HOT templates are defined in YAML and use the following structure: heat_template_version: 2013-05-23 description: # description of the template parameter_groups: # declaration of input parameter groups and order parameters: # declaration of input parameters resources: # declaration of template resources outputs: # declaration of output parameters heat_template_version This key with value 2013-05-23 (or a later date) indicates that the YAML document is a HOT template of the specified version. description This optional key gives a description of the template, or the workload that can be deployed using the template. parameter_groups This section specifies how the input parameters should be grouped and the order to provide the parameters in. This section is optional. parameters This section specifies input parameters that have to be provided when instantiating the template. This section is optional. resources This section contains the declaration of the resources of the template. This section with at least one resource must be defined in any HOT template, or the template would not really do anything when being instantiated. outputs This section specifies output parameters available to users once the template has been instantiated. This section is optional.

The parameter_groups section specifies how the input parameters should be grouped and the order to provide the parameters in. These groups are typically used to describe expected behavior for downstream user interfaces. These groups are specified in a list with each group containing a list of associated parameters. The lists are used to denote the expected order of the parameters. A parameter can only be included in one group. Use the name of the parameter to add it to a group. The parameters details are defined in the parameters section. parameter_groups: - label: <human-readable label of parameter group> description: <description of the parameter group> parameters: - <param name> - <param name> label A human-readable label that defines the associated group of parameters. description A human-readable description of the parameter group. parameters A list of parameters that belong with this parameter group. param name The name of a parameter defined in the parameters section.

The parameters section defines input parameters that have to be provided when instantiating the template. Such parameters are typically used to customize each deployment, or for binding to environment specifics like certain images. Each parameter is specified in a separated nested block with the name of the parameter defined in the first line and additional attributes such as a type or a default value defined as nested elements: parameters: <param name>: type: <string | number | json | comma_delimited_list | boolean> label: <human-readable name of the parameter> description: <description of the parameter> default: <default value for parameter> hidden: <true | false> constraints: <parameter constraints> param name The name of the parameter. type The type of the parameter. Supported types are string, number, comma_delimited_list, json and boolean. This attribute is required. label A human readable name for the parameter. This attribute is optional. description A human readable description for the parameter. This attribute is optional. default A default value for the parameter. This value is used if the user doesn't specify his own value during deployment. This attribute is optional. hidden Defines whether the parameters should be hidden when a user requests information about a stack created from the template. This attribute can be used to hide passwords specified as parameters. This attribute is optional and defaults to false. constraints A list of constraints to apply. The constraints are validated by the Orchestration engine when a user deploys a stack. The stack creation fails if the parameter value doesn't comply to the constraints. This attribute is optional. The following example shows a minimalistic definition of two parameters: parameters: user_name: type: string label: User Name description: User name to be configured for the application port_number: type: number label: Port Number description: Port number to be configured for the web server The description and the label are optional, but defining these attributes is good practice to provide useful information about the role of the parameter to the user.

The constraints block of a parameter definition defines additional validation constraints that apply to the value of the parameter. The parameter values provided by a user are validated against the contrainsts at instantiation time. The contrainsts are defined as a list with the following syntax: constraints: - <constraint type>: <constraint definition> description: <constraint description> constraint type Type of constraint to apply. The set of currently supported constraints is given below. constraint definition The actual constraint, depending on the constraint type. The concrete syntax for each constraint type is given below. description A description of the constraint. The text is presented to the user when the value he defines violates the constraint. If ommitted, a default validation message is presented to the user. This attribute is optional. The following example shows the definition of a string parameter with two constraints. Note that while the descriptions for each constraint are optional, it is good practice to provide concrete descriptions to present useful messages to the user at deployment time. parameters: user_name: type: string label: User Name description: User name to be configured for the application constraints: - length: { min: 6, max: 8 } description: User name must be between 6 and 8 characters - allowed_pattern: "[A-Z]+[a-zA-Z0-9]*" description: User name must start with an uppercase character While the descriptions for each constraint are optional, it is good practice to provide concrete descriptions so useful messages can be presented to the user at deployment time. The following sections list the supported types of parameter constraints, along with the syntax for each type.

The length constraint applies to parameters of type string. It defines a lower and upper limit for the length of the string value. The syntax of the length constraint is: length: { min: <lower limit>, max: <upper limit> } It is possible to define a length constraint with only a lower limit or an upper limit. However, at least one of min or max must be specified.

