Cloud-to-device communications guidance
IoT Hub provides three options for device apps to expose functionality to a back-end app:
Direct methods for communications that require immediate confirmation of the result. Direct methods are often used for interactive control of devices such as turning on a fan.
Twin's desired properties for long-running commands intended to put the device into a certain desired state. For example, set the telemetry send interval to 30 minutes.
Cloud-to-device messages for one-way notifications to the device app.
To learn how Azure IoT Plug and Play uses these options to control IoT Plug and Play devices, see IoT Plug and Play service developer guide.
Note
The features described in this article are available only in the standard tier of IoT Hub. For more information about the basic and standard/free IoT Hub tiers, see Choose the right IoT Hub tier for your solution.
Here is a detailed comparison of the various cloud-to-device communication options.
| Categories | Direct methods | Twin's desired properties | Cloud-to-device messages |
|---|---|---|---|
| Scenario | Commands that require immediate confirmation, such as turning on a fan. | Long-running commands intended to put the device into a certain desired state. For example, set the telemetry send interval to 30 minutes. | One-way notifications to the device app. |
| Data flow | Two-way. The device app can respond to the method right away. The solution back end receives the outcome contextually to the request. | One-way. The device app receives a notification with the property change. | One-way. The device app receives the message |
| Durability | Disconnected devices are not contacted. The solution back end is notified that the device is not connected. | Property values are preserved in the device twin. Device will read it at next reconnection. Property values are retrievable with the IoT Hub query language. | Messages can be retained by IoT Hub for up to 48 hours. |
| Targets | Single device using deviceId, or multiple devices using jobs. | Single device using deviceId, or multiple devices using jobs. | Single device by deviceId. |
| Size | Maximum direct method payload size is 128 KB for the request and 128 KB for the response. | Maximum desired properties size is 32 KB. | Up to 64 KB messages. |
| 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 using MQTT or AMQP. | Available using MQTT or AMQP. | Available on all protocols. Device must poll when using HTTPS. |
Learn how to use direct methods, desired properties, and cloud-to-device messages in the following tutorials: