Maximum length MQTT payload

Question

Hello AWS team!

I'm using AWS IoT Core with an ESP32.

When using MQTT test client with this JSON everything works as expected: 
{
 "pattern1": { "name":"Hello", "phases": {"position": "1", "speed":"1", "accel" : "0.2", "delay":"0"}},
 "pattern2": { "name":"World", "phases": {"position": "1", "speed":"1", "accel" : "0.2", "delay":"0"}}
}

The problem is that I want to send a lot of objects in this JSON, probably 50 or more. When I test it with 3 objects it doesn't work
{
 "pattern1": { "name":"Smooth", "phases": {"position": "1", "speed":"1", "accel" : "0.2", "delay":"0"}},
 "pattern2": { "name":"Yanking", "phases": {"position": "1", "speed":"1", "accel" : "0.2", "delay":"0"}},
 "pattern3": { "name":"Yanking", "phases": {"position": "1", "speed":"1", "accel" : "0.2", "delay":"0"}}
}

I realized that if I reduce the number of words then I'm able to send it, like this: 
{
 "1": { "n":"S", "p": {"p": "1", "sp":"1", "a" : "0.2", "d":"0"}},
 "2": { "n":"Y", "p": {"p": "1", "s":"1", "a" : "0.2", "d":"0"}},
 "3": { "n":"Y", "p": {"p": "1", "s":"1", "a" : "0.2", "d":"0"}},
 "4": { "n":"Y"}
}

I read that the MQTT payload limit is 128kb, but those 3 objects are smaller than 128kb.

On the ESP32 side, the callback function is triggered as expected. But, when the payload is too big it is not triggering.

Is this an AWS problem related to being able to send those larger payloads?

The AWS region is sa-east-1

Thanks a lot in advance!

Greg_B · Answer

Hi juanGomez. Although IoT Core can handle messages up to 128KB, practical limits, in regards to RAM and buffer sizes on the device, are usually hit first.

Assuming you're using AWS IoT Embedded C SDK libraries, please check the buffer size you are passing into `MQTT_Init()`:

https://aws.github.io/aws-iot-device-sdk-embedded-C/202012.00/libraries/standard/coreMQTT/docs/doxygen/output/html/mqtt_init_function.html.

If your message is bigger than that buffer size, you will have a problem. Although your failed message doesn't look very big.

Are you calling `MQTT_Publish()` to publish? Are you checking the return value?

https://aws.github.io/aws-iot-device-sdk-embedded-C/202012.00/libraries/standard/coreMQTT/docs/doxygen/output/html/mqtt_publish_function.html.

This call may be failing. Hence the message is perhaps never sent, and that may be why you don't get the callback.

On the cloud side, you can enable AWS IoT logging in CloudWatch to get greater visibility:

https://docs.aws.amazon.com/iot/latest/developerguide/configure-logging.html

Darren R · Answer

You are correct about the max payload of an MQTT message being 128KB, and if you were to run up against this limit, you would receive a CLIENT_ERROR with reason of PAYLOAD_LIMIT_EXCEEDED (see references below).

https://docs.aws.amazon.com/iot/latest/developerguide/topicdata.html
https://docs.aws.amazon.com/general/latest/gr/iot-core.html#message-broker-limits

I am able to successfully publish the 2nd example you've given above...Are you receiving a payload limit exception error? I suspect there is some other reason you are running into this issue.

Please accept answer if at all helpful!

rePost-User-2143012 · Answer

Hi,
    I am facing problem to build HTTPS API for AWS IoT core. and Facing  problem with GPRS through Arduino. How can I solve this?

juanGomez · Answer

Thanks a lot for the help.

Problem was that MQTT_MAX_PACKET_SIZE was set to 256 bytes.

I changed it to 65k bytes using setBufferSize() function and it solved the issue.

Thanks a lot

Maximum length MQTT payload

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