The range constraint applies to parameters of type number. It defines a lower and upper limit for the numeric value of the parameter. The syntax of the range constraint is: range: { min: <lower limit>, max: <upper limit> } It is possible to define a range constraint with only a lower limit or an upper limit. However, at least one of min or max must be specified. The minimum and maximum boundaries are included in the range. For example, the following range constraint would allow for all numeric values between 0 and 10: range: { min: 0, max: 10 }

The allowed_values constraint applies to parameters of type string or number. It specifies a set of possible values for a parameter. At deployment time, the user-provided value for the respective parameter must match one of the elements of the list. The syntax of the allowed_values constraint is: allowed_values: [ <value>, <value>, ... ] Alternatively, the following YAML list notation can be used: allowed_values: - <value> - <value> - ... For example: parameters: instance_type: type: string label: Instance Type description: Instance type for compute instances constraints: - allowed_values: - m1.small - m1.medium - m1.large

The allowed_pattern constraint applies to parameters of type string. It specifies a regular expression against which a user-provided parameter value must evaluate at deployment. The syntax of the allowed_pattern constraint is: allowed_pattern: <regular expression> For example: parameters: user_name: type: string label: User Name description: User name to be configured for the application constraints: - allowed_pattern: "[A-Z]+[a-zA-Z0-9]*" description: User name must start with an uppercase character

The custom_constraint constraint adds an extra step of validation, generally to check that the specified resource exists in the backend. Custom constraints get implemented by plug-ins and can provide any kind of advanced constraint validation logic. The syntax of the custom_constraint constraint is: custom_constraint: <name> The name attribute specifies the concrete type of custom constraint. It corresponds to the name under which the respective validation plugin has been registered in the Orchestration engine. For example: parameters: key_name type: string description: SSH key pair constraints: - custom_constraint: nova.keypair

In addition to parameters defined by a template author, the Orchestration module also creates two parameters for every stack that allow referential access to the stack's name and identifier. These parameters are named OS::stack_name for the stack name and OS::stack_id for the stack identifier. These values are accessible via the get_param intrinsic function, just like user-defined parameters.

The resources section defines actual resources that make up a stack deployed from the HOT template (for instance compute instances, networks, storage volumes). Each resource is defined as a separate block in the resources section with the following syntax: resources: <resource ID>: type: <resource type> properties: <property name>: <property value> metadata: <resource specific metadata> depends_on: <resource ID or list of ID> update_policy: <update policy> deletion_policy: <deletion policy> resource ID A resource ID which must be unique within the resources section of the template. type The resource type, such as OS::Nova::Server or OS::Neutron::Port. This attribute is required. properties A list of resource-specific properties. The property value can be provided in place, or via a function (see Intrinsic functions). This section is optional. metadata Resource-specific metadata. This section is optional. depends_on Dependencies of the resource on one or more resources of the template. See Resource dependencies for details. This attribute is optional. update_policy Update policy for the resource, in the form of a nested dictionary. Whether update policies are supported and what the exact semantics are depends on the type of the current resource. This attribute is optional. deletion_policy Deletion policy for the resource. Which type of deletion policy is supported depends on the type of the current resource. This attribute is optional. Depending on the type of resource, the resource block might include more resource specific data. All resource types that can be used in CFN templates can also be used in HOT templates, adapted to the YAML structure as outlined above. The following example demonstrates the definition of a simple compute resource with some fixed property values: resources: my_instance: type: OS::Nova::Server properties: flavor: m1.small image: F18-x86_64-cfntools

The depends_on attribute of a resource defines a dependency between this resource and one or more other resources. If a resource depends on just one other resource, the ID of the other resource is specified as string of the depends_on attribute, as shown in the following example: resources: server1: type: OS::Nova::Server depends_on: server2 server2: type: OS::Nova::Server If a resource depends on more than one other resources, the value of the depends_on attribute is specified as a list of resource IDs, as shown in the following example: resources: server1: type: OS::Nova::Server depends_on: [ server2, server3 ] server2: type: OS::Nova::Server server3: type: OS::Nova::Server

The outputs section defines output parameters that should be available to the user once a stack has been created. This would be, for example, parameters such as IP addresses of deployed instances, or URLs of web applications deployed as part of a stack. Each output parameter is defined as a separate block within the outputs section according to the following syntax: outputs: <parameter name>: description: <description> value: <parameter value> parameter name The output parameter name, which must be unique within the outputs section of a template. description A short description of the output parameter. This attribute is optional. parameter value The value of the output parameter. This value is usually resolved by means of a function. See Intrinsic functions for details about the functions. This attribute is required. The example below shows how the IP address of a compute resource can be defined as an output parameter: outputs: instance_ip: description: IP address of the deployed compute instance value: { get_attr: [my_instance, first_address] }

HOT provides a set of intrinsic functions that can be used inside templates to perform specific tasks, such as getting the value of a resource attribute at runtime. The following section describes the role and syntax of the intrinsic functions.

