) to be used
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constraints:
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- allowed_values: [ m1.medium, m1.large, m1.xlarge ]
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description: Value must be one of m1.medium, m1.large or m1.xlarge.
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The *constraints* section allows for defining a list of constraints that must
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all be fulfilled by user input. For example, the following list of constraints
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could be used to clearly specify format requirements on a password to be
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provided by users:
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::
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parameters:
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database_password:
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type: string
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label: Database Password
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description: Password to be used for database
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hidden: true
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constraints:
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- length: { min: 6, max: 8 }
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description: Password length must be between 6 and 8 characters.
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- allowed_pattern: "[a-zA-Z0-9]+"
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description: Password must consist of characters and numbers only.
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- allowed_pattern: "[A-Z]+[a-zA-Z0-9]*"
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description: Password must start with an uppercase character.
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Note that you can define multiple constraints of the same type. Especially in
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the case of allowed patterns this not only allows for keeping regular
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expressions simple and maintainable, but also for keeping error messages to be
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presented to users precise.
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Providing template outputs
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--------------------------
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In addition to template customization through input parameters, you will
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typically want to provide outputs to users, which can be done in the
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*outputs* section of a template (see also :ref:`hot_spec_outputs`).
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For example, the IP address by which the instance defined in the example
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above can be accessed should be provided to users. Otherwise, users would have
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to look it up themselves. The definition for providing the IP address of the
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compute instance as an output is shown in the following snippet:
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::
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outputs:
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instance_ip:
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description: The IP address of the deployed instance
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value: { get_attr: [my_instance, first_address] }
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Output values are typically resolved using intrinsic function such as
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the *get_attr* function in the example above (see also
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:ref:`hot_spec_intrinsic_functions`).
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