Skip to main content

Device Presence

DeviceChain keeps a live presence signal for every device — whether it is currently online, and when it last connected, disconnected, or reported activity. Presence is part of a device's last-known state (the same projection that holds its most recent measurements and location), and it surfaces on the device's Connectivity tab in the console.

The important thing to understand is how DeviceChain decides a device is online, because it depends on the transport.

Two ways presence is known​

Every device carries a presence source that says how its online/offline state is determined:

  • Inferred (the default) — DeviceChain has no explicit connect/disconnect signal from the transport, so it infers presence from activity. A device is considered online while it is sending data; if it goes quiet for longer than its inactivity timeout, a background sweep marks it offline. This is the right model for connectionless transports (plain HTTP, CoAP) and for simple MQTT clients that don't announce themselves.

  • Asserted — the transport tells DeviceChain explicitly when a device connects and disconnects, so presence is authoritative rather than guessed. The first time such a signal arrives for a device, DeviceChain switches that device to the asserted source and, from then on:

    • its online/offline state is driven only by explicit connect/disconnect signals — a stray data packet can never mark a device online that the platform has been told is offline;
    • the inactivity sweep leaves it alone — an asserted device that goes quiet is not assumed dead, because if it had died the transport would have said so.

A device stays inferred until an asserting transport produces for it, so nothing changes for existing devices unless they start arriving over a transport that asserts presence. Today two transports assert presence: Sparkplug-B, whose BIRTH and DEATH messages are exactly these explicit connect/disconnect signals, and LwM2M, whose registration lifecycle — register, periodic update, and deregister (or a lapsed lifetime) — does the same.

Why the distinction matters​

Inferred presence is convenient but laggy and ambiguous: "offline" only means "hasn't spoken recently," which is slow to notice a real disconnect and blind for devices that report on a long interval. Asserted presence is immediate and unambiguous — a disconnect is a disconnect the instant the transport reports it — which is what you want for anything you'll alarm or act on.

Keeping the two modes as an explicit per-device flag means a device on a connectionless transport keeps its familiar timeout behavior, while a device on a presence-aware transport gets the authoritative signal, and the two never interfere.

Status

Device presence — both inferred and asserted — is available, with Sparkplug-B and LwM2M as presence-asserting transports. A detection rule can be authored directly on a connect/disconnect edge: the Connectivity condition raises an alarm the instant an authoritative disconnect arrives and resolves it on reconnect — no timeout to tune. It complements the timeout-based Absence rule (authoritative death vs. inferred silence), and the two are meant to be paired. An authoritative disconnect also updates the device's live state, visible on the Connectivity tab.