Module twin read without update

Athira Gopinath(UST,IN) 96

Hi,
We have tried to test a custom(sensing) CModules based on the tutorial "Develop a C IoT Edge module using Linux containers "tutorial-c-module". Raspberry PI 4B arm32v7 is used as an environment for the testing purposes.
We are using azure iot sdk _c and edge version 1.1
Our goal is to sense the temperature and compare it with a predefined threshold. Predefined threshold can be fetched from the module twin. Using Module twin callback function, we are able to receive the module twin.
We observed that after the device restarts, the module twin receives as null instead of the threshold sets over there. Also it is noted that ,module twin is receiving a callback only when a desired property is updated.
Please help us to understand how can we get the module twin call backs during a device restart or start up when there is no update, as we don't have provision to save the read values, to reuse .

QuantumCache 20,106 Reputation points

2022-09-02T18:05:51.603+00:00

Hello @Athira Gopinath(UST,IN) Just checking in if you have had a chance to see the previous response.

If the response is helpful, please click "Accept Answer" and upvote it.
AshokPeddakotla-MSFT 28,751 Reputation points

2022-09-06T06:27:24.267+00:00

@Athira Gopinath(UST,IN) Wanted to follow up with you to understand whether the below information helped. Please let us know if there are any other queries.
If below suggestions answers your query, do click Accept Answer and Up-Vote for the same. And, if you have any further query do let us know.

1 answer

Sander van de Velde 29,856 Reputation points MVP

2022-08-04T07:31:15.023+00:00

Hello

I observed this behavior too while using the C# SDK.

When a module receives a twin update, a copy of the module twin is kept by the EdgeHub.

So, you are able to request your 'last' module twin on startup, even if the device is not connected to the cloud anymore.

As seen here, I actively read the module twin from the edgeHub:

Can you try this using your SDK too?
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Athira Gopinath(UST,IN) 96 Reputation points

2022-08-04T10:59:32.967+00:00

Thanks SandervandeVelde42
We have noticed that there is a payload difference after restart and update mentioned as below

Twin after restart:
Twin callback called with (state=DEVICE_TWIN_UPDATE_COMPLETE, size=197):
{"desired":{"gid":"UEG0012","cid":"a808-1","ReportingInterval":60,"TemperatureSampling":10,"HumiditySampling":10,"LTemp":7,"HTemp":30,"Lhumi":40,"Hhumi":90,"$version":37},"reported":{"$version":1}}
Twin after update:
Twin callback called with (state=DEVICE_TWIN_UPDATE_PARTIAL, size=159):
{"gid":"UEG0013","cid":"a808-1","ReportingInterval":60,"TemperatureSampling":10,"HumiditySampling":10,"LTemp":7,"HTemp":30,"Lhumi":40,"Hhumi":90,"$version":38}
Based on this we have made changes in parson logic and it is working fine for both conditions
Thanks & Regards
Athira Gopinath

Sander van de Velde 29,856 Reputation points MVP

2022-08-04T11:11:04.78+00:00

Hello @Athira Gopinath(UST,IN) ,

great to hear it's working as expected now.

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Module twin read without update

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