Flux Streaming subscriptions with generic type

[alicejgibbons]

Today using Flux with streaming subscriptions in Java, you can only publish/subscribe using Dapr's CloudEvent type as shown in the example here:

java-sdk/examples/src/main/java/io/dapr/examples/pubsub/stream/Subscriber.java

Line 46 in 8f11972

client.subscribeToEvents(

The latest PR by @artur-ciocanu is an improvement since subscribeToEvents() uses a flux based method but the limitation here is that only the CloudEvent can be sent.

We should also support something like the following as many users will be sending generic types (not cloud event based) on the message broker and this is supported in Dapr Pubsub generally.

public <T> Flux<T> subscribeToEvents(...

One way to standardize this would be to add an override that uses the DaprAppCallbackProtos.TopicEventRequest topicEventRequest type which can be deserialized as per the example here: https://github.com/artur-ciocanu/java-sdk/blob/26d215342bf541135c200ba22fd085b8a4e1c010/examples/src/main/java/io/dapr/examples/pubsub/grpc/SubscriberGrpcService.java#L67

[alicejgibbons]

Note even the old subscribeToEvents() method that doesn't use Flux seems to convert to CloudEvents: https://github.com/dapr/java-sdk/blob/master/sdk/src/main/java/io/dapr/client/Subscription.java#L94

[artur-ciocanu]

@alicejgibbons exactly, I tried to follow the current pattern for anything related to subscription. Since the Flux based API is pretty new, it still marked as "preview", I am wondering if there is any value in exposing the CloudEvent based API or should replace the current API with the generic one. I want to make that we keep the API surface small.

Let me know your thoughts and what customers are looking for.

[alicejgibbons]

@artur-ciocanu Replacing the current API could be a good option since users could still use the Dapr Java CloudEvent object as the generic type anyways and get the same result. This also avoids the code around having to rebuild the CoudEvent object which doesn't take into account all of the cloudevent fields here.

Lots of users DO use cloudevents though so leaving both in is also fine for ease of use :)