Docs on general precedence (#50201) (#59120)

* Add docs/docsite/rst/reference_appendices/general_precedence.rst

Co-Authored-By: Sandra McCann <samccann@redhat.com>
(cherry picked from commit c2469648e4)

docs/docsite/rst/index.rst

@@ -69,6 +69,7 @@ Ansible releases a new major release of Ansible approximately three to four time
    reference_appendices/galaxy
    reference_appendices/common_return_values
    reference_appendices/config
+   reference_appendices/general_precedence
    reference_appendices/YAMLSyntax
    reference_appendices/python_3_support
    reference_appendices/interpreter_discovery

docs/docsite/rst/reference_appendices/general_precedence.rst

@@ -0,0 +1,140 @@
+.. _general_precedence_rules:
+
+Controlling how Ansible behaves: precedence rules
+=================================================
+
+To give you maximum flexibility in managing your environments, Ansible supports many ways to control how Ansible behaves: how it connects to managed nodes, how it works once it has connected.
+If you use Ansible to manage a large number of servers, network devices, and cloud resources, you may define Ansible behavior in several different places and pass that information to Ansible in several different ways.
+This flexibility is convenient, but it can backfire if you do not understand the precedence rules.
+
+These precedence rules apply to any setting that can be defined in multiple ways (by configuration settings, command-line options, playbook keywords, variables).
+
+.. contents::
+   :local:
+
+Precedence categories
+---------------------
+
+Ansible supports four sources for controlling its behavior. In order of precedence from lowest (most easily overridden) to highest (overrides all others), the categories are:
+
+ * Configuration settings
+ * Command-line options
+ * Playbook keywords
+ * Variables
+
+Each category overrides any information from all lower-precedence categories. For example, a playbook keyword will override any configuration setting.
+
+Within each precedence category, specific rules apply. However, generally speaking, 'last defined' wins and overrides any previous definitions.
+
+Configuration settings
+^^^^^^^^^^^^^^^^^^^^^^
+
+:ref:`Configuration settings<ansible_configuration_settings>` include both values from the ``ansible.cfg`` file and environment variables. Within this category, values set in configuration files have lower precedence. Ansible uses the first ``ansible.cfg`` file it finds, ignoring all others. Ansible searches for ``ansible.cfg`` in these locations in order:
+
+ * ``ANSIBLE_CONFIG`` (environment variable if set)
+ * ``ansible.cfg`` (in the current directory)
+ * ``~/.ansible.cfg`` (in the home directory)
+ * ``/etc/ansible/ansible.cfg``
+
+Environment variables have a higher precedence than entries in ``ansible.cfg``. If you have environment variables set on your control node, they override the settings in whichever ``ansible.cfg`` file Ansible loads. The value of any given environment variable follows normal shell precedence: the last value defined overwrites previous values.
+
+Command-line options
+^^^^^^^^^^^^^^^^^^^^
+
+Any command-line option will override any configuration setting.
+
+When you type something directly at the command line, you may feel that your hand-crafted values should override all others, but Ansible does not work that way. Command-line options have low precedence - they override configuration only. They do not override playbook keywords, variables from inventory or variables from playbooks.
+
+You can override all other settings from all other sources in all other precedence categories at the command line by  :ref:`general_precedence_extra_vars`, but that is not a command-line option, it is a way of passing a :ref:`variable<general_precedence_variables>`.
+
+At the command line, if you pass multiple values for a parameter that accepts only a single value, the last defined value wins. For example, this :ref:`ad-hoc task<intro_adhoc>` will connect as ``carol``, not as ``mike``::
+
+      ansible -u mike -m ping myhost -u carol
+
+Some parameters allow multiple values. In this case, Ansible will append all values from the hosts listed in inventory files inventory1 and inventory2::
+
+   ansible -i /path/inventory1 -i /path/inventory2 -m ping all
+
+The help for each :ref:`command-line tool<command_line_tools>` lists available options for that tool.
+
+Playbook keywords
+^^^^^^^^^^^^^^^^^
+
+Any :ref:`playbook keyword<playbook_keywords>` will override any command-line option and any configuration setting.
+
+Within playbook keywords, precedence flows with the playbook itself; the more specific wins against the more general:
+
+- play (most general)
+- blocks/includes/imports/roles (optional and can contain tasks and each other)
+- tasks (most specific)
+
+A simple example::
+
+   - hosts: all
+     connection: ssh
+     tasks:
+       - name: This task uses ssh.
+         ping:
+
+       - name: This task uses paramiko.
+         connection: paramiko
+         ping:
+
+In this example, the ``connection`` keyword is set to ``ssh`` at the play level. The first task inherits that value, and connects using ``ssh``. The second task inherits that value, overrides it, and connects using ``paramiko``.
+The same logic applies to blocks and roles as well. All tasks, blocks, and roles within a play inherit play-level keywords; any task, block, or role can override any keyword by defining a different value for that keyword within the task, block, or role.
+
+Remember that these are KEYWORDS, not variables. Both playbooks and variable files are defined in YAML but they have different significance.
+Playbooks are the command or 'state description' structure for Ansible, variables are data we use to help make playbooks more dynamic.
+
+.. _general_precedence_variables:
+
+Variables
+^^^^^^^^^
+
+Any variable will override any playbook keyword, any command-line option, and any configuration setting.
+
+Variables that have equivalent playbook keywords, command-line options, and configuration settings are known as :ref:`connection_variables`. Originally designed for connection parameters, this category has expanded to include other core variables like the temporary directory and the python interpreter.
+
+Connection variables, like all variables, can be set in multiple ways and places. You can define variables for hosts and groups in :ref:`inventory<intro_inventory>`. You can define variables for tasks and plays in ``vars:`` blocks in :ref:`playbooks<about_playbooks>`. However, they are still variables - they are data, not keywords or configuration settings. Variables that override playbook keywords, command-line options, and configuration settings follow the same rules of :ref:`variable precedence <ansible_variable_precedence>` as any other variables.
+
+When set in a playbook, variables follow the same inheritance rules as playbook keywords. You can set a value for the play, then override it in a task, block, or role::
+
+   - hosts: cloud
+     gather_facts: false
+     become: yes
+     vars:
+       ansible_become_user: admin
+     tasks:
+       - name: This task uses admin as the become user.
+         dnf:
+           name: some-service
+           state: latest
+       - block:
+           - name: This task uses service-admin as the become user.
+             # a task to configure the new service
+           - name: This task also uses service-admin as the become user, defined in the block.
+             # second task to configure the service
+         vars:
+           ansible_become_user: service-admin
+       - name: This task (outside of the block) uses admin as the become user again.
+         service:
+           name: some-service
+           state: restarted
+
+Variable scope: how long is a value available?
+""""""""""""""""""""""""""""""""""""""""""""""
+
+Variable values set in a playbook exist only within the playbook object that defines them. These 'playbook object scope' variables are not available to subsequent objects, including other plays.
+
+Variable values associated directly with a host or group, including variables defined in inventory, by vars plugins, or using modules like :ref:`set_fact<set_fact_module>` and :ref:`include_vars<include_vars_module>`, are available to all plays. These 'host scope' variables are also available via the ``hostvars[]`` dictionary.
+
+.. _general_precedence_extra_vars:
+
+Using ``-e`` extra variables at the command line
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+To override all other settings in all other categories, you can use extra variables: ``--extra-vars`` or ``-e`` at the command line. Values passed with ``-e`` are variables, not command-line options, and they will override configuration settings, command-line options, and playbook keywords as well as variables set elsewhere. For example, this task will connect as ``brian`` not as ``carol``::
+
+   ansible -u carol -e 'ansible_user=brian' -a whoami all
+
+You must specify both the variable name and the value with ``--extra-vars``.

