Built in functions

There are a number of functions that you can use to help you write templates.

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

Ref

Return 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 returns the Base64 representation of the input string.

Parameters

value : String

The string to convert.

Usage

{Base64: "convert this string please."}

Returns the Base64 of the 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: [email protected]}
    jim: {phone: 908, email: [email protected]}

{"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

Return 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 an replace one string with another.

Parameters

subsitutions : map

A map of subsitutions.

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'}