deb-heat/doc/source/template_guide/functions.rst

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==================
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: 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
----------
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'}