cylc.flow.exceptions.CylcError: could not start Cylc ZMQ server: Could not bind socket to random port

Rose/Cylc and FCM
1

Dear Helpdesk,

I have been running JULES on JASMIN. Everything works fine until today. I did not make any change but this error started to occur but not for every run. Sometimes I re-trigger the run and the error goes away Could you please take a look. Thanks a lot for your help.

Exception in thread Thread-1 (start):
Traceback (most recent call last):
File “/apps/jasmin/metomi/cylc-8.6.0-1/lib/python3.12/threading.py”, line 1075, in _bootstrap_inner
self.run()
File “/apps/jasmin/metomi/cylc-8.6.0-1/lib/python3.12/threading.py”, line 1012, in run
self._target(*self.args, **self.kwargs)
File “/apps/jasmin/metomi/cylc-8.6.0-1/lib/python3.12/site-packages/cylc/flow/network/server.py”, line 206, in start
self.replier.start(min, max)
File “/apps/jasmin/metomi/cylc-8.6.0-1/lib/python3.12/site-packages/cylc/flow/network/init.py”, line 188, in start
self._socket_bind(*args, **kwargs)
File “/apps/jasmin/metomi/cylc-8.6.0-1/lib/python3.12/site-packages/cylc/flow/network/init.py”, line 250, in _socket_bind
raise CylcError(
cylc.flow.exceptions.CylcError: could not start Cylc ZMQ server: Could not bind socket to random port.
2025-10-31T16:07:15Z ERROR -
Traceback (most recent call last):
File “/apps/jasmin/metomi/cylc-8.6.0-1/lib/python3.12/site-packages/cylc/flow/scheduler_cli.py”, line 719, in cylc_play
asyncio.get_running_loop()
RuntimeError: no running event loop
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File “/apps/jasmin/metomi/cylc-8.6.0-1/lib/python3.12/site-packages/cylc/flow/scheduler.py”, line 791, in start
barrier.wait()
File “/apps/jasmin/metomi/cylc-8.6.0-1/lib/python3.12/threading.py”, line 725, in wait
self._wait(timeout)
File “/apps/jasmin/metomi/cylc-8.6.0-1/lib/python3.12/threading.py”, line 763, in _wait
raise BrokenBarrierError
threading.BrokenBarrierError
2025-10-31T16:07:15Z CRITICAL - Workflow shutting down
2025-10-31T16:07:15Z INFO - DONE

Best,

Wenyao

2

Hi Wenyao

I’m getting the same errors myself. I think it’s a JASMIN problem. I was composing a message to the jasmin_general Slack channel when your message arrived. Since you describe the problem, I’ll post a link to this thread instead.

David

3

Dear David,

Thanks. Glad to know it’s not just on my end. Hopefully JASMIN can sort it out soon.

Best,

Wenyao