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Heat Orchestration Template (HOT) specification

HOT is a new template format meant to replace the Heat CloudFormation-compatible format (CFN) as the native format supported by the Heat over time. This specification explains in detail all elements of the HOT template format. An example driven guide to writing HOT templates can be found at hot_guide.

Status

HOT is considered reliable, supported, and standardized as of our Icehouse (April 2014) release. The Heat core team may make improvements to the standard, which very likely would be backward compatible. The template format is also versioned. Since Juno release, Heat supports multiple different versions of the HOT specification.

Template structure

HOT templates are defined in YAML and follow the structure outlined below.

heat_template_version: 2013-05-23

description:
  # a description of the template

parameter_groups:
  # a 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 allows for giving a description of the template, or the workload that can be deployed using the template.
parameter_groups: This section allows for specifying how the input parameters should be grouped and the order to provide the parameters in. This section is optional and can be omitted when necessary.
parameters: This section allows for specifying input parameters that have to be provided when instantiating the template. The section is optional and can be omitted when no input is required.
resources: This section contains the declaration of the single resources of the template. This section with at least one resource should be defined in any HOT template, or the template would not really do anything when being instantiated.
outputs: This section allows for specifying output parameters available to users once the template has been instantiated. This section is optional and can be omitted when no output values are required.

Heat template version

The value of heat_template_version tells Heat not only the format of the template but also features that will be validated and supported. For example, Heat currently supports the following values for the heat_template_version key:

2013-05-23

The key with value 2013-05-23 indicates that the YAML document is a HOT template and it may contain features implemented until the Icehouse release. This version supports the following functions (some are back ported to this version):

get_attr
get_file
get_param
get_resource
list_join
resource_facade
str_replace
Fn::Base64
Fn::GetAZs
Fn::Join
Fn::MemberListToMap
Fn::Replace
Fn::ResourceFacade
Fn::Select
Fn::Split
Ref

2014-10-16

The key with value 2014-10-16 indicates that the YAML document is a HOT template and it may contain features added and/or removed up until the Juno release. This version removes most CFN functions that were supported in the Icehouse release, i.e. the 2013-05-23 version. So the supported functions now are:

get_attr
get_file
get_param
get_resource
list_join
resource_facade
str_replace
Fn::Select

2015-04-30

The key with value 2015-04-30 indicates that the YAML document is a HOT template and it may contain features added and/or removed up until the Kilo release. This version adds the repeat function. So the complete list of supported functions is:

get_attr
get_file
get_param
get_resource
list_join
repeat
digest
resource_facade
str_replace
Fn::Select

2015-10-15

The key with value 2015-10-15 indicates that the YAML document is a HOT template and it may contain features added and/or removed up until the Liberty release. This version removes the Fn::Select function, path based get_attr/get_param references should be used instead.

get_attr
get_file
get_param
get_resource
list_join
repeat
digest
resource_facade
str_replace
str_split

Parameter groups section

The parameter_groups section allows for specifying 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. Each parameter should be associated to a specific group only once using the parameter name to bind it to a defined parameter 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: This attribute allows for giving a human-readable description of the parameter group.
parameters: A list of parameters associated with this parameter group.
param name: The name of the parameter that is defined in the associated parameters section.

Parameters section

The parameters section allows for specifying input parameters that have to be provided when instantiating the template. Such parameters are typically used to customize each deployment (e.g. by setting custom user names or passwords) or for binding to environment-specifics like certain images.

Each parameter is specified in a separated nested block with the name of the parameters defined in the first line and additional attributes such as type or 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 table below describes all currently supported types with examples:

Type	Description	Examples
string	A literal string.	"String param"
number	An integer or float.	"2"; "0.2"
comma_delimited_list	An array of literal strings that are separated by commas. The total number of strings should be one more than the total number of commas.	["one", "two"]; "one, two"; Note: "one, two" returns ["one", " two"]
json	A JSON-formatted map or list.	{"key": "value"}
boolean	Boolean type value, which can be equal "t", "true", "on", "y", "yes", or "1" for true value and "f", "false", "off", "n", "no", or "0" for false value.	"on"; "n"

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

Note

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.

Parameter Constraints

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 constraints at instantiation time. The constraints 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 omitted, 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

Note

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 concrete syntax for each type.

length

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.

range

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 }

allowed_values

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

allowed_pattern

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

custom_constraint

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

Pseudo parameters

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

Note

OS::project_id is available since 2015.1 (Kilo).

Resources section

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 hot_spec_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 hot_spec_resources_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

Resource dependencies

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

Outputs section

The outputs section defines output parameters that should be available to the user after 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 hot_spec_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] }

Intrinsic functions

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.

get_attr

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.

Path based attribute referencing using keys or indexes requires heat_template_version 2014-10-16 or higher.

