Do leaf/downstream devices connect directly to iot-hub even when edge is used as gateway?

Question

I am trying to setup an iot-edge device as an edge gateway. We wouldn't want our leaf/sensor/downstream devices directly connecting to the internet/cloud, and thus I would expect the iot-edge-gateway(as it name suggests) to bridge the connection between downstream devices and the cloud/iot-hub. However, I realize that the connection string for iot-hub/edge at any device level is simply

connection-string-for-iothub-with-gatewayhostwayAppended

This makes me assume that downstream devices transmit messages to an endpoint (prolly messages/* )to cloud/iot-hub and it is from there that gateway gets it(works with that data maybe then) and forwards it back to the $upstream, which shuns the whole point of a gateway.

Here in the message routing section of IOT-EDGE-GATEWAY https://docs.microsoft.com/en-us/azure/iot-edge/how-to-create-transparent-gateway, in the ROUTE MESSAGES FROM DOWNSTREAM DEVICES section

{
    "routes":{
        "sensorToAIInsightsInput1":"FROM /messages/* WHERE NOT IS_DEFINED($connectionModuleId) INTO BrokeredEndpoint(\"/modules/ai_insights/inputs/input1\")", 
        "AIInsightsToIoTHub":"FROM /messages/modules/ai_insights/outputs/output1 INTO $upstream" 
    } 
}

makes it sound like the gateway is routing messages falling on the built-in-endpoint(Default) to $upstream. I can't find any other clearer documentations over the web on this subject. I would really appreciate if someone clears this up. I was expecting the connection string for edge-gateway(that i'd mention in the device end to be something along the lines of localhost:port and not cloudaddress+gatewayhostname)

silent silent · Accepted Answer · 2019-07-12T07:24:57

If your connection string contains a gateway hostname - and the SDK you are using on the device properly handles this, the device only connects to the gateway, not to the IoT Hub.

You can see the example from the .NET SDK here:

this.HostName = builder.GatewayHostName == null || builder.GatewayHostName == "" ? builder.HostName : builder.GatewayHostName;

https://github.com/Azure/azure-iot-sdk-csharp/blob/f86cb76470326f5af8426f3c2695279f51f6e0c8/iothub/device/src/IotHubConnectionString.cs#L30

If the gateway hostname is set, it actually overwrites the IoT Hub hostname for the connection.

Do leaf/downstream devices connect directly to iot-hub even when edge is used as gateway?

1 Answers