GraphQL UI does not show exceptions that are thrown inside subscription

[Bazalii]

After successful connection to server with subscription endpoint graphql UI in dev mode doesn't show exceptions that are thrown by subscription. Is it an expected behaviour?

For example, I have created a subscription inside which an exception is thrown when given token is not "123"

@Subscription
fun subscribeToUsersWithFilter(context: Context, token: String, userName: String): Multi<User> {
    if (token != "123"){
        throw CustomException("Invalid token!")
    }

    return _broadcastProcessor.filter { user -> user.name == userName }
}

But dev UI shows only that connection was successful and nothing else.

Furthermore, I don't see any error messages in my message history of websocket connection in browser

[quarkus-bot[bot]]

/cc @jmartisk (graphql), @phillip-kruger (graphql)

[phillip-kruger]

Are there any errors in the browser's console ? I don't think (even though the ui say it did) a connection has been established. The Error happens before that. Can you share a reproducer ?

[phillip-kruger]

We can probably move this discussion to SmallRye.

[Bazalii]

This is the reproducer.

First, send request for subscription with invalid "token":

subscription subscribeToUsersWithFilter($token: String!, $userName: String!) {
  subscribeToUsersWithFilter(token: $token, userName: $userName) {
    id
    name
    roles
  }
}

variables:

{
  "userName": "Test",
  "token": "12"
}

After that send mutation:

mutation addUser($user: UserInput!) {
    addUser(user: $user) {
    id
    name
    roles
  }

variables:

{
  "user": {
    "id": "00000000-0000-0100-0000-000000000000",
    "name": "Test",
    "roles": [
      "Worker",
      "User"
    ]
  }
}

Exception is thrown inside the application but nothing is send to the client.

Another thing here is If you comment line 37 in UserController, you will see that exception is thrown when the first User is added using mutation and it is sent after connection and processed correctly.

[jmartisk]

It seems we're not properly handling the situation that the subscription method itself throws an exception, instead of returning a failed Multi. Generally, if a method returns a Multi, it should always return it, and preferably an exception should be wrapped in it, instead of being thrown from the method itself (so, you return Multi.createFrom().failure(exception). In that case the server sends a proper error message and ends the operation.

But yes, the library should expect that this can happen. I'll look into fixing this.

[jmartisk]

I've submitted a PR that should help with this case: smallrye/smallrye-graphql#1705
In any case I still recommend returning a failed Multi instead of throwing an exception directly.

[Bazalii]

Okay, thank you!