Add MQTT Trigger

[lenny-goodell]

This triggers function pipeline from data subscribed from MQTT Broker.

Will handle JSON or CBOR which are identified by the initial bytes in the data.

If ResponseTopic is provided in configuration, the results from the function pipeline (if any) are publish to the configured topic.

Secure connections will follow the same patter/options used for the MqttSecretSend used to export to an MQTT Broker.

[cloudxxx8]

add @FelixTing for awareness

[FelixTing]

IMHO, the premise of this issue is whether we allow app-service to connect to an external MQTT broker?
Current app-funcitons-sdk use go-mod-messaging library to interact with message bus, so it already supports MQTT(https://github.com/edgexfoundry/go-mod-messaging/blob/master/messaging/factory.go#L42). But go-mod-messaging is for EdgeX internal use, it only accepts the message payload that can be unmarshaled to EdgeX MessageEnvelope(https://github.com/edgexfoundry/go-mod-messaging/blob/master/internal/pkg/mqtt/client.go#L192). This limitation restricts the opportunity for app-service to connect to an external message bus.

[lenny-goodell]

@FelixTing , As you have pointed out, go-mod-messaging is for internal MessageBus.

For allowing incoming data from external MQTT, we need to handle it similar to the HTTP trigger but with secure connection similar to MQTTSecretSender for exporting data with a secure connection, i.e. the MQTT Trigger will also use the exclusive secrets for the secure connection.

[lenny-goodell]

@FelixTing , What is the Use Case for receiving data from external broker? Are you sending Event/Readings or custom data type? What is the end goal of receiving this data?

[FelixTing]

@lenny-intel We tried to use app service to interact with AWS IoT Device Shadow. The initial thought is to use app service to subscribe to AWS IoT Device Shadow topics. When receiving messages, analyze the topic name and the payload(custom data) via a built-in or custom function to determine the action needs to be taken, such as calling core-command API to update device resource values.

[cloudxxx8]

@lenny-intel we are finding another way to deal with the downstream request, so please feel free to close this issue if it is not the case app service designed for.

[lenny-goodell]

When receiving messages, analyze the topic name and the payload(custom data) via a built-in or custom function to determine the action needs to be taken, such as calling core-command API

@FelixTing , @cloudxxx8 , This MQTT trigger seems appropriate for your use case when creating a custom App Service with custom function to operate on the custom data received from the external MQTT broker. The app-functions-sdk has the ability for a custom App Service to specify the TargetType for the incoming data, which can be your custom data type or simply raw data (i.e. []byte). We would add new Trigger configuration type of mqtt and new config section for the MQTT Trigger connection details (similar to MessageBus configuration). This MQTT Trigger configuration would follow the pattern used to config the MQTTSecretSender and use the exclusive SecrectProvider to get the secrets required for secure connection.

Does this make sense for your use case or is finding another way to deal with the downstream request better for you?

[cloudxxx8]

Thanks, @lenny-intel , this would be very useful.

[jpwhitemn]

Implement similar to how HTTP trigger works (per @lenny-intel )