Error Handling In Playbooks

• Ignoring Failed Commands
• Resetting Unreachable Hosts
• Handlers and Failure
• Controlling What Defines Failure
• Overriding The Changed Result
• Aborting the play

Ansible normally has defaults that make sure to check the return codes of commands and modules and it fails fast – forcing an error to be dealt with unless you decide otherwise.

Sometimes a command that returns different than 0 isn’t an error. Sometimes a command might not always need to report that it ‘changed’ the remote system. This section describes how to change the default behavior of Ansible for certain tasks so output and error handling behavior is as desired.

Ignoring Failed Commands

Generally playbooks will stop executing any more steps on a host that has a task fail. Sometimes, though, you want to continue on. To do so, write a task that looks like this:

- name: this will not be counted as a failure
  command: /bin/false
  ignore_errors: yes

Note that the above system only governs the return value of failure of the particular task, so if you have an undefined variable used or a syntax error, it will still raise an error that users will need to address. Note that this will not prevent failures on connection or execution issues. This feature only works when the task must be able to run and return a value of ‘failed’.

Resetting Unreachable Hosts

Connection failures set hosts as ‘UNREACHABLE’, which will remove them from the list of active hosts for the run. To recover from these issues you can use meta: clear_host_errors to have all currently flagged hosts reactivated, so subsequent tasks can try to use them again.

Handlers and Failure

When a task fails on a host, handlers which were previously notified will not be run on that host. This can lead to cases where an unrelated failure can leave a host in an unexpected state. For example, a task could update a configuration file and notify a handler to restart some service. If a task later on in the same play fails, the service will not be restarted despite the configuration change.

You can change this behavior with the --force-handlers command-line option, or by including force_handlers: True in a play, or force_handlers = True in ansible.cfg. When handlers are forced, they will run when notified even if a task fails on that host. (Note that certain errors could still prevent the handler from running, such as a host becoming unreachable.)

Controlling What Defines Failure

Suppose the error code of a command is meaningless and to tell if there is a failure what really matters is the output of the command, for instance if the string “FAILED” is in the output.

Ansible in 1.4 and later provides a way to specify this behavior as follows:

- name: Fail task when the command error output prints FAILED
  command: /usr/bin/example-command -x -y -z
  register: command_result
  failed_when: "'FAILED' in command_result.stderr"

or based on the return code:

- name: Fail task when both files are identical
  raw: diff foo/file1 bar/file2
  register: diff_cmd
  failed_when: diff_cmd.rc == 0 or diff_cmd.rc >= 2

In previous version of Ansible, this can be still be accomplished as follows:

- name: this command prints FAILED when it fails
  command: /usr/bin/example-command -x -y -z
  register: command_result
  ignore_errors: True

- name: fail the play if the previous command did not succeed
  fail: msg="the command failed"
  when: "'FAILED' in command_result.stderr"

Overriding The Changed Result

When a shell/command or other module runs it will typically report “changed” status based on whether it thinks it affected machine state.

Sometimes you will know, based on the return code or output that it did not make any changes, and wish to override the “changed” result such that it does not appear in report output or does not cause handlers to fire:

tasks:

  - shell: /usr/bin/billybass --mode="take me to the river"
    register: bass_result
    changed_when: "bass_result.rc != 2"

  # this will never report 'changed' status
  - shell: wall 'beep'
    changed_when: False

Aborting the play

Sometimes it’s desirable to abort the entire play on failure, not just skip remaining tasks for a host.

The any_errors_fatal play option will mark all hosts as failed if any fails, causing an immediate abort:

- hosts: somehosts
  any_errors_fatal: true
  roles:
    - myrole

for finer-grained control max_fail_percentage can be used to abort the run after a given percentage of hosts has failed.