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docs: Add example of anchor and aliases in playbook (#54079)

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* docs: Add example of anchor and aliases in playbook

YAML supports anchors and aliases, add examples for explaining
this functionality with respect to Ansible playbook. Fix headers, fix 
local TOC, remove unused section on YAML tags and Python types.
This commit is contained in:
Abhijeet Kasurde 2019-03-29 20:17:38 +05:30 committed by Alicia Cozine
parent 889f423a22
commit d131332cbd
1 changed files with 62 additions and 11 deletions
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73
docs/docsite/rst/user_guide/playbooks_advanced_syntax.rst
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@ -1,23 +1,18 @@
.. _playbooks_advanced_syntax:
***************
Advanced Syntax
===============
***************
.. contents:: Topics
The advanced YAML syntax examples on this page give you more control over the data placed in YAML files used by Ansible. You can find additional information about Python-specific YAML in the official `PyYAML Documentation <https://pyyaml.org/wiki/PyYAMLDocumentation#YAMLtagsandPythontypes>`_.
This page describes advanced YAML syntax that enables you to have more control over the data placed in YAML files used by Ansible.
.. _yaml_tags_and_python_types:
YAML tags and Python types
``````````````````````````
The documentation covered here is an extension of the documentation that can be found in the `PyYAML Documentation <https://pyyaml.org/wiki/PyYAMLDocumentation#YAMLtagsandPythontypes>`_
.. contents::
:local:
.. _unsafe_strings:
Unsafe or Raw Strings
~~~~~~~~~~~~~~~~~~~~~
=====================
Ansible provides an internal data type for declaring variable values as "unsafe". This means that the data held within the variables value should be treated as unsafe preventing unsafe character substitution and information disclosure.
@ -49,7 +44,63 @@ For complex variables such as hashes or arrays, ``!unsafe`` should be used on th
my_unsafe_hash:
unsafe_key: !unsafe 'unsafe value'
.. _anchors_and_aliases:
YAML anchors and aliases: sharing variable values
=================================================
`YAML anchors and aliases <https://yaml.org/spec/1.2/spec.html#id2765878>`_ help you define, maintain, and use shared variable values in a flexible way.
You define an anchor with ``&``, then refer to it using an alias, denoted with ``*``. Here's an example that sets three values with an anchor, uses two of those values with an alias, and overrides the third value::
---
...
vars:
app1:
jvm: &jvm_opts
opts: '-Xms1G -Xmx2G'
port: 1000
path: /usr/lib/app1
app2:
jvm:
<<: *jvm_opts
path: /usr/lib/app2
...
Here, ``app1`` and ``app2`` share the values for ``opts`` and ``port`` using the anchor ``&jvm_opts`` and the alias ``*jvm_opts``.
The value for ``path`` is merged by ``<<`` or `merge operator <https://yaml.org/type/merge.html>`_.
Anchors and aliases also let you share complex sets of variable values, including nested variables. If you have one variable value that includes another variable value, you can define them separately::
vars:
webapp_version: 1.0
webapp_custom_name: ToDo_App-1.0
This is inefficient and, at scale, means more maintenance. To incorporate the version value in the name, you can use an anchor in ``app_version`` and an alias in ``custom_name``::
vars:
webapp:
version: &my_version 1.0
custom_name:
- "ToDo_App"
- *my_version
Now, you can re-use the value of ``app_version`` within the value of ``custom_name`` and use the output in a template::
---
- name: Using values nested inside dictionary
hosts: localhost
vars:
webapp:
version: &my_version 1.0
custom_name:
- "ToDo_App"
- *my_version
tasks:
- name: Using Anchor value
debug:
msg: My app is called "{{ webapp.custom_name | join('-') }}".
You've anchored the value of ``version`` with the ``&my_version`` anchor, and re-used it with the ``*my_version`` alias. Anchors and aliases let you access nested values inside dictionaries.
.. seealso::