Handle exceptions in Ansible Playbooks with block and rescue

Have you ever gotten to the end of your Ansible Playbook execution and found you needed to:

1. Rescue from errors or partial execution of your tasks
2. Capture a summary of the results per host for further revision

If you have Ansible Automation Platform (AAP), you can use the techniques I described in How to use workflow job templates in Ansible to handle #1, and you will need something like the set_stats module for #2 (to be able to persist variables between workflow nodes).

In this article, I will cover the block/rescue feature in Ansible. You can incorporate it into your playbooks, whether or not you have AAP. You can then take these playbooks and run then in your AAP instance.

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By learning this new technique, you can use the approach that suits you best.

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What is a block?

A block is a logical grouping of tasks within a playbook that can be executed as a single unit. This makes it easy to manage complex playbooks by breaking them down into smaller, more manageable parts.

You can use blocks to apply options to a group of tasks and avoid repeating code, like in this example from the documentation.

 tasks:
   - name: Install, configure, and start Apache
     block:
       - name: Install httpd and memcached
         ansible.builtin.yum:
           name:
           - httpd
           - memcached
           state: present

       - name: Apply the foo config template
         ansible.builtin.template:
           src: templates/src.j2
           dest: /etc/foo.conf

       - name: Start service bar and enable it
         ansible.builtin.service:
           name: bar
           state: started
           enabled: True
     when: ansible_facts['distribution'] == 'CentOS'
     become: true
     become_user: root
     ignore_errors: true

Notice that the keywords when, become, become_user, and ignore_errors are all applied to the block.

[ Write your first Ansible Playbook in this hands-on interactive lab. ]

How to use blocks and rescue in Ansible

Blocks and rescue work together to provide error-handling capabilities in Ansible. Use the rescue keyword in association with a block to define a set of tasks that will be executed if an error occurs in the block. You can use the rescue tasks to handle errors, log messages, or take other actions to recover from the error.

Here is a high-level example:

---
- hosts: <hosts>
  tasks:
    - block:
        - <task1>
        - <task2>
        - <task3>
      rescue:
        - <rescue_task1>
        - <rescue_task2>
        - <rescue_task3>
      always:
        - <always_task>

You define tasks under the block keyword, which could be as simple as invoking the ansible.builtin.ping module, or you could have a combination of multiple tasks and including/importing roles.

The associated rescue keyword is where the playbook execution will be sent, for each host, if anything fails along the block.

Finally, the always section executes for all nodes, no matter if they succeed or fail.

Some key ideas from this structure:

1. rescue and always are optional features, which I will use for the specific purpose of demonstrating this "recover and summary" logic.
2. When your playbook runs against a considerable number of hosts, handling the individual results becomes harder to track. This is how the ideas discussed in this article can help.

For the following example, the inventory file contains:

[nodes]
node1
node2
node3

Here is the playbook:

---
- name: Test block/rescue
  hosts: nodes
  gather_facts: false

  tasks:

    - name: Main block
      block:
        - name: Role 1
          ansible.builtin.include_role:
            name: role1

        - name: Role 2
          ansible.builtin.include_role:
            name: role2

        - name: Accumulate success
          ansible.builtin.set_fact:
            _result:
              host: "{{ inventory_hostname }}"
              status: "OK"
              interfaces: "{{ ansible_facts['interfaces'] }}"

      rescue:
        - name: Accumulate failure
          ansible.builtin.set_fact:
            _result:
              host: "{{ inventory_hostname }}"
              status: "FAIL"

      always:
        - name: Tasks that will always run after the main block
          block:
            - name: Collect results
              ansible.builtin.set_fact:
                _global_result: "{{ (_global_result | default([])) + [hostvars[item]['_result']] }}"
              loop: "{{ ansible_play_hosts }}"

            - name: Classify results
              ansible.builtin.set_fact:
                _result_ok: "{{ _global_result | selectattr('status', 'equalto', 'OK') | list }}"
                _result_fail: "{{ _global_result | selectattr('status', 'equalto', 'FAIL') | list }}"

            - name: Display results OK
              ansible.builtin.debug:
                msg: "{{ _result_ok }}"
              when: (_result_ok | length ) > 0

            - name: Display results FAIL
              ansible.builtin.debug:
                msg: "{{ _result_fail }}"
              when: (_result_fail | length ) > 0
          delegate_to: localhost
          run_once: true
...

Think about this playbook as an illustration of some logic that could be applied to a complex automation in the real world. Yes, you could run simpler actions to recover or issue a notification about the failure, but you want a summary of all results. Then you can use this summary in the always section to automate sending a notification by email or writing the individual results into a database.

Also, the variables starting with _ are my personal naming convention preference... there's no special meaning in Ansible for that.

1. For this example, the roles in the main block don't do anything special. They represent the actions that you would put in the main block, which could fail at different points. In this simplified example, if a node succeeds, there will be a list of interfaces in the _result variable. Otherwise, the status will be set to FAIL.
2. For each host the playbook is running on:
   1. If the actions proceed without errors, the task Accumulate success will execute.
   2. If the action fails in any of the roles, the flow goes to the rescue block for each host.
3. The always section collects the results saved in the variable _result. Here is a little breakdown of the logic:
   1. Up to this point, each host has a variable in its hostvars structure, either with a success or failed status information.
   2. In the Collect results task, which runs once and is delegated to localhost, it captures the individual results and adds them to the list _global_result.
   3. The loop is done using the Ansible magic variable ansible_play_hosts_all, which is a list of all hosts targeted by this playbook.
   4. Classify results does some filtering to create a list of all OK and failed results. You can use these in notifications, reports, or to send to a database (this example just displays them).

