Google Cloud Function pulling from Pub/Sub subscription throws exception - Deadline Exceeded

Question

I have a Google Cloud Function in Python 3.7 reading from a Pub/Sub subscription in synchronous pull mode.

After running fine 1/hour for 24 hours, it threw this exception stack trace:

Traceback (most recent call last): File
"/env/local/lib/python3.7/site-packages/google/api_core/grpc_helpers.py",
line 57, in error_remapped_callable
return callable_(*args, **kwargs) File "/env/local/lib/python3.7/site-packages/grpc/_channel.py", line 824,
in call
return _end_unary_response_blocking(state, call, False, None) File "/env/local/lib/python3.7/site-packages/grpc/_channel.py", line
726, in _end_unary_response_blocking
raise _InactiveRpcError(state) grpc._channel._InactiveRpcError: <_InactiveRpcError of RPC that terminated with: status =
StatusCode.DEADLINE_EXCEEDED details = "Deadline Exceeded"
debug_error_string =
"{"created":"@1580454091.145703535","description":"Error received from
peer
ipv4:74.125.202.95:443","file":"src/core/lib/surface/call.cc","file_line":1056,"grpc_message":"Deadline
Exceeded","grpc_status":4}"

The above exception was the direct cause of the following exception:

Traceback (most recent call last): File
"/env/local/lib/python3.7/site-packages/google/cloud/functions/worker.py",
line 346, in run_http_function
result = _function_handler.invoke_user_function(flask.request) File
"/env/local/lib/python3.7/site-packages/google/cloud/functions/worker.py",
line 217, in invoke_user_function
return call_user_function(request_or_event) File "/env/local/lib/python3.7/site-packages/google/cloud/functions/worker.py",
line 210, in call_user_function
return self._user_function(request_or_event) File "/user_code/main.py", line 39, in iteration
response = sub.pull(sub_path, MAX_MESSAGES) File "/env/local/lib/python3.7/site-packages/google/cloud/pubsub_v1/_gapic.py",
line 40, in
fx = lambda self, *a, **kw: wrapped_fx(self.api, *a, **kw) # noqa File
"/env/local/lib/python3.7/site-packages/google/cloud/pubsub_v1/gapic/subscriber_client.py",
line 1005, in pull
request, retry=retry, timeout=timeout, metadata=metadata File "/env/local/lib/python3.7/site-packages/google/api_core/gapic_v1/method.py",
line 143, in call
return wrapped_func(*args, **kwargs) File "/env/local/lib/python3.7/site-packages/google/api_core/retry.py",
line 286, in retry_wrapped_func
on_error=on_error, File "/env/local/lib/python3.7/site-packages/google/api_core/retry.py",
line 184, in retry_target
return target() File "/env/local/lib/python3.7/site-packages/google/api_core/timeout.py",
line 214, in func_with_timeout
return func(*args, **kwargs) File "/env/local/lib/python3.7/site-packages/google/api_core/grpc_helpers.py",
line 59, in error_remapped_callable
six.raise_from(exceptions.from_grpc_error(exc), exc) File "", line 3, in raise_from
google.api_core.exceptions.DeadlineExceeded: 504 Deadline Exceeded

What is this about? Is it to be expected or a result of some configuration problem? If to be expected, how should it be handled?

The documentation ( view-source:https://googleapis.dev/python/pubsub/latest/subscriber/api/client.html ) on pull has nothing about this being a possible exception.

I ack the messages immediately after the pull completes. I only permit one function execution at a time. I have a 600 second acknowledgement deadline. A block of messages pulled at one time seem to be less than 100 in number. If this is about failing to ack a message, it seems like the error could be done much better.

It seems to mean that there were no more messages to pull. As a workaround I suggest to catch the exception and retry the operation — Tlaquetzal
Hmm, that seems strange. The documentation says the pull may return zero messages, so I wouldn't expect an exception in that case. — Kevin Buchs
Yes, it seems to be an issue in the PubSub library. Apparently in the version v0.45.0 the error does not occur. My guess is that the issue is related to the behavior of return_immediately. This flag is being deprecated, and the behavior is set to the equivalent of return_immediately=False, so the client just keeps the connection alive while waiting for new messages, until the underlying grpc deadline raises the exception — Tlaquetzal
Ah, gotcha. I thought about using return immediately, because I just want to pull a batch of everything available at the current time, but I have actually left it unspecified, which should be the =False scenario. But maybe I should just be setting the timeout, as I was letting that default. It is hard, because the Python docs really leave a lot to guesswork. — Kevin Buchs

Tlaquetzal Tlaquetzal · Accepted Answer · 2020-02-03T21:24:21

This exception is raised by the client when there's no messages to read in the subscription. It is a known issue from the latest PubSub library versions >= 1.0.0. If necessary, you can downgrade to the version 0.45.0 where this issue was not present.

However, as a workaround you can catch the DeadlineExceeded exception and retry the operation again. Also, based on the comment of Hemang, here's a small monkeypatch that you can add to your running code, which might help to get the same behavior as in version 0.45.0.

from google.cloud.pubsub_v1.gapic import subscriber_client_config as sub_config
sub_config.config['interfaces']['google.pubsub.v1.Subscriber']['retry_params']['messaging']['initial_rpc_timeout_millis'] = 25000

Finally, keep in mind that when using synchronous pull, having many outstanding pull requests helps lower the delivery latency, which in turn might result in higher latency pull requests (and DeadlineExceeded errors). Although, if latency is crucial for the application, you could consider using StreamingPull

Google Cloud Function pulling from Pub/Sub subscription throws exception - Deadline Exceeded

1 Answers