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I'm trying to find code examples to build a translation gateway (NOT an Edge system) using C, preferably the C SDK with task control using FreeRTOS, although I could probably wrap everything RTOS myself if I had the rest of the example code working.
The goal is to have one device connected to Azure IoT Central via WiFi which can (1) provision existing but not-yet provisioned devices and (2) publish telemetry on their behalf (they cannot connect to the internet directly but will be on buses, local radios, filesystems, etc.).
The purpose of using the C SDK is for compatibility with a few microcontrollers that are relatively less constrained but still cannot run Linux, Windows, etc. (e.g. Espressif ESP32/Pico-D4). The purpose of avoiding Edge is hardware constraints (power, cost, etc.) of the project.
Are there any examples of this so so I don't have to build one from scratch?
Is there any way to do this other than AMQP "multiplexing" using multiple IoT Hub device clients objects that take turns sending?
Is there any way to publish telemetry messages using MQTT that will appear as separate devices in IoT Central? I'm reading here that "Any new MQTT connection on behalf of the same device ID causes IoT Hub to drop the existing connection"... but is there a way to on behalf of other device IDs?
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I can provide you with some guidance and recommendations to help you achieve your goals.
Azure IoT Central Provisioning: To provision devices in Azure IoT Central, you can use the Device Provisioning Service (DPS) provided by Azure IoT. The DPS enables you to securely provision devices to connect to an IoT hub, such as Azure IoT Central. You would need to implement the necessary logic in your C application to interact with the DPS APIs and handle the device provisioning flow.
Telemetry Publication: To publish telemetry on behalf of devices that cannot connect to the internet directly, you can use the Azure IoT Hub SDK for C. The SDK provides libraries and APIs to connect your device to Azure IoT Hub and send telemetry messages. You can find the Azure IoT Hub SDK for C on GitHub.
Regarding your question about using MQTT to publish telemetry messages that appear as separate devices in IoT Central, you are correct that IoT Hub drops existing connections when a new MQTT connection is made using the same device ID. This limitation means that you cannot directly achieve separate device identities in IoT Central using MQTT. Each device must have a unique device ID.
One possible approach to work around this limitation is to use IoT Hub device-to-cloud (D2C) message routing capabilities. You can configure message routing in IoT Hub to route telemetry messages based on specific properties or message content to different endpoints or custom Azure Functions. By leveraging this feature, you can route the telemetry messages from your translation gateway to separate endpoints or Azure Functions that can handle the messages as if they were coming from different devices.
Here's a high-level overview of the suggested approach:
By using this approach, you can achieve the desired separation of telemetry messages in IoT Central, even though they are sent from a single translation gateway.
I hope this guidance helps you in building your translation gateway using the C SDK with FreeRTOS for Azure IoT Central. Remember to consult the Azure IoT documentation and resources for the latest updates and examples specific to your use case.
To publish telemetry on behalf of devices that cannot connect to the internet directly, you can use the Azure IoT Hub SDK for C
How? I assume you mean the C99 Azure IoT C SDK ("azure-iot-sdk-c)? There is are no "IoT Hub SDKs" that I've seen. We're already using the C SDK to connect, provision, and publish but I don't see a way to publish directly using device IDs aside from an AMQP sample that has compile-time errors, and Microsoft is recommending against using AMQP at all, and they don't seem to be supporting it well for existing clients.
Route the telemetry messages from your translation gateway to different endpoints or Azure Functions based on your routing rules.
I don't want to route anything to other endpoints. They should be stored directly in IoT Central as telemetry events under each unique Device ID so they can be grouped and viewed by template, etc.
Each device must have a unique device ID.
Yes, that isn't a problem. We already have unique device IDs in the system generated before the translation gateway connects. The problem is publishing on their behalf. They remain unprovisioned until seen by the translation gateway, which then must provision them, I assume by DPS. But then how do we publish as different devices?
Is REST API able to do this?
Does anyone know the combination of APIs and functions I'd need, or the various code samples I could stitch together to make this work?