How to (re)use the single MQTT connection in a custom app service for subscribe and publish topics?

[jiekechoo]

I followed the example service cloud-export-mqtt code, a custom app-service to connect my cloud was worked, but I need the same MQTT connection to subscribe a few topics. How to do it?

Thanks.

[lenny-goodell]

@jiekechoo , to subscribe to topics from the cloud broker you must use the MQTT Trigger. It will not share the MQTT connection use to export to the Cloud. You will need to use unique ClientID for each connection.

Example MQTT trigger configuration can be found here: https://github.com/edgexfoundry/app-service-configurable/blob/main/res/external-mqtt-trigger/configuration.yaml#L49-L69

Note that the SubscribeTopics: is a comma separated list of topics.

[jiekechoo]

@lenny-intel ,thanks for your help.

I tried to develop a new app-service for the enterprise/cloud, not using two services. The cloud only allows one connection with the mqtt client, so i need to make a new app-service using one mqtt connection.

[jiekechoo]

I would like to try, add a feature about this issue, maybe someone needs it.

[lenny-goodell]

The MQTT broker will allow multiple connections if the client IDs for each connection are unique. Please try this.

[lenny-goodell]

Actually, I don't think the external trigger is what you want here since you need the event data flowing from the EdgeX MessageBus to the MQTT Export pipeline. Since this is a custom App Service you can create your own MQTT Export where to the client (already connected) is passed into the constructor for the your MQTT Export pipeline function. Then you also have the client available to subscribe to topics and take actions based on data received from the cloud.

[jiekechoo]

The MQTT broker will allow multiple connections if the client IDs for each connection are unique. Please try this.

Some cloud platform does not verify the client ID, every device has its unique token and all communications must be in one MQTT connection to make sure the device status(such as: up/down, online/offline), it's very common.

[jiekechoo]

Actually, I don't think the external trigger is what you want here since you need the event data flowing from the EdgeX MessageBus to the MQTT Export pipeline. Since this is a custom App Service you can create your own MQTT Export where to the client (already connected) is passed into the constructor for the your MQTT Export pipeline function. Then you also have the client available to subscribe to topics and take actions based on data received from the cloud.

@lenny-intel Yes, it's right. If I use the external-mqtt-trigger and the custom App Service in two separate containers, it can't share the MQTT connection. It seems I have to modify the MQTTSecretConfig struct. Do you have any other advise?

[lenny-goodell]

You can simply add a GetClient() to return the client from the MQTTSecretSender. Call it after calling ConnectToBroker().

[jiekechoo]

You can simply add a GetClient() to return the client from the MQTTSecretSender. Call it after calling ConnectToBroker().

Thanks a lot. I'll try.

[jiekechoo]

@lenny-intel I have followed your advise, adding two functions: SetClient(opts MQTT.ClientOptions) and GetClient()(unsed) in the MQTTSecretSender, before calling ConnectToBroker() some opts added to the MQTT client, the Subscribe in single MQTT connection works good. May I create a pr for this feature?

[lenny-goodell]

Please rename SetClient to SetClientOpts. Yes, please create the PR.

[jiekechoo]

solved.