The Delivery Engine is where a stored command becomes an actual MQTT message. It pulls the next command for a device, publishes it on the command topic, starts an ack timer, and decides whether the result is success, retry or failure.
At-least-once, made safe
Delivery is at-least-once, so a device can legitimately receive the same command twice (e.g. ack lost on the way back). Commands carry a stable id and devices are expected to de-duplicate on it.
failed.DELIVERY_ATTEMPT in the
Command Repository,
and command bodies are encoded with Protobuf to keep wire frames compact
for constrained devices.