Can´t use a MQTT message payload greater than 63KB with AWS IoT Device SDK for Python

When working with MQTT messages in AWS IoT Greengrass, there are several limitations to be aware of regarding message payload size.

While AWS IoT Core has a message size limit of 128 KB, AWS IoT MQTT-based file delivery specifically limits the size of a block to 128 KB. If you make a request for a block that exceeds this size, the request will fail.

In the Greengrass environment, there are additional considerations. When publishing MQTT messages from Greengrass devices to AWS IoT Core, there are quotas of 100 transactions per second and 512 KB of data per second. Messages that exceed these quotas are queued in a spooler in memory.

The behavior you're experiencing where messages larger than 63 KB get stuck might be related to how the Greengrass MQTT bridge handles message payloads. The bridge component has its own limitations that may differ from the direct AWS IoT Core limits.

For applications that need to send larger messages or messages at a higher rate, AWS recommends using the Stream manager component instead of direct MQTT publishing. Stream manager is specifically designed to transfer high-volume data to the AWS Cloud, such as through Kinesis Data Streams.

If you absolutely need to send larger payloads, you might need to implement a chunking mechanism to break your data into smaller pieces, or consider using the Stream manager component as an alternative approach.
Sources
MQTT message payload - AWS IoT Core
Publish/subscribe AWS IoT Core MQTT messages - AWS IoT Greengrass
Using AWS IoT MQTT-based file delivery in devices - AWS IoT Core

Hi, the mqtt.maxMessageSizeInBytes configuration can be used to set the max limit for MQTT messages, but the default value is 128 KB so that should not be the issue here unless your core has it configured to a lower value. I would recommend opening an issue on the https://github.com/aws/aws-iot-device-sdk-python-v2/ repo since it could be a bug there.

If you could grab the logs for your component, that would be helpful (they are likely in /greengrass/v2/logs).

Sure. This is the content extracted with sudo tail -f /greengrass/v2/logs/greengrass.log:

2025-06-25T17:38:44.784Z [INFO] (pool-3-thread-17) com.aws.greengrass.mqtt.bridge.clients.MQTTClient: Connected to broker. {clientId=mqtt-bridge-2puf2f3ahsk, brokerUri=ssl://localhost:8883}
2025-06-25T17:38:44.789Z [INFO] (nioEventLoopGroup-3-1) io.moquette.broker.metrics.MQTTMessageLogger: C->B SUBSCRIBE <mqtt-bridge-2puf2f3ahsk> to topics [MqttTopicSubscription[topicFilter=clients/+/hello/world, option=SubscriptionOption[qos=AT_LEAST_ONCE, noLocal=false, retainAsPublished=false, retainHandling=SEND_AT_SUBSCRIBE]]]. {}
2025-06-25T17:45:12.799Z [INFO] (nioEventLoopGroup-3-2) io.moquette.broker.metrics.MQTTMessageLogger: C->B CONNECT <null>. {}
2025-06-25T17:45:13.067Z [INFO] (nioEventLoopGroup-3-2) com.aws.greengrass.mqtt.moquette.ClientDeviceAuthorizer: Successfully authenticated client device. {clientId=Local_IoT_device, sessionId=93ff2b84-06dc-4588-aeed-bd1ca59da079}
2025-06-25T17:45:13.077Z [INFO] (nioEventLoopGroup-3-2) io.moquette.broker.metrics.MQTTMessageLogger: C->B SUBSCRIBE <Local_IoT_device> to topics [MqttTopicSubscription[topicFilter=clients/Local_IoT_device/hello/world, option=SubscriptionOption[qos=AT_MOST_ONCE, noLocal=false, retainAsPublished=false, retainHandling=SEND_AT_SUBSCRIBE]]]. {}
2025-06-25T17:45:23.639Z [INFO] (nioEventLoopGroup-3-2) io.moquette.broker.metrics.MQTTMessageLogger: C->B DISCONNECT <Local_IoT_device>. {}
2025-06-25T17:45:23.645Z [INFO] (nioEventLoopGroup-3-2) io.moquette.broker.metrics.MQTTMessageLogger: Channel Inactive. {}
2025-06-25T17:50:37.684Z [INFO] (nioEventLoopGroup-3-3) io.moquette.broker.metrics.MQTTMessageLogger: C->B CONNECT <null>. {}
2025-06-25T17:50:37.986Z [INFO] (nioEventLoopGroup-3-3) com.aws.greengrass.mqtt.moquette.ClientDeviceAuthorizer: Successfully authenticated client device. {clientId=Local_IoT_device, sessionId=7838c109-7325-46ab-8176-67cb1e750ef9}
2025-06-25T17:50:37.997Z [INFO] (nioEventLoopGroup-3-3) io.moquette.broker.metrics.MQTTMessageLogger: C->B SUBSCRIBE <Local_IoT_device> to topics [MqttTopicSubscription[topicFilter=clients/Local_IoT_device/hello/world, option=SubscriptionOption[qos=AT_MOST_ONCE, noLocal=false, retainAsPublished=false, retainHandling=SEND_AT_SUBSCRIBE]]]. {}
2025-06-25T17:51:19.660Z [INFO] (nioEventLoopGroup-3-3) io.moquette.broker.metrics.MQTTMessageLogger: Channel Inactive. {}

Remember that this is the log right after a failed execution with a 64KB payload which, of course, got stucked until I canceled the execution with Ctrl + C.