The syntax of the get_attr function is:

get_attr:
  - <resource name>
  - <attribute name>
  - <key/index 1> (optional)
  - <key/index 2> (optional)
  - ...

resource name

The resource name for which the attribute needs to be resolved.

The resource name 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_attr 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 get_attr function would resolve to 10.0.0.1 (first item of the private entry in the networks map).

get_file

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 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 populates the files dictionary with the actual content of fetched paths and URLs. The Orchestration client command supports relative paths and transforms these to the absolute URLs required by the Orchestration API.

Note

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

get_param

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.

get_resource

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 example:

resources:
  instance_port:
    type: OS::Neutron::Port
    properties: ...

  instance:
    type: OS::Nova::Server
    properties:
      ...
      networks:
        port: { get_resource: instance_port }

list_join

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.

digest

The digest function allows for performing digest operations on a given value. This function has been introduced in the Kilo release and is usable with HOT versions later than 2015-04-30.

The syntax of the digest function is:

digest:
  - <algorithm>
  - <value>

algorithm: The digest algorithm. Valid algorithms are the ones provided natively by hashlib (md5, sha1, sha224, sha256, sha384, and sha512) or any one provided by OpenSSL.
value: The value to digest. This function will resolve to the corresponding hash of the value.

For example:

# from a user supplied parameter
pwd_hash: { digest: ['sha512', { get_param: raw_password }] }

The value of the digest function would resolve to the corresponding hash of the value of raw_password.

repeat

The repeat function allows for dynamically transforming lists by iterating over the contents of one or more source lists and replacing the list elements into a template. The result of this function is a new list, where the elements are set to the template, rendered for each list item.

The syntax of the repeat function is:

repeat:
  template:
    <template>
  for_each:
    <var>: <list>

template: The template argument defines the content generated for each iteration, with placeholders for the elements that need to be replaced at runtime. This argument can be of any supported type.
for_each: The for_each argument is a dictionary that defines how to generate the repetitions of the template and perform substitutions. In this dictionary the keys are the placeholder names that will be replaced in the template, and the values are the lists to iterate on. On each iteration, the function will render the template by performing substitution with elements of the given lists. If a single key/value pair is given in this argument, the template will be rendered once for each element in the list. When more than one key/value pairs are given, the iterations will be performed on all the permutations of values between the given lists. The values in this dictionary can be given as functions such as get_attr or get_param.

The following example shows how a security group resource can be defined to include a list of ports given as a parameter:

parameters:
  ports:
    type: comma_delimited_list
    label: ports
    default: "80,443,8080"

resources:
  security_group:
    type: OS::Neutron::SecurityGroup
    properties:
      name: web_server_security_group
      rules:
        repeat:
          for_each:
            %port%: { get_param: ports }
          template:
            protocol: tcp
            port_range_min: %port%
            port_range_max: %port%

The following example demonstrates how the use of multiple lists enables the security group to also include parameterized protocols:

parameters:
  ports:
    type: comma_delimited_list
    label: ports
    default: "80,443,8080"
  protocols:
    type: comma_delimited_list
    label: protocols
    default: "tcp,udp"

resources:
  security_group:
    type: OS::Neutron::SecurityGroup
    properties:
      name: web_server_security_group
      rules:
        repeat:
          for_each:
            %port%: { get_param: ports }
            %protocol%: { get_param: protocols }
          template:
            protocol: %protocol%
            port_range_min: %port%

Note how multiple entries in the for_each argument are equivalent to nested for-loops in most programming languages.

resource_facade

The resource_facade function retrieves data in a parent provider template.

A provider template provides a custom definition of a resource, called its facade. For more information about custom templates, see composition. The syntax of the resource_facade function is:

resource_facade: <data type>

data type can be one of metadata, deletion_policy or update_policy.

str_replace

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 }

In the example above, one can imagine that MySQL is being configured on a compute instance and the root password is going to be set based on a user provided parameter. The script for doing this is provided as userdata to the compute instance, leveraging the str_replace function.

str_split

The str_split function allows for splitting a string into a list by providing an arbitrary delimiter, the opposite of list_join.

The syntax of the str_split function is as follows:

str_split:
  - ','
  - string,to,split

Or:

str_split: [',', 'string,to,split']

The result of which is:

['string', 'to', 'split']

Optionally, an index may be provided to select a specific entry from the resulting list, similar to get_attr/get_param:

str_split: [',', 'string,to,split', 0]

The result of which is:

'string'

Note: The index starts at zero, and any value outside the maximum (e.g the length of the list minus one) will cause an error.

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Heat Orchestration Template (HOT) specification

Status

Template structure

Heat template version

Parameter groups section

Parameters section

Parameter Constraints

length

range

allowed_values

allowed_pattern

custom_constraint

Pseudo parameters

Resources section

Resource dependencies

Outputs section

Intrinsic functions

get_attr

get_file

get_param

get_resource

list_join

digest

repeat

resource_facade

str_replace

str_split

34 KiB

Raw Blame History