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.. highlight: yaml
:linenothreshold: 5
..
Licensed under the Apache License, Version 2.0 (the "License"); you may
not use this file except in compliance with the License. You may obtain
a copy of the License at
http://www.apache.org/licenses/LICENSE-2.0
Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
License for the specific language governing permissions and limitations
under the License.
.. _hot_spec:
===============================================
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 :ref:`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.
.. _hot_spec_template_version:
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
.. _hot_spec_parameter_groups:
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.
.. _hot_spec_parameters:
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 | ["one", "two"]; |
| | that are separated by commas. | "one, two"; |
| | The total number of strings | Note: "one, two" |
| | should be one more than the | returns |
| | total number of commas. | ["one", " two"] |
+----------------------+-------------------------------+------------------+
| json | A JSON-formatted map or list. | {"key": "value"} |
+----------------------+-------------------------------+------------------+
| boolean | Boolean type value, which can | "on"; "n" |
| | be equal "t", "true", "on", | |
| | "y", "yes", or "1" for true | |
| | value and "f", "false", | |
| | "off", "n", "no", or "0" for | |
| | false value. | |
+----------------------+-------------------------------+------------------+
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.
.. _hot_spec_parameters_constraints:
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
.. _hot_spec_pseudo_parameters:
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).
.. _hot_spec_resources:
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 :ref:`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 :ref:`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
.. _hot_spec_resources_dependencies:
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
.. _hot_spec_outputs:
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 :ref:`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] }
.. _hot_spec_intrinsic_functions:
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 :code:`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 :code:`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:`file:///path/to/my_instance_user_data.sh`
* :file:`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 :ref:`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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