How to read out message content of the EventGridEvent object that is passed from IotHub

Question

I have an IoT device that sends a message to its IotHub like this in Python:

device_client = IoTHubDeviceClient.create_from_connection_string(conn_str)
 ...
message = "Message sent from my device"
await device_client.send_message(message)

In the IoTHub, there is an event grid subscription that is an event-grid triggered function app. In the function app, I am trying to get the message out from the event but doesn't seem the message "Message sent from my device" can be located?

def main(event: func.EventGridEvent):
    ...
    received_message = {"id": event.id, "message": str(event.get_json()), "}

I also tried event.topic, event.subject and they seem not the message that sending from the device. If uses event.get_json()["body"], it seems it is an encrypted message like VGhpcyBpcyBhIG1lc3NhZ2UgdGhhdCBpcyBiZWluZyBzZW50IGZyb20gVG9kZCdzIGRldmljZQ==. What's a good way to decrypt?

Also, if the message sent from a device is a json format, how to retrieve it? My intention is to save the message/json into a cosmos db with some process in the function app.

Answer 1

Hello @Hsieh, Todd T ,

The best way is to decode the base64 string. For the example provided in your question, I can see the below-decoded message.

VGhpcyBpcyBhIG1lc3NhZ2UgdGhhdCBpcyBiZWluZyBzZW50IGZyb20gVG9kZCdzIGRldmljZQ==
"This is a message that is being sent from Todd's device"

import base64

base64_message = 'VGhpcyBpcyBhIG1lc3NhZ2UgdGhhdCBpcyBiZWluZyBzZW50IGZyb20gVG9kZCdzIGRldmljZQ=='
base64_bytes = base64_message.encode('ascii')
message_bytes = base64.b64decode(base64_bytes)
message = message_bytes.decode('ascii')

print(message)

See also the examples in C# to get an idea here:

The below screengrab shows that my Function is triggered and the Telemetry data is in base64 encoded format, i need to decode the 'body' content to read the actual message/telemetry.

Answer 2

Hello @Hsieh, Todd T , the 'encrypted' message 'VGhpcyBpcyBhIG1lc3NhZ2UgdGhhdCBpcyBiZWluZyBzZW50IGZyb20gVG9kZCdzIGRldmljZQ==' is just a base64 encoded string which contains 'This is a message that is being sent from Todd's device'.

Base64 is just a binary-to-text encoding scheme that represents binary data in an ASCII string format. No data is lost in this encode/decode process.

Check it out for yourself with any online base64 decoder.

So all you need to do is decoding the message from base64 to a text string.

I recommend using Microsoft Azure SDKs code.

Once you have the text string which is a JSON text, you could try to convert JSON to an object.

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