Same message is getting published to subscriber multiple times even after success acknowledgment

[Tinshuk Singh]

Hi Team, 

I'm struggling with a behavior of pub/sub, which is, the same message is getting published to subscriber multiple times.

Please refer below table to understand the issue:

#	Msg Id	Received at Subscriber		Response Returned after processing	Status
1	12345	2020-05-05 20:38:02.246 IST		2020-05-05 20:38:37.677 IST	Failed
2	12345	2020-05-05 20:38:38.685 IST		2020-05-05 20:39:32.417 IST	Success
3	12345	2020-05-05 20:38:39.359 IST		2020-05-05 20:39:52.151 IST	Success

GCP Subscriber properties (PUSH):

Acknowledgement deadline : 300 Seconds

Message retention duration : 7 days

Subscriber : Java API (running on kubernetes with 3 instances )

#1, a message received from topic to subscriber which got failed due to some system failure

#2, as the message was failed subscriber tried to process the same message again which was successful

#3, the same message was received again to the subscriber and was processed successfully again.

As per my understanding, if the message was delivered at mentioned time at #2, it should not send the same message again at #3.

As per my use case, this subscriber is not expected to process the same message multiple times. Please let me know if this is something which can be handled.

[Kamal Aboul-Hosn]

Hi, Tinshuk,

Cloud Pub/Sub guarantees at-least-once delivery. That means that even if a message is processed by a subscriber and acknowledged, it is still possible that the message will be redelivered. It is a relatively rare occurrence (for a well-behaved subscriber, it tends to happen less than 0.5% of the time), but the subscriber still needs to be handle redelivery of messages. 

Kamal

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[Ondřej Fuchs]

Hi Kamal,

I have the same problem in Google PubSub with duplicity of messages. But I got then more then 0,5%. Thanks for any advice.

I have one topic and one subscriber. I publish one message per ten minutes and I got one duplicity message every two hours. Duplicity messages hava the same message_ID. I also check nonACK messages in Google metrics and I didn't find any = Every message was ACK successfully. I also thought about ACK deadline, but process of one message < 2s and I set ack deadline to 20s. 

Here is piece of logs:

INFO:__main__:ID: 1181259733624335, Pubtime: 2020-05-05 16:24:58.357000+00:00, Received message: Message {} at time 2020-05-05 16:24:59.314107
INFO:__main__:MSG was ack at 2020-05-05 16:24:59.829352

INFO:__main__:ID: 1181259733624335, Pubtime: 2020-05-05 16:24:58.357000+00:00, Received message: Message {} at time 2020-05-05 16:25:10.725601
INFO:__main__:MSG was ack at 2020-05-05 16:25:11.195541

I know that google Cloud Pub/Sub guarantees at-least-once delivery, but this is too much I guess. 

Again thanks for any advice.

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[Kamal Aboul-Hosn]

Hi, Ondřej,

Take a look at an answer I posted on Stack Overflow: https://stackoverflow.com/a/58001511/1121288. It talks about some causes of duplicates. You may need to reach out to support with your subscription information for further assistance.

Kamal

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[Tinshuk Singh]

Thanks Kamal for the reply. Just to confirm, is this behavior applicable for "Pull" subscriber as well?

Tinshuk.

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[Kamal Aboul-Hosn]

Yes, it is generally applicable for pull subscribers as well. The point about large backlogs and streaming pull subscribers doesn't apply if you are using the synchronous Pull API.

Kamal

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