Representing devices remotely.
WHAT A SHADOW IS
A stored representation of a device's state, available even when the device is not.
WHAT IT CONTAINS
Reported state: what the device last said
Desired state: what it should be
The difference between them
WHY IT MATTERS
Applications can read and set state without the device being connected.
WHAT RECONCILIATION IS
The device retrieving the desired state on connection and applying it.
WHAT TO REPORT AFTER APPLYING
The new reported state, confirming.
WHAT DIVERGENCE INDICATES
A device unable or unwilling to apply the configuration.
WHAT TO ALERT ON
Divergence persisting beyond an expected period.
WHAT TO BE CAREFUL WITH
Conflicting desired states set by several sources Configuration changes applied before the device is ready Large state documents on constrained links
WHAT TO KEEP IN THE SHADOW
Configuration and status.
WHAT NOT TO KEEP THERE
Time-series measurements.
WHY
Shadows represent current state; history belongs in storage designed for it.
WHAT TO VERSION
The state document, so concurrent updates are detected.
WHAT TO TEST
Configuration applied to a device that was offline for a long period.
