Commands for intermittently connected devices?

Geoff Bruce-Payne 1

In certain applications, particularly for low power consumption, the device will not maintain a constant connection to IoT Hub/Central. Examples in the cellular world include using an SMS to "wake" or even send an asynchronous command, and conceptual parallels exist in satellite systems. Commands appear to assume the device is always connected on IP, while twin desired properties may not be appropriate for on-demand remote operations (e.g. "wake up").

Is there a recommended approach to enable such "push" Commands? For example an ability to generate a "pendingCommand" event from Hub/Central which could be picked up in something like EventGrid to trigger a Function App. Alternatively a kind of virtual proxy Function App that maintains device connections for all such devices within Azure, is that a scalable approach?

AshokPeddakotla-MSFT 27,726 Reputation points

2020-09-23T06:52:16.63+00:00

@Geoff Bruce-Payne Welcome to Microsoft Q&A forum!
I have reached out to our internal team on the issue and once I have an update will keep you posted on same.
Geoff Bruce-Payne 1 Reputation point

2020-10-13T15:48:10.153+00:00

Apologies I mistakenly put this in the answers section initially...

@AshokPeddakotla-MSFT Can you provide some more guidance on how properties are managed in terms of "Pending" and the use of $version?

What I'm seeing in the Central portal is that properties are marked as Pending when I update them, and the device twin desired properties wants $version=1. It's not clear to me how I baseline that set of version 1 properties, and how/when version is updated...per property or in aggregate? Also when I update the twin with a "pending" change and then subsequently updated with "complete" the desired property seems to still persist rather than closing.
Geoff Bruce-Payne 1 Reputation point

2020-10-13T15:48:53.753+00:00
For a standing instruction the behaviour I would like is as follows:

I define a set of "proxy" commands in the capability model let's say as a boolean intended to trigger a remote command

When the user on Central sets the boolean flag (using a form), the proxy device picks that up from the twin, triggers a message to be sent to the real device, and updates the twin to acknowledge and reset the boolean flag back to false. Alternatively this could flag the desired property as "pending"

The real device eventually sends back a set of telemetry and/or reported properties via the proxy which could set the command property back to its original state such that it is no longer pending.

What I'm unclear on is the structure of the reported property updates to achieve this behaviour. When I just send a list of property names and values it seems everything shows up as "pending".

1 answer

AshokPeddakotla-MSFT 27,726 Reputation points

2020-09-23T09:31:24.053+00:00

@Geoff Bruce-Payne Below is the response from Product Team, I hope this helps with your query.

Wake-up via IoT Hub is not possible as you will need an additional channel for activating the device (client) to connect via TCP to IoT Hub. SMS are still used, while definitely not a very secure way. Ripple control is also a way used by utilities to activate a more powerful communication channel. Alternatively the device could automatically connect based on a local event or regularly via timer.
When a device is connected, syncing the Device Twin's desired properties is a good way for "standing instructions" or you utilize the "device connect" event (EventGrid) to trigger a Azure Function with a method call. Few customers also use the "device connect" event to en-queue C2D messages due to the missing support of methods or Device Twins in their devices.

Do let us know if you have any further queries.

If an answer is helpful, please "Accept answer" or "Up-Vote" for the same which might be beneficial to other community members reading this thread.
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Geoff Bruce-Payne 1 Reputation point

2020-09-23T12:04:58.767+00:00

Understood, so the 2 workarounds proposed seem like viable approaches i.e.

1) "standing instructions" using twin desired properties delivered on device connect, or
2) "proxy connection" that pushes Commands on a non-IP channel.

FYI for satellite messaging on the Inmarsat network, the SMS "like" approach for push does not have the security issues associated with conventional SMS. It is all authenticated and encrypted over the system API.

AshokPeddakotla-MSFT 27,726 Reputation points

2020-09-24T09:39:52.057+00:00

@Geoff Bruce-Payne Thanks for the information. Feel free to reach out to us if you have any other queries.

Geoff Bruce-Payne 1 Reputation point

2020-10-14T21:30:43.647+00:00

1) It turns out that since Desired and Reported properties are not correlated*, the "standing instructions" approach is problematic since the device twin effectively keeps asking for a property change since it can't sync with the reported value.
2) Device connect event is also problematic since the devices are "connecting" only intermittently so commands time out.

Could the command not trigger an EventGrid event? Then a function app could pick up the event, connect to the device twin and handle the command asynchronously.

see: https://learn.microsoft.com/en-us/answers/questions/37240/query-regarding-reporteddesired-states-of-device-t.html
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