Summary

You can now explain how Diyar keeps a kilowatt-class heater safe with no network connection, and how a single sensor reading becomes evidence an inspector can trust.

In this module, you:

• Learned the core rule, de-energize-to-trip, and the four layers that back it: an independent hardware thermal cutoff (mandatory, survives a wedged CPU), a hardware watchdog with fail-safe wiring, the software typestate interlock (Idle -> Armed -> Heating), and worker enforcement that cuts the heater on every exit path - plus the comms-loss watchdog a Modbus-driven heat output must declare before profile validation will accept it.
• Explained why a remote start from the cloud or a mobile client is a signed, vetoable request the edge alone validates - never a command - and why the device shadow's desired state is deliberately config-class only, never actuation.
• Traced a reading from the local journal (written before anything is published) through the SHA-256 hash chain, the Merkle history tree that proves completeness without shipping a whole session's history, and the cloud's independent reconciliation and quarantine of anything that does not match.
• Learned, honestly, what is defense-in-depth versus a hardware guarantee: the layer-3/4 heartbeat is cooperative and does not catch a thread parked in a blocking call, the hash chain is tamper-evident rather than tamper-proof against root access, and the Merkle root is not signed yet.

Next steps

• How a device becomes a Diyar solution - the next module in Introduction to Cogitave Diyar, on the signed device profile, the certified verdict-engine registry, and how ISPM-15 became Diyar's first certified solution.
• Keep "the edge is the only thing that can ever actuate" in mind - the next module builds a device's whole authority model on top of exactly that boundary.