docs/docsite/rst/reference_appendices/special_variables.rst

@@ -121,10 +121,13 @@ ansible_local
     The keys available depend on the custom facts created.
     See the :ref:`setup <setup_module>` module for more details.
 
+.. _connection_variables:
+
 Connection variables
 ---------------------
 Connection variables are normally used to set the specifics on how to execute actions on a target. Most of them correspond to connection plugins, but not all are specific to them; other plugins like shell, terminal and become are normally involved.
 Only the common ones are described as each connection/become/shell/etc plugin can define its own overrides and specific variables.
+See :ref:`general_precedence_rules` for how connection variables interact with :ref:`configuration settings<ansible_configuration_settings>`, :ref:`command-line options<command_line_tools>`, and :ref:`playbook keywords<playbook_keywords>`.
 
 ansible_become_user
     The user Ansible 'becomes' after using privilege escalation. This must be available to the 'login user'.

docs/docsite/rst/user_guide/playbooks_variables.rst

@@ -1047,7 +1047,7 @@ Basically, anything that goes into "role defaults" (the defaults folder inside t
           This last one can be superceeded by the user via ``ansible_group_priority``, which defaults to ``1`` for all groups.
           This variable, ``ansible_group_priority``, can only be set in the inventory source and not in group_vars/ as the variable is used in the loading of group_vars/.
 
-Another important thing to consider (for all versions) is that connection variables override config, command line and play/role/task specific options and keywords.  For example, if your inventory specifies ``ansible_user: ramon`` and you run::
+Another important thing to consider (for all versions) is that connection variables override config, command line and play/role/task specific options and keywords. See :ref:`general_precedence_rules` for more details. For example, if your inventory specifies ``ansible_user: ramon`` and you run::
 
     ansible -u lola myhost
 

docs/templates/config.rst.j2

@@ -8,9 +8,9 @@
 {{name}}
 {{ '=' * name_len }}
 
-Ansible supports a few ways of providing configuration variables, mainly through environment variables, command line switches and an ini file named ``ansible.cfg``.
+Ansible supports several sources for configuring its behavior, including an ini file named ``ansible.cfg``, environment variables, command-line options, playbook keywords, and variables. See :ref:`general_precedence_rules` for details on the relative precedence of each source.
 
-Starting at Ansible 2.4 the ``ansible-config`` utility allows users to see all the configuration settings available, their defaults, how to set them and
+The ``ansible-config`` utility allows users to see all the configuration settings available, their defaults, how to set them and
 where their current value comes from. See :ref:`ansible-config` for more information.
 
 .. _ansible_configuration_settings_locations:

docs/templates/playbooks_keywords.rst.j2

@@ -3,7 +3,8 @@
 Playbook Keywords
 =================
 
-These are the keywords available on common playbook objects.
+These are the keywords available on common playbook objects. Keywords are one of several sources for configuring Ansible behavior. See :ref:`general_precedence_rules` for details on the relative precedence of each source.
+
 
 .. note:: Please note: