I'm using PubSubClient per this example to connect to AWS IoT. Specifically, I'm trying to use AWS IoT Fleet Provisioning, which involves subscribing and publishing to some special $aws/ topics.
The connection is being established correctly, and CloudWatch logs show successful Connect, Subscribe, Publish-In and Publish-Out events. However, whenever the MCU receives a message, it appears to drop the connection. What gives?
Debugging PubSubClient showed that the underlying WiFiClientSecure instance was losing connection due to the following error:
BR_ERR_TOO_LARGE: Incoming record is too large to be processed, or buffer is too small for the handshake message to send.
The message I was receiving was about 4 KB, so I had to add the following:
wifi_ = new WiFiClientSecure();
wifi_->setBufferSizes(4096, 512);
From there, it turned out PubSubClient had its own buffer, which I also had to raise:
mqtt_ = new PubSubClient(*wifi_);
mqtt_->setBufferSize(4096);
Making these changes allowed me to receive these messages from AWS IoT successfully.