How to Define and Use Variable Values in a Topic Namespace

Satyam Chauhan 567

Hi,

I am using azure event grid namespace mqtt broker. The broker has topic spaces configured. I want the topic template to support variable which can be replaced by client while publishing.

For example,

The topic template should be something like this - myhome/{roomname}/{sensorid}, here roomname and sensorid should be dynamic based on the value provided by client.

Let's say client1 is publishing some payload to topic - "myhome/livingroom/009"

and client2 is publishing to topic - "myhome/bedroom/001"

and client3 is subscribing to topic - "myhome/livingroom/009", then client3 should only receive messages from "myhome/livingroom/009" and not from any other topics.

Please help how should I create such topic templates and how to utilize these templates while publishing and subscribing.

I am developing in .NET using mqttnet package.

Thanks

1 answer

Sander van de Velde | MVP 33,231 Reputation points MVP

2024-11-11T18:31:13.67+00:00
Hello @Satyam Chauhan,

welcome to this moderated Azure community forum.

Can you check out this part of the documentation?

Topic templates support MQTT wildcards (+ and #) and the following variables: ${client.attributes.x}: this variable represents an assigned attribute to a client during client creation/update, such as "x" would be equal to the exact string of the attribute key

An example seen there is:

area/${client.attributes.area}/telemetry

If you want to limit access to certain topics, you probably need to add the 'client.authenticationName' name too in the topics (either for publishing or subscribing).

If the response helped, do "Accept Answer". If it doesn't work, please let us know the progress. All community members with similar issues will benefit by doing so. Your contribution is highly appreciated.
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Satyam Chauhan 567 Reputation points

2024-11-12T05:13:07.5733333+00:00

Hello @Sander van de Velde | MVP ,

Thanks for your response,

I tried to implement this but facing issue, Can you please help me figure out what could be missing.

Added this topic template - "area/${client.attributes.area}/telemetry" in Event grid topic space.

And in the .net app I tried the below code :

string topic = "area/${client.attributes.area}/telemetry"; string payload = "Hello from client 1"; if (mqttClient.IsConnected) { var message = new MqttApplicationMessageBuilder() .WithTopic(topic) .WithPayload(payload) .WithAtLeastOnceQoS() .WithUserProperty("area", "east_us") .Build(); var pubResult = await mqttClient.PublishAsync(message, CancellationToken.None); }

The public async line throws exception.

Stacktrace : at MQTTnet.Client.MqttClient.ThrowIfNotConnected()

at MQTTnet.Client.MqttClient.PublishAsync(MqttApplicationMessage applicationMessage, CancellationToken cancellationToken)

at ClientMQTT.MqttClientService.PublishAsync(String topic, String payload) in D:\project\mqtt_publish_client1\MQTTClientService.cs:line 124

Please help how to access such topics.

Sander van de Velde | MVP 33,231 Reputation points MVP

2024-11-12T07:12:15.4866667+00:00

Hello @Satyam Chauhan,

you try to publish to :

area/${client.attributes.area}/telemetry

but the wildcards are not available for clients.

Clients must publish to eg:

area/1/telemetry

where '1' must be equal to the value of attribute 'area'.

Can you try this?

Satyam Chauhan 567 Reputation points

2024-11-12T07:40:51.2633333+00:00

hello @Sander van de Velde | MVP ,

Tried that as well, on starting the program the client get connected to mqtt broker. But when client tries to publish the message it gets disconnected from broker.

Please help

Sander van de Velde | MVP 33,231 Reputation points MVP

2024-11-13T23:25:52.7666667+00:00

Hello @Satyam Chauhan,

I have tested this with the MQTTX tool.

I try to publish using a topic that is related to an attribute (line:1).

I constructed:

az eventgrid namespace create -g mqtt-test-weu-rg -n mqtt-test-weu-egns --topic-spaces-configuration "{state:Enabled}" az eventgrid namespace client create -g mqtt-test-weu-rg --namespace-name mqtt-test-weu-egns -n client1-authnID --authentication-name client1-authnID --client-certificate-authentication "{validationScheme:ThumbprintMatch,allowed-thumbprints:[23etc]}" --attributes "{'line':1}" az eventgrid namespace client-group create -g mqtt-test-weu-rg --namespace-name mqtt-test-weu-egns -n line1group --group-query "attributes.line=1" az eventgrid namespace topic-space create -g mqtt-test-weu-rg --namespace-name mqtt-test-weu-egns -n line1topicspace --topic-templates 'line/${client.attributes.line}/telemetry' az eventgrid namespace permission-binding create -g mqtt-test-weu-rg --namespace-name mqtt-test-weu-egns -n line1permissionbinding --client-group-name line1group --permission publisher --topic-space-name line1topicspace

So, Client 1 should be able to publish on topic 'line/1/telemetry' only!

In MQTTX, I try this:

The message is sent:

I then try to publish on topic 'line/2/telemetry' and this should fail...

So:

gives an error (the client disconnects):

I add a second client that subscribes topic 'line/2/telemetry' and the faulty message was not forwarded to the second client.

I also notice the second client was only allowed to subscribe to 'line/2/telemetry' and not to 'line/3/telemetry'. The 'line/3/telemetry' gave an error and reconnect when setting it in MQTTX.

I think this is the behavior you expect so please check this with a tool like MQTTX before you try it with your coded solution.

See this blog post on how to install and configure MQTTX.

Sander van de Velde | MVP 33,231 Reputation points MVP

2024-11-15T12:11:04.0233333+00:00

Hello @Satyam Chauhan,

can you confirm you see the same behavior in MQTTX?

If the answer helped, do "Accept Answer". If it doesn't work, please let me know the progress.

All community members with similar issues will benefit by doing so. Your contribution is highly appreciated.
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