Is there a way to subscribe to topic and get data

[blublinsky]

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An example here shows how to publish event, but provides no information on how to subscribe and receive events?
I can see Node and Python examples at https://github.com/dapr/samples/tree/master/4.pub-sub,
But they are not easily transferable to java

[amanbha]

@blublinsky The subscription in dapr is similar across all languages:
Dapr will call into the app over http or grpc.

1. The app should have a /subscribe route which will return a json list of Topics it wants to subscribe to (https://github.com/dapr/docs/blob/master/reference/api/pubsub.md#handling-topic-subscriptions)
   2 Lets say, the app subscribes to 2 topics: Deposit & Withdraw, then the app also needs to have 2 routes on /Deposit & /Withdraw to recieve the events for the subscribed topics, (https://github.com/dapr/docs/blob/master/reference/api/pubsub.md#delivering-events-to-subscribers)

[blublinsky]

Thanks @amanbha.
It is a bit confusing, because publish is a GRPC API, but subscribe is not. Why is it asymmetric?
I see some of the methods for subscribe are in GRPCClient, but they seem not been implemented .
Am I missing something?

[amanbha]

Thanks @amanbha.
It is a bit confusing, because publish is a GRPC API, but subscribe is not. Why is it asymmetric?
I see some of the methods for subscribe are in GRPCClient, but they seem not been implemented .
Am I missing something?

@blublinsky I understand the confusion, allow me to explain a bit more. User App and Dapr can interact with each other over gRPC or HTTP. The Java SDK is autogenerated from the proto files of gRPC apis in Dapr runtime.
You can also make http calls directly to publish the events to a topic

There is an HTTP and gRPC server running in Dapr runtime and your app acts as a client when making calls to it (so it can make calls over http or gRPC). When Dapr needs to call into your app over HTTP or gRPC, you need to have a HTTP or gRPC server running in your app. The examples which I gave you earlier explained about doing it over HTTP(your app running an HTTP server). You can do similar things over gRPC(your app running as gRPC server) as well.

[blublinsky]

Thanks @amanbha.
Did some more experiments. I finally realized that DaprClient.proto defines an interface that I need to implement to respond to Dapr requests. The issue that I am getting into with this approach is that I need 2 GRPC channels one for invoking DAPR and one for responding to DAPR request.
Is there a way to tell DAPR environment to use these 2 channels, defining which one to use for what?

[blublinsky]

After more experimentations, it looks like, the right configuration is:
dapr run --app-id scalaTest --grpc-port 50001 --app-port 50002 --protocol grpc --port 3500 -- sbt run,
but it does not work -
Caused by: io.netty.channel.AbstractChannel$AnnotatedConnectException: Connection refused: localhost/0:0:0:0:0:0:0:1:50001
If I remove
--app-port 50002 --protocol grpc
DAPR obviously does not know how to access my app.
So it looks like 2 GRCP channels are not supported

[amanbha]

@blublinsky The command looks correct, when you provide the --protocol grpc flag, the app-port is treated as gRPC port. There will be 2 channels, App-> Dapr(Dapr runtime listening on port 50001) & Dapr->app(App listening on port 50002). It should have worked.
cc @yaron2 & @youngbupark for further help.

[blublinsky]

I can provide a project if this will help

[amanbha]

I can provide a project if this will help

Thanks @blublinsky , that would be very helpful.

[blublinsky]

dapr.zip
Uploaded

[blublinsky]

Its Scala, not Java, but should be the same

[blublinsky]

With more experiments it looks like Dapr currently does not support bidirectional communications with an applications. I tried to use HTTP to talk to Darp instead of GRPC, but with the same result.
Additionally, once this is fixed, feel free to use this example as scala APIs

[yaron2]

Hey @blublinsky , in order for Dapr to talk to your app via gRPC, your app needs to implement the daprclient service.
For Java/Scala, that would be:

https://github.com/dapr/java-sdk/blob/master/src/main/java/io/dapr/DaprClientProtos.java

See this document on how to use gRPC and Dapr:
https://github.com/dapr/docs/tree/master/howto/create-grpc-app

[blublinsky]

Its implemented, take a look at the code provided and works by itself.
The issue is that bidirectional communications do not work. I can either talk to runtime or receive runtime messages, but not both in the same app

[yaron2]

In gRPC bidirectional communications implies a streaming nature, but that's just a design decision.

For example, Dapr will call your app on the GetTopicSubscriptions method to get your topic subscriptions, and then when a message comes it will call you via OnTopicEvent.

Regardless, you could publish a message by invoking Dapr over gRPC with the PublishEvent method.

[blublinsky]

But it does not work. Please, take a look at enclosed code. I might be missing something,but I am creating two channels - one for listening on Dapr and one for accessing Dapr. each channel works by itself, but both in one app fail