An IoT device goes offline. A backend service wants to read its current temperature. The device isn’t there to respond. A device shadow (also called digital twin) solves this: a server-side representation of the device’s last-known state that can be read and written to even when the device is offline. When the device reconnects, it synchronizes with the shadow. The Problem IoT devices are intermittently connected. Cellular, wifi, and industrial networks all have gaps.

A factory floor has 10,000 sensors each sending readings every second. Sending all 10,000 readings per second to the cloud costs bandwidth money and creates a massive ingestion problem. Most readings are uninteresting: the temperature was 23.4C last second, it’s 23.4C this second. Edge processing filters, aggregates, and transforms data before it reaches the cloud, pushing computation closer to where data originates. What to Do at the Edge Filtering: only forward readings that cross a threshold or change by more than a delta.

A temperature sensor in a factory sends a reading every 30 seconds. It runs on a microcontroller with 256KB of RAM and a 2G cellular connection. It can’t maintain a persistent HTTP connection. It can’t handle TLS handshakes with 10KB certificates. IoT devices require a protocol built for constrained environments. MQTT is that protocol, and device registration is the first problem you solve before any data flows. MQTT Basics MQTT is a publish/subscribe protocol over TCP, designed for low-bandwidth, high-latency, unreliable networks.