@Marvin I had a worker running and killed it. Now I can't start a new worker because it fails

It seems you're facing an issue where a Prefect worker fails to restart after being killed. This is a known issue and can occur in various scenarios, particularly when using Dask with Prefect. Here are some common causes and potential solutions: 1. Dask Cluster State: If you're using a Dask-based environment, ensure that all worker nodes are properly shut down and restarted. Sometimes, the cluster state can become inconsistent if workers are killed abruptly. 2. Zombie Processes: Prefect tasks may be marked as "zombies" if they don't send heartbeats for a period. This can lead to unexpected behavior when restarting workers. Disabling heartbeats might help in some configurations. You can refer to the Prefect documentation on disabling heartbeats. 3. Resource Management: If you're using Kubernetes or another orchestrator, ensure that all resources (like pods) are properly cleaned up before attempting to start new workers. This can help avoid resource conflicts or scheduling issues. 4. Logs and Errors: Check the logs for any specific errors or assertions that might give more insight into why the worker fails to start. For example, an error related to the worker's status being

Status.created

instead of

Status.closed

can indicate issues with the shutdown process. For more detailed guidance, you might want to explore this GitHub issue related to Dask worker shutdown and restart problems. If these suggestions don't resolve your issue, providing more context about your environment (e.g., are you using Dask, Kubernetes, etc.) could help in troubleshooting further.

@Marvin Not seeing any dask or distributed references on the traceback. When doing

prefect worker start -p my-work-pool -l 4

I get this error:

16:43:49.173 | DEBUG   | prefect.workers.process.processworker aca1e820-0c09-45f3-bacc-2990e8620a31 - Discovered 0 scheduled_flow_runs
16:43:49.179 | INFO    | prefect.workers.process.processworker aca1e820-0c09-45f3-bacc-2990e8620a31 - Found 1 flow runs awaiting cancellation.
16:43:49.180 | DEBUG   | prefect.workers.process.processworker aca1e820-0c09-45f3-bacc-2990e8620a31 - Worker synchronized with the Prefect API server.
16:43:49.215 | WARNING | prefect.workers.process.processworker aca1e820-0c09-45f3-bacc-2990e8620a31 - Unable to kill process 26924: The process was not found. Marking flow run as cancelled.
16:43:49.216 | DEBUG   | prefect.workers.process.processworker aca1e820-0c09-45f3-bacc-2990e8620a31 - Exiting worker context...
16:43:49.216 | DEBUG   | prefect.workers.process.processworker aca1e820-0c09-45f3-bacc-2990e8620a31 - Tearing down worker...
Traceback (most recent call last):
  File "/Users/david/.virtualenvs/hc-refactor/lib/python3.10/site-packages/prefect/workers/process.py", line 185, in start
    async with anyio.create_task_group() as loops_task_group:
  File "/Users/david/.virtualenvs/hc-refactor/lib/python3.10/site-packages/anyio/_backends/_asyncio.py", line 771, in __aexit__
    raise exc_val
  File "/Users/david/.virtualenvs/hc-refactor/lib/python3.10/site-packages/anyio/_backends/_asyncio.py", line 743, in __aexit__
    await self._on_completed_fut
asyncio.exceptions.CancelledError: Cancelled by cancel scope 1066c3430

Your message was too long by 209 tokens...

@Marvin Not seeing any dask or distributed references on the traceback. When doing

prefect worker start -p my-work-pool -l 4

I get this error:

