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In most workflow steps, when an operation is initiated, it completes and returns a result within a few seconds. However, if you have (for example) a longer-lived operation that is triggered through a HTTP request, then holding the HTTP connection open for the duration is fragile, and could result in missing the response without the ability to recover.

To handle this scenario, Workflow Orchestrator supports callbacks.

Using callbacks

When you wish to use a callback, Orchestrator supplies a callback URL that should be passed to the remote service. When the operation is triggered, the step immediately completes.

The workflow then pauses until the remote service performs the callback. At that point, the callback URL is disabled (so further triggers will not result in a repeat), a validation step is performed, and the result is provided to the UI.

If the validation step fails, then the step fails and execution of the workflow is stopped. If the validation step succeeds, then execution of the workflow is paused, and the operator is prompted for confirmation before it continues.

Writing a callback

The only immediate difference between a regular step and one that uses a callback is the addition of the callback_route parameter. This will be populated by orchestrator.

Here is an example that makes a HTTP request to a service that executes an Ansible playbook:

from orchestrator import step

@step("Execute an ansible playbook")
def call_ansible_playbook(
    subscription: L2vpnProvisioning,
    callback_route: str,
    dry_run: bool,
) -> None:
    inventory = f"{port_A.node.node_name}\n{port_B.node.node_name}"
    port_A = subscription.virtual_circuit.saps[0].port
    port_B = subscription.virtual_circuit.saps[1].port

    extra_vars = {
        "vlan": subscription.virtual_circuit.saps[0].vlan,
        "SiteA": f"{port_A.node.node_name}",
        "interfaceA": port_A.port_name,
        "SiteB": f"{port_B.node.node_name}",
        "interfaceB": port_B.port_name,

    callback_url = f"http://orchestrator{callback_route}"

    parameters = {
        "playbook_name": "playbook.yml",
        "inventory": inventory,
        "extra_vars": extra_vars,
        "callback": callback_url,

    url = f"http://ansible-proxy/api/playbook/"
    request =, json=parameters, timeout=10)

However, this step is not included directly in the step list. To supplement it, we will need an additional function which also provides the validation step.

from orchestrator.workflow import Step, callback_step

def callback_interaction(provisioning_step: Step) -> StepList:
    return (
        >> callback_step(
        >> _show_callback_results

We also have to provide the evaluation function, and the confirmation step.

The remote service, when performing the callback, should provide a JSON payload containing the fields that will be evaluated and output in the UI. In the above example, the service responds with this payload:

    "job_id": "example", // UUID
    "output": "example", // From ansible_playbook_run.stdout.readlines()
    "return_code": 0,    // From int(ansible_playbook_run.rc)
    "status": "example"  // From ansible_playbook_run.status

This allows us, in this example, to use the return code from Ansible to make the pass/fail decision on the step:

from orchestrator import step
from orchestrator.utils.errors import ProcessFailureError

@step("Evaluate callback result")
def _evaluate_callback_results(callback_result: dict) -> State:
    if callback_result["return_code"] != 0:
        raise ProcessFailureError(message="Callback failure", details=callback_result)

    return {"callback_result": callback_result}

Then we also need the step that presents the results to the operator and requests confirmation before proceeding:

from orchestrator.config.assignee import Assignee
from orchestrator.forms import FormPage
from orchestrator.workflow import Step, inputstep
from pydantic_forms.validators import LongText

@inputstep("Confirm provisioning proxy results", assignee=Assignee("SYSTEM"))
def _show_callback_results(state: State) -> FormGenerator:
    if "callback_result" not in state:
        return state

    class ConfirmRunPage(FormPage):
        class Config:
            title: str = (
                f"Execution for {state['subscription']['product']['name']} completed."

        run_status: str = state["callback_result"]["status"]
        run_results: LongText = json.dumps(state["callback_result"], indent=4)

    yield ConfirmRunPage
    return state

Finally, we wire this all up in our StepList. Instead of including the step directly, provide the step as a parameter to the interaction function:

from orchestrator.types import SubscriptionLifecycle
from orchestrator.workflows.utils import create_workflow

@create_workflow("Example workflow", initial_input_form=initial_input_form_generator)
def create_l2vpn() -> StepList:
    return (
        >> construct_model
        >> store_process_subscription(Target.CREATE)
        >> callback_interaction(call_ansible_playbook)
        >> set_status(SubscriptionLifecycle.ACTIVE)