If you run this playbook and no node fails, there is no need for rescue, and the display should show that results are OK in all nodes:

PLAY [Test block/rescue] *******************************************************

TASK [Role 1] ******************************************************************

TASK [role1 : Execution of role 1] *********************************************
ok: [node1] => {
    "changed": false,
    "msg": "All assertions passed"
}
ok: [node2] => {
    "changed": false,
    "msg": "All assertions passed"
}
ok: [node3] => {
    "changed": false,
    "msg": "All assertions passed"
}

TASK [Role 2] ******************************************************************

TASK [role2 : Execution of role 2] *********************************************
ok: [node1]
ok: [node2]
ok: [node3]

TASK [role2 : Show network information] ****************************************
skipping: [node1]
skipping: [node2]
skipping: [node3]

TASK [Accumulate success] ******************************************************
ok: [node1]
ok: [node2]
ok: [node3]

TASK [Collect results] *********************************************************
ok: [node1 -> localhost] => (item=node1)
ok: [node1 -> localhost] => (item=node2)
ok: [node1 -> localhost] => (item=node3)

TASK [Classify results] ********************************************************
ok: [node1 -> localhost]

TASK [Display results OK] ******************************************************
ok: [node1 -> localhost] => {
    "msg": [
        {
            "host": "node1",
            "interfaces": [
                "lo",
                "enp7s0",
                "enp1s0"
            ],
            "status": "OK"
        },
        {
            "host": "node2",
            "interfaces": [
                "lo",
                "enp7s0",
                "enp1s0"
            ],
            "status": "OK"
        },
        {
            "host": "node3",
            "interfaces": [
                "enp7s0",
                "lo",
                "enp1s0"
            ],
            "status": "OK"
        }
    ]
}

TASK [Display results FAIL] ****************************************************
skipping: [node1]

PLAY RECAP *********************************************************************
node1   : ok=6    changed=0    unreachable=0    failed=0    skipped=2    rescued=0    ignored=0   
node2   : ok=3    changed=0    unreachable=0    failed=0    skipped=1    rescued=0    ignored=0   
node3   : ok=3    changed=0    unreachable=0    failed=0    skipped=1    rescued=0    ignored=0   

If you force a failure in some nodes, they will invoke the rescue section, and the summary will show the ones that succeeded and those that failed:

PLAY [Test block/rescue] *******************************************************

TASK [Role 1] ******************************************************************

TASK [role1 : Execution of role 1] *********************************************
ok: [node1] => {
    "changed": false,
    "msg": "All assertions passed"
}
fatal: [node2]: FAILED! => {
    "assertion": "inventory_hostname in nodes_ok",
    "changed": false,
    "evaluated_to": false,
    "msg": "Assertion failed"
}
fatal: [node3]: FAILED! => {
    "assertion": "inventory_hostname in nodes_ok",
    "changed": false,
    "evaluated_to": false,
    "msg": "Assertion failed"
}

TASK [Role 2] ******************************************************************

TASK [role2 : Execution of role 2] *********************************************
ok: [node1]

TASK [role2 : Show network information] ****************************************
skipping: [node1]

TASK [Accumulate success] ******************************************************
ok: [node1]

TASK [Accumulate failure] ******************************************************
ok: [node2]
ok: [node3]

TASK [Collect results] *********************************************************
ok: [node1 -> localhost] => (item=node1)
ok: [node1 -> localhost] => (item=node2)
ok: [node1 -> localhost] => (item=node3)

TASK [Classify results] ********************************************************
ok: [node1 -> localhost]

TASK [Display results OK] ******************************************************
ok: [node1 -> localhost] => {
    "msg": [
        {
            "host": "node1",
            "interfaces": [
                "enp7s0",
                "enp1s0",
                "lo"
            ],
            "status": "OK"
        }
    ]
}

TASK [Display results FAIL] ****************************************************
ok: [node1 -> localhost] => {
    "msg": [
        {
            "host": "node2",
            "status": "FAIL"
        },
        {
            "host": "node3",
            "status": "FAIL"
        }
    ]
}

PLAY RECAP *********************************************************************
node1   : ok=7    changed=0    unreachable=0    failed=0    skipped=1    rescued=0    ignored=0   
node2   : ok=1    changed=0    unreachable=0    failed=0    skipped=0    rescued=1    ignored=0   
node3   : ok=1    changed=0    unreachable=0    failed=0    skipped=0    rescued=1    ignored=0   

Notice that even though there were failures, at the end, the Ansible output counts them as rescued.

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Handle exceptions

I hope this article has given you some ideas about how to handle exceptions in your playbooks.

You can also think about what actions you want in your rescue section, such as displaying a message or performing some "undo" action, depending on what stage it reached before the failure.

Finally, you can execute the always section for each host or, as in my example, one time only.