16:43:49.215 | WARNING | prefect.workers.process.processworker aca1e820-0c09-45f3-bacc-2990e8620a31 - Unable to kill process 26924: The process was not found. Marking flow run as cancelled.
16:43:49.216 | DEBUG   | prefect.workers.process.processworker aca1e820-0c09-45f3-bacc-2990e8620a31 - Exiting worker context...
16:43:49.216 | DEBUG   | prefect.workers.process.processworker aca1e820-0c09-45f3-bacc-2990e8620a31 - Tearing down worker...
Traceback (most recent call last):
  File "/Users/david/.virtualenvs/hc-refactor/lib/python3.10/site-packages/prefect/workers/process.py", line 185, in start
    async with anyio.create_task_group() as loops_task_group:
  File "/Users/david/.virtualenvs/hc-refactor/lib/python3.10/site-packages/anyio/_backends/_asyncio.py", line 771, in __aexit__
    raise exc_val
  File "/Users/david/.virtualenvs/hc-refactor/lib/python3.10/site-packages/anyio/_backends/_asyncio.py", line 743, in __aexit__
    await self._on_completed_fut
asyncio.exceptions.CancelledError: Cancelled by cancel scope 1066c3430

Your message was too long by 64 tokens...

@Marvin Not seeing any dask references on the traceback. When doing

prefect worker start -p my-work-pool -l 4

I get this error:

16:43:49.215 | WARNING | prefect.workers.process.processworker aca1e820-0c09-45f3-bacc-2990e8620a31 - Unable to kill process 26924: The process was not found. Marking flow run as cancelled.
16:43:49.216 | DEBUG   | prefect.workers.process.processworker aca1e820-0c09-45f3-bacc-2990e8620a31 - Exiting worker context...
16:43:49.216 | DEBUG   | prefect.workers.process.processworker aca1e820-0c09-45f3-bacc-2990e8620a31 - Tearing down worker...
Traceback (most recent call last):
  File "/Users/david/.virtualenvs/hc-refactor/lib/python3.10/site-packages/prefect/workers/process.py", line 185, in start
    async with anyio.create_task_group() as loops_task_group:
  File "/Users/david/.virtualenvs/hc-refactor/lib/python3.10/site-packages/anyio/_backends/_asyncio.py", line 771, in __aexit__
    raise exc_val
  File "/Users/david/.virtualenvs/hc-refactor/lib/python3.10/site-packages/anyio/_backends/_asyncio.py", line 743, in __aexit__
    await self._on_completed_fut
asyncio.exceptions.CancelledError: Cancelled by cancel scope 1066c3430

Your message was too long by 62 tokens...

@Marvin Not seeing any dask references on the traceback. When doing

prefect worker start -p my-work-pool -l 4

I get this error:

16:43:49.215 | WARNING | prefect.workers.process.processworker aca1e820-0c09-45f3-bacc-2990e8620a31 - Unable to kill process 26924: The process was not found. Marking flow run as cancelled.
16:43:49.216 | DEBUG   | prefect.workers.process.processworker aca1e820-0c09-45f3-bacc-2990e8620a31 - Exiting worker context...
16:43:49.216 | DEBUG   | prefect.workers.process.processworker aca1e820-0c09-45f3-bacc-2990e8620a31 - Tearing down worker...
asyncio.exceptions.CancelledError: Cancelled by cancel scope 1066c3430

The error you're encountering when starting a Prefect worker is related to the process cancellation mechanism. It seems the worker is trying to kill a process that it cannot find, leading to the flow run being marked as canceled. Here's what might be happening: 1. Process Not Found: The worker attempts to kill a process with a specific PID, but the process is no longer running. This can happen if the process was terminated outside of Prefect's control or if there's a mismatch in tracking the process ID. 2. Cancellation Scope: Prefect uses a cancellation scope to ensure it doesn't terminate the wrong infrastructure. If the scope doesn't match (e.g., due to overlapping PIDs on different machines), the worker may fail to cancel the intended process. 3. Infrastructure Mismatch: If the infrastructure associated with the flow run is no longer available or was already terminated, Prefect marks the run as canceled but cannot enforce the cancellation. To address this, you might consider: - Ensuring that any previous processes related to Prefect workers are fully terminated and not lingering in the background. - Checking your system's process management to confirm that Prefect has the necessary permissions to manage and terminate processes. - Reviewing any custom infrastructure configurations to ensure they align with Prefect's expectations for process management. For more details, you can refer to the Prefect documentation on cancellation. If the issue persists, further investigation into your specific environment setup might be necessary.