When sending information from the device app to the solution back end, IoT Hub exposes three options:
- Device-to-cloud messages for time series telemetry and alerts.
- Reported properties for reporting device state information such as available capabilities, conditions, or the state of long-running workflows. For example, configuration and software updates.
- File uploads for media files and large telemetry batches uploaded by intermittently connected devices or compressed to save bandwidth.
Here is a detailed comparison of the various device-to-cloud communication options.
|Device-to-cloud messages||Reported properties||File uploads|
|Scenario||Telemetry time series and alerts. For example, 256-KB sensor data batches sent every 5 minutes.||Available capabilities and conditions. For example, the current device connectivity mode such as cellular or WiFi. Synchronizing long-running workflows, such as configuration and software updates.||Media files. Large (typically compressed) telemetry batches.|
|Storage and retrieval||Temporarily stored by IoT Hub, up to 7 days. Only sequential reading.||Stored by IoT Hub in the device twin. Retrievable using the IoT Hub query language.||Stored in user-provided Azure Storage account.|
|Size||Up to 256-KB messages.||Maximum reported properties size is 8 KB.||Maximum file size supported by Azure Blob Storage.|
|Frequency||High. For more information, see IoT Hub limits.||Medium. For more information, see IoT Hub limits.||Low. For more information, see IoT Hub limits.|
|Protocol||Available on all protocols.||Currently available only when using MQTT.||Available when using any protocol, but requires HTTP on the device.|
It is possible that an application requires to both send information as a telemetry time series or alert and also to make it available in the device twin. In this scenario, you can chose one of the following options:
- The device app sends a device-to-cloud message and reports a property change.
- The solution back end can store the information in the device twin's tags when it receives the message.
Since device-to-cloud messages enable a much higher throughput than device twin updates, it is sometimes desirable to avoid updating the device twin for every device-to-cloud message.