The get_attr function references an attribute of a resource. The attribute value is resolved at runtime using the resource instance created from the respective resource definition. The syntax of the get_attr function is: get_attr: - <resource ID> - <attribute name> - <key/index 1> (optional) - <key/index 2> (optional) - ... resource ID The resource ID for which the attribute needs to be resolved. The resource ID must exist in the resources section of the template. attribute name The attribute name to be resolved. If the attribute returns a complex data structure such as a list or a map, then subsequent keys or indexes can be specified. These additional parameters are used to navigate the data structure to return the desired value. The following example demonstrates how to use the get_param function: resources: my_instance: type: OS::Nova::Server # ... outputs: instance_ip: description: IP address of the deployed compute instance value: { get_attr: [my_instance, first_address] } instance_private_ip: description: Private IP address of the deployed compute instance value: { get_attr: [my_instance, networks, private, 0] } In this example, if the networks attribute contained the following data: {"public": ["2001:0db8:0000:0000:0000:ff00:0042:8329", "1.2.3.4"], "private": ["10.0.0.1"]} then the value of the get_attr function would resolve to 10.0.0.1 (first item of the private entry in the networks map).

The get_file function returns the content of a file into the template. It is generally used as a file inclusion mechanism for files containing scripts or configuration files. The syntax of the get_file function is: get_file: <content key> The content key is used to look up the files dictionary that is provided in the REST API call. The Orchestration client command (heat) is get_file aware and will populate the files dictionnary with the actual content of fetched paths and URLs. The Orchestration client command supports relative paths and will transform these to the absolute URLs required by the Orcestration API. The get_file argument must be a static path or URL and not rely on intrinsic functions like get_param. the Orchestration client does not process intrinsic functions (they are only processed by the Orchestration engine). The example below demonstrates the get_file function usage with both relative and absolute URLs: resources: my_instance: type: OS::Nova::Server properties: # general properties ... user_data: get_file: my_instance_user_data.sh my_other_instance: type: OS::Nova::Server properties: # general properties ... user_data: get_file: http://example.com/my_other_instance_user_data.sh The files dictionary generated by the Orchestration client during instantiation of the stack would contain the following keys: * file:///path/to/my_instance_user_data.sh * http://example.com/my_other_instance_user_data.sh*

The get_param function references an input parameter of a template. It resolves to the value provided for this input parameter at runtime. The syntax of the get_param function is: get_param: - <parameter name> - <key/index 1> (optional) - <key/index 2> (optional) - ... parameter name The parameter name to be resolved. If the parameters returns a complex data structure such as a list or a map, then subsequent keys or indexes can be specified. These additional parameters are used to navigate the data structure to return the desired value. The following example demonstrates the use of the get_param function: parameters: instance_type: type: string label: Instance Type description: Instance type to be used. server_data: type: json resources: my_instance: type: OS::Nova::Server properties: flavor: { get_param: instance_type} metadata: { get_param: [ server_data, metadata ] } key_name: { get_param: [ server_data, keys, 0 ] } In this example, if the instance_type and server_data parameters contained the following data: {"instance_type": "m1.tiny", {"server_data": {"metadata": {"foo": "bar"}, "keys": ["a_key","other_key"]}}} then the value of the property flavor would resolve to m1.tiny, metadata would resolve to {"foo": "bar"} and key_name would resolve to a_key.

The get_resource function references another resource within the same template. At runtime, it is resolved to reference the ID of the referenced resource, which is resource type specific. For example, a reference to a floating IP resource returns the respective IP address at runtime. The syntax of the get_resource function is: get_resource: <resource ID> The resource ID of the referenced resource is given as single parameter to the get_resource function. For exemple: resources: instance_port: type: OS::Neutron::Port properties: ... instance: type: OS::Nova::Server properties: ... networks: port: { get_resource: instance_port }

The list_join function joins a list of strings with the given delimiter. The syntax of the list_join function is: list_join: - <delimiter> - <list to join> For example: list_join: [', ', ['one', 'two', 'and three']] This resolve to the string one, two, and three.

The resource_facade function retrieves data in a parent provider template. A provider template provdes a custom definition of a resource, called its facade. For more information about custom templates, see . The syntax of the resource_facade function is: resource_facade: <data type> data type can be one of metadata, deletion_policy or update_policy.

The str_replace function dynamically constructs strings by providing a template string with placeholders and a list of mappings to assign values to those placeholders at runtime. The placeholders are replaced with mapping values wherever a mapping key exactly matches a placeholder. The syntax of the str_replace function is: str_replace: template: <template string> params: <parameter mappings> template Defines the template string that contains placeholders which will be substituted at runtime. params Provides parameter mappings in the form of dictionary. Each key refers to a placeholder used in the template attribute. The following example shows a simple use of the str_replace function in the outputs section of a template to build a URL for logging into a deployed application: resources: my_instance: type: OS::Nova::Server # general metadata and properties ... outputs: Login_URL: description: The URL to log into the deployed application value: str_replace: template: http://host/MyApplication params: host: { get_attr: [ my_instance, first_address ] } The following examples show the use of the str_replace function to build an instance initialization script: parameters: DBRootPassword: type: string label: Database Password description: Root password for MySQL hidden: true resources: my_instance: type: OS::Nova::Server properties: # general properties ... user_data: str_replace: template: | #!/bin/bash echo "Hello world" echo "Setting MySQL root password" mysqladmin -u root password $db_rootpassword # do more things ... params: $db_rootpassword: { get_param: DBRootPassword }