How can I find out why my client is being throttled when subscribing in mqtt?

Question

I have recently begun to see subscribe requests fail with reason "throttled" in the aws iot logs in cloudwatch.

I'm unsure what is causing this or why and need to find out immediately!

How can I learn what the cause for throttling is so I can quickly resolve it?

On that note it would be nice if AWS returned the specific reason instead of the generic "throttled" in the logs!

Example log:
```
{
    "timestamp": "2023-08-08 22:43:33.727",
    "logLevel": "ERROR",
    "traceId": "X",
    "accountId": "X",
    "status": "Failure",
    "eventType": "Subscribe",
    "protocol": "MQTT",
    "topicName": "users/X/status",
    "clientId": "",
    "principalId": "USER",
    "sourceIp": "X",
    "sourcePort": X,
    "reason": "throttled"
}
```

Greg · Answer

Hi borg. Is it intermittent? I see from your question history that this is probably a chat application? How many clients?

Please also check your CloudWatch metrics. The `.Throttle` metrics will only exist if you have indeed exceeded that particular quota at some point. Description of metrics here: https://docs.aws.amazon.com/iot/latest/developerguide/metrics_dimensions.html#message-broker-metrics

From the error you've shared, I'm not certain which quota you've exceeded, but please be aware of these limits: https://docs.aws.amazon.com/general/latest/gr/iot-core.html#message-broker-limits

![Enter image description here](/media/postImages/original/IMj9LYVpgWSnKHtl56HeFS7w)

![Enter image description here](/media/postImages/original/IMNpuy5k7zSSCjhKquW_Os2A)

And there are other subscription limits if you use shared subscriptions at all.

How can I find out why my client is being throttled when subscribing in mqtt?

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