ansible/docs/docsite/rst/user_guide/playbooks_blocks.rst
Caio Ramos 1de1d081a5 Adding more information about blocks and blocks error handling. (#54429)
* Adding more information about blocks and blocks error handling.
* Update docs/docsite/rst/user_guide/playbooks_error_handling.rst and playbooks_blocks.rst
* Removing undefined variables as not rescuable errors.

Signed-off-by: Caio Ramos <caioramos97@gmail.com>
Signed-off-by: Gabriely Pereira <gabriely.pereira@usp.br>

* Apply suggestions from code review

Co-Authored-By: caiohsramos <caioramos97@gmail.com>
2019-04-05 16:49:20 -05:00

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.. _playbooks_blocks:
Blocks
======
Blocks allow for logical grouping of tasks and in play error handling. Most of what you can apply to a single task (with the exception of loops) can be applied at the block level, which also makes it much easier to set data or directives common to the tasks. This does not mean the directive affects the block itself, but is inherited by the tasks enclosed by a block. i.e. a `when` will be applied to the tasks, not the block itself.
.. code-block:: YAML
:emphasize-lines: 3
:caption: Block example with named tasks inside the block
tasks:
- name: Install, configure, and start Apache
block:
- name: install httpd and memcached
yum:
name: "{{ item }}"
state: present
loop:
- httpd
- memcached
- name: apply the foo config template
template:
src: templates/src.j2
dest: /etc/foo.conf
- name: start service bar and enable it
service:
name: bar
state: started
enabled: True
when: ansible_facts['distribution'] == 'CentOS'
become: true
become_user: root
ignore_errors: yes
In the example above, each of the 3 tasks will be executed after appending the `when` condition from the block
and evaluating it in the task's context. Also they inherit the privilege escalation directives enabling "become to root"
for all the enclosed tasks. Finally, ``ignore_errors: yes`` will continue executing the playbook even if some of the tasks fail.
Names for tasks within blocks have been available since Ansible 2.3. We recommend using names in all tasks, within blocks or elsewhere, for better visibility into the tasks being executed when you run the playbook.
.. _block_error_handling:
Blocks error handling
`````````````````````
Blocks also introduce the ability to handle errors in a way similar to exceptions in most programming languages.
Blocks only deal with 'failed' status of a task. A bad task definition or an unreachable host are not 'rescuable' errors.
.. _block_rescue:
.. code-block:: YAML
:emphasize-lines: 3,10
:caption: Block error handling example
tasks:
- name: Handle the error
block:
- debug:
msg: 'I execute normally'
- name: i force a failure
command: /bin/false
- debug:
msg: 'I never execute, due to the above task failing, :-('
rescue:
- debug:
msg: 'I caught an error, can do stuff here to fix it, :-)'
This will 'revert' the failed status of the task for the run and the play will continue as if it had succeeded.
There is also an ``always`` section, that will run no matter what the task status is.
.. _block_always:
.. code-block:: YAML
:emphasize-lines: 2,9
:caption: Block with always section
- name: Always do X
block:
- debug:
msg: 'I execute normally'
- name: i force a failure
command: /bin/false
- debug:
msg: 'I never execute :-('
always:
- debug:
msg: "This always executes, :-)"
They can be added all together to do complex error handling.
.. code-block:: YAML
:emphasize-lines: 2,9,16
:caption: Block with all sections
- name: Attempt and graceful roll back demo
block:
- debug:
msg: 'I execute normally'
- name: i force a failure
command: /bin/false
- debug:
msg: 'I never execute, due to the above task failing, :-('
rescue:
- debug:
msg: 'I caught an error'
- name: i force a failure in middle of recovery! >:-)
command: /bin/false
- debug:
msg: 'I also never execute :-('
always:
- debug:
msg: "This always executes"
The tasks in the ``block`` would execute normally, if there is any error the ``rescue`` section would get executed
with whatever you need to do to recover from the previous error.
The ``always`` section runs no matter what previous error did or did not occur in the ``block`` and ``rescue`` sections.
It should be noted that the play continues if a ``rescue`` section completes successfully as it 'erases' the error status (but not the reporting),
this means it won't trigger ``max_fail_percentage`` nor ``any_errors_fatal`` configurations but will appear in the playbook statistics.
Another example is how to run handlers after an error occurred :
.. code-block:: YAML
:emphasize-lines: 6,10
:caption: Block run handlers in error handling
tasks:
- name: Attempt and graceful roll back demo
block:
- debug:
msg: 'I execute normally'
notify: run me even after an error
- command: /bin/false
rescue:
- name: make sure all handlers run
meta: flush_handlers
handlers:
- name: run me even after an error
debug:
msg: 'This handler runs even on error'
.. versionadded:: 2.1
Ansible also provides a couple of variables for tasks in the ``rescue`` portion of a block:
ansible_failed_task
The task that returned 'failed' and triggered the rescue. For example, to get the name use ``ansible_failed_task.name``.
ansible_failed_result
The captured return result of the failed task that triggered the rescue. This would equate to having used this var in the ``register`` keyword.
.. seealso::
:doc:`playbooks`
An introduction to playbooks
:doc:`playbooks_reuse_roles`
Playbook organization by roles
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