Child Workflows

A workflow can start other workflows as children. The parent can wait for the child to complete or fire-and-forget it and track it independently.

Execute and await

execute_child_workflow starts a child workflow and waits for its result:

from datetime import timedelta
from cadence.workflow import execute_child_workflow

@registry.workflow()
class ParentWorkflow:
    @workflow.run
    async def run(self, order_id: str) -> str:
        result = await execute_child_workflow(
            "ProcessOrderWorkflow",   # child workflow type name
            str,                      # expected return type
            order_id,                 # argument to child
            task_list="order-workers",
            execution_start_to_close_timeout=timedelta(minutes=30),
        )
        return result

Start without immediately awaiting

start_child_workflow returns a ChildWorkflowFuture you can await later or signal before awaiting:

from cadence.workflow import start_child_workflow

@registry.workflow()
class ParentWorkflow:
    @workflow.run
    async def run(self) -> None:
        future = await start_child_workflow(
            "ChildWorkflow",
            str,
            task_list="child-workers",
            execution_start_to_close_timeout=timedelta(hours=1),
        )

        # Send a signal before awaiting
        await future.signal("start-processing")

        result = await future

ChildWorkflowFuture

Method / Property	Description
await future	Wait for the child to complete and return its result
future.workflow_id	The child's workflow ID
future.run_id	The child's run ID
future.cancel()	Request cancellation of the child
await future.signal(name, *args)	Send a signal to the child

Child workflow options

Option	Description
workflow_id	ID for the child execution (auto-generated if omitted)
task_list	Task list for the child worker
execution_start_to_close_timeout	Max total duration
task_start_to_close_timeout	Max time per decision task
retry_policy	Retry policy for the child
cron_schedule	Run the child as a recurring cron workflow
domain	Domain for the child (defaults to parent's domain)
parent_close_policy	What to do with the child when the parent closes
memo	Key-value metadata attached to the child execution

Parent close policy

parent_close_policy controls what happens to the child when the parent workflow closes (completes, fails, or is cancelled):

from cadence.api.v1 import workflow_pb2

future = await start_child_workflow(
    "ChildWorkflow",
    str,
    task_list="child-workers",
    execution_start_to_close_timeout=timedelta(hours=1),
    parent_close_policy=workflow_pb2.PARENT_CLOSE_POLICY_ABANDON,
)

Policy	Behavior
PARENT_CLOSE_POLICY_TERMINATE (default)	Terminate the child
PARENT_CLOSE_POLICY_ABANDON	Leave the child running
PARENT_CLOSE_POLICY_REQUEST_CANCEL	Request cancellation of the child