Subscribing to n device under one application

Can we subscribe to multiple devices under one application? If so how the result will be? If the data comes from different devices at a time how mqtt subscribe will taken care?

Yes, for example:

mosquitto_sub -t "application/1234/node/+/rx" -v

Note that + is a wildcard for one level. You can also use #, e… application/1234/# to subscribe to everything under the application/1234/ prefix.

Thanks for your reply:grinning::grinning:. I got the clear idea now.

Hi brocaar,
I need to subscribe for 16 devices under one application. I am planning to use the below command
mosquitto_sub -t “application/1234/node/+/rx” -v
but can you please tell me what will be the result of above command if two devices sends data simultaneously.

I believe you should see 2 strings simultaneously. Each string will contain node_ID field which refers to a particular device.

Something like this:
application/14/node/0000000000000001/rx {“applicationID”:“14”,“applicationName”:“dragino_app”,“nodeName”:“dragino_node”,“devEUI”:“0000000000000001”, ETC…}

Mosquitto_sub subscribes to pointed topic and listens it all the time. It is not a one time command. =)

Hi lance ,
Thank you for your reply.
My question is if i subscribe to multiple devices ( mosquitto_sub -t “application/1234/node/+/rx” -v), and two devices sends data simultaneously how the json format will be.

Whether we will get one after the another like below?
{“applicationID”:“14”,“applicationName”:“dragino_app”,“nodeName”:“dragino_node1”,“devEUI”:“0000000000000001”, ETC…}
{“applicationID”:“14”,“applicationName”:“dragino_app”,“nodeName”:“dragino_node2”,“devEUI”:“0000000000000002”, ETC…}

I believe we will. Unfortunately I have no devices to test, but there has to be a mechanic which manages free time slots inside the server to prevent collisions, if that is what are you talking about.

Thank you lance . :grinning:
If someone have tested with two devices please give me the output of mqtt_sub when two devices sends data at a time.

You will get both, one after the other, like this:



You can see that they both have the same timestamp. This was tested and confirmed, as you can see in this screenshot, where some modified mqtt messages are received by a javascript client and shown in a map:

So, in summary, don’t worry about two nodes sending at the same time: if your gateway has multiple channels, it will handle them correctly and any of the two will be seen just before the other at the application level.


Thank you very much iegomez :grinning: