MQTT subscription failure

[m.saadat]

Hi,

I'm working on the Thingy91 'asset tracker v2' project. I see the data on the nRF cloud. Now, I want to subscribe for incoming data over my Android or Windows app. The connection breaks when I try to subscribe to the MQTT endpoint.

The thing I did is as follows:

• Flashed the Asset Tracker V2 firmware on Thingy91
• Got the certificate and clientId from 'POST api.nrfcloud.com/.../certificates'
• Converted the received JSON to separate files using the nRF LTE Link Monitor app
• Subscribing with mosquito using a command like this:

mosquitto_sub.exe -h ${mqttEndPoint} -t ${deviceId}/shadow/get/accepted --cert ./ssl\client.crt --cafile ./ssl\ca.crt -p 8883 --key ./ssl\client.key -d -i ${clientId}

Where the ${mqttEndPoint}, ${deviceId} and, ${clientId} is replaced with the real values obtained from nRF cloud.

When I subscribe to the topic mentioned above, everything seems fine, and the result is like this (the clientId and other information is replaced with ****):

Client nrf-*************** sending CONNECT
Client nrf-*************** received CONNACK (0)
Client nrf-*************** sending SUBSCRIBE (Mid: 1, Topic: nrf-***************/shadow/get/accepted, QoS: 0, Options: 0x00)
Client nrf-*************** received SUBACK
Subscribed (mid: 1): 0

But the moment something is published on that topic from Thingy91, it breaks the connection:

Client nrf-*************** sending CONNECT
Client nrf-*************** received CONNACK (0)
Client nrf-*************** sending SUBSCRIBE (Mid: 1, Topic: nrf-***************/shadow/get/accepted, QoS: 0, Options: 0x00)
Client nrf-*************** received SUBACK
Subscribed (mid: 1): 0
Error: The connection was lost.

However if I subscribe to the 'prod/${tenantId}/m/#' topic or any other topic starting with m, the connection breaks right away; the output is as follows:

Client nrf-*************** sending CONNECT
Client nrf-*************** received CONNACK (0)
Client nrf-*************** sending SUBSCRIBE (Mid: 1, Topic: prod/***********/m/#, QoS: 0, Options: 0x00)
Error: The connection was lost.

As described here, I should be able to subscribe to this topic.

One more thing, after I try to connect via mosquitto or MQTT Explorer, the Thingy91 gets disconnected, and I can not connect again. I have to remove the device from the nRF cloud and add it again!

Any idea what is wrong?

[Didrik Rokhaug]

Hi,

Could you try to subscribe to the specific topics you want to subscribe to, instead of using wildcards?

You might not be allowed to subscribe to all subtopics of prod/${tenantId}/m, which is probably why you get kicked out.

Best regards,

Didrik

[m.saadat]

Hi. I should have added that. I tried all the topics. I just added that one as a sample. The same result is happening even when I tried, for example, this one:

m/d/${deviceId}/c2d

Or any other topic in that table (I even tested the ones that are only for publishing). And for the topic 

${device_id}/shadow/get/accepted

I can connect, but as soon as Thingy91 publishes something to it, the connection breaks.

[Didrik Rokhaug]

m.saadat said:

I can connect, but as soon as Thingy91 publishes something to it, the connection breaks.

Are the Thingy and the mosquitto client using the same ID and credentials?

If that is the case, being disconnected is expected. Client IDs must be unique, i.e. cannot be used by two different clients at the same time.

[m.saadat]

That is the case. I use the same clientId and credentials that are received by calling the 'POST on api.nrfcloud.com/.../certificates'. If I use a different clientId (added '-2' at the end), it does not connect (connection breaks before the connection is complete):

Client nrf-**************-2 sending CONNECT
Error: The connection was lost.

When the Thingy91 is disconnected, the same thing happens, and the connection breaks (even when using the same clientId as Thingy91).

Where can I get the new clientId for new connections? Do I need a different certificate to be able to connect to the MQTT broker?

[Didrik Rokhaug]

That API creates JITP certificates for the client. Part of the provisioning process involeves disconnecting and reconnecting the client.

I recommend you follow this guide for how to create credentials for your client: https://docs.nrfcloud.com/Guides/GettingStarted/Devices#generating-credentials-on-a-computer

(note that step 3, 4 and 5 is not necesary for a desktop client. In step 2, you can use the nrf_cloud_provision.py script if you don'w want to call the REST API manually)

With credentials created that way, I have been successfully subscribed to prod/<team ID>/m/d/<client id>/c2d and <client id>/shadow/get/accepted. I was also able to receive the shadow on the latter topic by publishing to $aws/things/<client id>/shadow/get

Finally, you should note that client IDs starting with "nrf-" is reserved for evices created by Nordic Semiconductor (e.g. DKs and Thingys). nRF Cloud therefore treats them slightly differently. You should choose client IDs that do not start with "nrf-"

[m.saadat]

Thanks. I followed the link and now I'm able to connect.