Understand extended offline capabilities for IoT Edge devices, modules, and child devices

Artikkel
07/26/2022

Applies to: IoT Edge 1.1

Important

IoT Edge 1.1 end of support date was December 13, 2022. Check the Microsoft Product Lifecycle for information about how this product, service, technology, or API is supported. For more information about updating to the latest version of IoT Edge, see Update IoT Edge.

Azure IoT Edge supports extended offline operations on your IoT Edge devices, and enables offline operations on downstream devices too. As long as an IoT Edge device has had one opportunity to connect to IoT Hub, that device and any downstream devices can continue to function with intermittent or no internet connection.

How it works

When an IoT Edge device goes into offline mode, the IoT Edge hub takes on three roles. First, it stores any messages that would go upstream and saves them until the device reconnects. Second, it acts on behalf of IoT Hub to authenticate modules and downstream devices so that they can continue to operate. Third, it enables communication between downstream devices that normally would go through IoT Hub.

The following example shows how an IoT Edge scenario operates in offline mode:

Configure devices

IoT Edge devices automatically have offline capabilities enabled. To extend that capability to other devices, you need to configure the downstream devices to trust their assigned parent device and route the device-to-cloud communications through the parent as a gateway.
Sync with IoT Hub

At least once after installation of the IoT Edge runtime, the IoT Edge device needs to be online to sync with IoT Hub. In this sync, the IoT Edge device gets details about any downstream devices assigned to it. The IoT Edge device also securely updates its local cache to enable offline operations and retrieves settings for local storage of telemetry messages.
Go offline

While disconnected from IoT Hub, the IoT Edge device, its deployed modules, and any downstream devices can operate indefinitely. Modules and downstream devices can start and restart by authenticating with the IoT Edge hub while offline. Telemetry bound upstream to IoT Hub is stored locally. Communication between modules or between downstream devices is maintained through direct methods or messages.
Reconnect and resync with IoT Hub

Once the connection with IoT Hub is restored, the IoT Edge device syncs again. Locally stored messages are delivered to the IoT Hub right away, but are dependent on the speed of the connection, IoT Hub latency, and related factors. They are delivered in the same order in which they were stored.

Any differences between the desired and reported properties of the modules and devices are reconciled. The IoT Edge device updates any changes to its set of assigned downstream devices.

Restrictions and limits

The extended offline capabilities described in this article are available in IoT Edge version 1.0.7 or higher. Earlier versions have a subset of offline features. Existing IoT Edge devices that don't have extended offline capabilities can't be upgraded by changing the runtime version, but must be reconfigured with a new IoT Edge device identity to gain these features.

Only non-IoT Edge devices can be added as downstream devices.

IoT Edge devices and their assigned downstream devices can function indefinitely offline after the initial, one-time sync. However, storage of messages depends on the time to live (TTL) setting and the available disk space for storing the messages.

A device's EdgeAgent updates its reported properties whenever there is a change in the deployment status such as a new or failed deployment. When a device is offline, the EdgeAgent can't report status to the Azure portal. Therefore, the device status in the Azure portal may remain 200 OK when IoT Edge device has no internet connectivity.

Set up parent and child devices

By default, a parent device can have up to 100 children. You can change this limit by setting the MaxConnectedClients environment variable in the edgeHub module. A child device only has one parent.

Note

A downstream device emits data directly to the Internet or to gateway devices (IoT Edge-enabled or not). A child device can be a downstream device or a gateway device in a nested topology.

Downstream devices can be any non-IoT Edge device registered to the same IoT Hub.

If you're unfamiliar with creating a parent-child relationship between an IoT Edge device and an IoT device, see Authenticate a downstream device to Azure IoT Hub. The symmetric key, self-signed X.509, and CA-signed X.509 sections show examples of how to use the Azure portal and Azure CLI to define the parent-child relationships when creating devices. For existing devices, you can declare the relationship from the device details page of either the parent or child device.

Set up the parent device as a gateway

You can think of a parent/child relationship as a transparent gateway, where the child device has its own identity in IoT Hub but communicates through the cloud via its parent. For secure communication, the child device needs to be able to verify that the parent device comes from a trusted source. Otherwise, third-parties could set up malicious devices to impersonate parents and intercept communications.

One way to create this trust relationship is described in detail in the following articles:

Specify DNS servers

To improve robustness, it is highly recommended you specify the DNS server addresses used in your environment. To set your DNS server for IoT Edge, see the resolution for Edge Agent module reports 'empty config file' and no modules start on the device in the troubleshooting article.

Optional offline settings

If your devices go offline, the IoT Edge parent device stores all device-to-cloud messages until the connection is reestablished. The IoT Edge hub module manages the storage and forwarding of offline messages. For devices that may go offline for extended periods of time, optimize performance by configuring two IoT Edge hub settings.

First, increase the time to live setting so that the IoT Edge hub will keep messages long enough for your device to reconnect. Then, add additional disk space for message storage.

Time to live

The time to live setting is the amount of time (in seconds) that a message can wait to be delivered before it expires. The default is 7200 seconds (two hours). The maximum value is only limited by the maximum value of an integer variable, which is around 2 billion.

This setting is a desired property of the IoT Edge hub, which is stored in the module twin. You can configure it in the Azure portal or directly in the deployment manifest.

"$edgeHub": {
    "properties.desired": {
        "schemaVersion": "1.0",
        "routes": {},
        "storeAndForwardConfiguration": {
            "timeToLiveSecs": 7200
        }
    }
}

Host storage for system modules

Messages and module state information are stored in the IoT Edge hub's local container filesystem by default. For improved reliability, especially when operating offline, you can also dedicate storage on the host IoT Edge device. For more information, see Give modules access to a device's local storage

Next steps

Learn more about how to set up a transparent gateway for your parent/child device connections:

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