Messages published over MQTT from an IoT Greengrass device arrive out-of-order

Out-of-order MQTT messages should be expected and solutions should be designed with this principle in mind. Messages published with QoS1 retries from the client when ACK is not received is a common way that messages get out of order. Many things can cause the ACK to not be received - Internet down, power outage, network congestion.

Strategies for handling out-of-order messages include use of sequencing numbers generated by the client and ignoring messages received with a timestamp older than the last processed message. For more guidance, consult the IoT Lens for AWS Well-Architected. https://docs.aws.amazon.com/wellarchitected/latest/iot-lens/welcome.html