[IoT Core] Duplicate messages from topic forwarding rule

Question

Hi,

I have created a topic forwarding rule in AWS IoT Core using this rule.

SELECT * from 'data/device01' WHERE temperature > 35

This rule forwards messages to the topic 'data/alarms'. I used QoS 0.

I observed duplicate messages like this from the subscription of 'data/alarms'

data/alarms
    August 05, 2020, 14:36:01 (UTC+0900)
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    {
      "device": "device02",
      "temperature": 35.02,
      "brightness": 12,
      "humidity": 87,
      "time": "2020.08.05 07:36:01"
    }
    data/alarms
    August 05, 2020, 14:36:01 (UTC+0900)
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    Hide
    {
      "device": "device02",
      "temperature": 35.02,
      "brightness": 12,
      "humidity": 87,
      "time": "2020.08.05 07:36:01"
    }

Interesting part is where sometimes there are duplicates and sometimes not.

**Question**: Is this expected behavior of IoT Core?

Thanks,

Accepted Answer

The AWS IoT Core message broker defines MQTT Quality of Service level zero like so:

> In AWS IoT Core, subscribing to a topic with QoS level 0 means that a
> message is delivered zero or more times. A message might be delivered
> more than once. Messages delivered more than once might be sent with a
> different packet ID.

https://docs.aws.amazon.com/iot/latest/developerguide/mqtt.html

[IoT Core] Duplicate messages from topic forwarding rule

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