Device shadow
A cloud-side JSON document that stores the last reported and desired state of a device so applications can interact with it even when it is offline.
A device shadow decouples applications from device connectivity. The device reports its state (“valve: open”) to the shadow whenever it connects; an application writes a desired state (“valve: closed”) to the same document. When the device next wakes up it reads the difference and acts on it, then reports back. The term comes from AWS IoT Core; Azure calls the same idea a device twin.
Shadows are especially useful for battery-powered devices that spend most of their time in deep sleep and for mobile assets with patchy coverage. They are simpler than a full digital twin, which models behavior and relationships rather than just a state document.
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