CloudFormation Compatible Functions

There are a number of functions that you can use to help you write CloudFormation compatible templates. While most CloudFormation functions are supported in HOT version ‘2013-05-23’, Fn::Select is the only CloudFormation function supported in HOT templates since version ‘2014-10-16’ which is introduced in Juno.

All of these functions (except Ref) start with Fn::.

Ref

Returns the value of the named parameter or resource.

Parameters

name : String

The name of the resource or parameter.

Usage

{Ref: my_server}

Returns the nova instance ID. For example, d8093de0-850f-4513-b202-7979de6c0d55.

Fn::Base64

This is a placeholder for a function to convert an input string to Base64. This function in Heat actually performs no conversion. It is included for the benefit of CFN templates that convert UserData to Base64. Heat only accepts UserData in plain text.

Parameters

value : String

The string to convert.

Usage

{"Fn::Base64": "convert this string please."}

Returns the original input string.

Fn::FindInMap

Returns the value corresponding to keys into a two-level map declared in the Mappings section.

Parameters

map_name : String

The logical name of a mapping declared in the Mappings section that contains the keys and values.

top_level_key : String

The top-level key name. It’s value is a list of key-value pairs.

second_level_key : String

The second-level key name, which is set to one of the keys from the list assigned to top_level_key.

Usage

Mapping:
  MyContacts:
    jone: {phone: 337, email: a@b.com}
    jim: {phone: 908, email: g@b.com}

{"Fn::FindInMap": ["MyContacts", "jim", "phone" ] }

Returns 908.

Fn::GetAtt

Returns an attribute of a resource within the template.

Parameters

resource : String

The name of the resource.

attribute : String

The name of the attribute.

Usage

{Fn::GetAtt: [my_server, PublicIp]}

Returns an IP address such as 10.0.0.2.

Fn::GetAZs

Returns the Availability Zones within the given region.

Note: AZ’s and regions are not fully implemented in Heat.

Parameters

region : String

The name of the region.

Usage

{Fn::GetAZs: ""}

Returns the list provided by nova availability-zone-list.

Fn::Join

Like python join, it joins a list of strings with the given delimiter.

Parameters

delimiter : String

The string to join the list with.

list : list

The list to join.

Usage

{Fn::Join: [",", ["beer", "wine", "more beer"]]}

Returns beer, wine, more beer.

Fn::Select

Select an item from a list.

Heat extension: Select an item from a map

Parameters

selector : string or integer

The number of item in the list or the name of the item in the map.

collection : map or list

The collection to select the item from.

Usage

For a list lookup:

{ "Fn::Select" : [ "2", [ "apples", "grapes", "mangoes" ] ] }

Returns mangoes.

For a map lookup:

{ "Fn::Select" : [ "red", {"red": "a", "flu": "b"} ] }

Returns a.

Fn::Split

This is the reverse of Join. Convert a string into a list based on the delimiter.

Parameters

delimiter : string

Matching string to split on.

string : String

The string to split.

Usage

{ "Fn::Split" : [ ",", "str1,str2,str3,str4"]}

Returns {["str1", "str2", "str3", "str4"]}.

Fn::Replace

Find and replace one string with another.

Parameters

substitutions : map

A map of substitutions.

string: String

The string to do the substitutions in.

Usage

{"Fn::Replace": [
 {'$var1': 'foo', '%var2%': 'bar'},
  '$var1 is %var2%'
]}

Returns "foo is bar".

Fn::ResourceFacade

When writing a Template Resource:

• user writes a template that will fill in for a resource (the resource is the facade).
• when they are writing their template they need to access the metadata from the facade.

Parameters

attribute_name : String

One of Metadata, DeletionPolicy or UpdatePolicy.

Usage

{'Fn::ResourceFacade': 'Metadata'}
{'Fn::ResourceFacade': 'DeletionPolicy'}
{'Fn::ResourceFacade': 'UpdatePolicy'}

Example

Here is a top level template top.yaml

resources:
  my_server:
    type: OS::Nova::Server
    metadata:
      key: value
      some: more stuff

Here is a resource template my_actual_server.yaml

resources:
  _actual_server_:
    type: OS::Nova::Server
    metadata: {'Fn::ResourceFacade': Metadata}

The environment file env.yaml

resource_registry:
  resources:
    my_server:
      "OS::Nova::Server": my_actual_server.yaml

To use it

$ heat stack-create -f top.yaml -e env.yaml

What happened is the metadata in top.yaml (key: value, some: more stuff) gets passed into the resource template via the Fn::ResourceFacade function.

Fn::MemberListToMap

Convert an AWS style member list into a map.

Parameters

key name: string

The name of the key (normally “Name” or “Key”).

value name: string

The name of the value (normally “Value”).

list: A list of strings

The string to convert.

Usage

{'Fn::MemberListToMap': ['Name', 'Value', ['.member.0.Name=key',
                                           '.member.0.Value=door',
                                           '.member.1.Name=colour',
                                           '.member.1.Value=green']]}

Returns {'key': 'door', 'colour': 'green'}.