Filtering Subscriptions by Event Type

[PvtPuddles]

Hello,

I have a very noisy event bus that I would like users to be able to subscribe to and only get events that they asked for.

Using the example from the documentation, I have a userEvents field that is quite chatty.

A user may write a query like this:

subscription {
    userEvents {
        __typename
        ... on UserCreated {
            id
            name
        }
    }
}

My current understanding is that on UserUpdated events, the server will still send the event to the client, but the event body will be empty (aside from the __typename). On a busy bus, this could be hundreds of empty events.

Ideally, I would like the server to filter out events that don't have any requested fields, but I am not sure how to go about this.

[yanns]

I don't think you can use __typename as this is for introspection.
It will always return the type of id and name, and not the values

What you could do:

• userEvents returns an interface, that UserCreated implements.
• then the query would look like:

subscription {
    userEvents {
        ... on UserCreated {
            id
            name
    }
}

Inside the resolve function, you have access to ctx.astFields. With that, you could find the information UserCreated and adapt the resolution.

Another way is to use parameters:

subscription {
    userEvents(eventTypes: "UserCreated") {
        ... on UserCreated {
            id
            name
    }
}

but it's a bit redundant.

[PvtPuddles]

I was able to access the requested events via:

val requestedEvents = ctx.astFields.head.selections.collect {
  case InlineFragment(Some(typeCondition), _, _, _, _, _) =>
    typeCondition.name
  }

This allowed me to use server-side filtering to only send events to the client if there is a corresponding on clause, which enables things like:

subscription {
  snapshots {
    timestamp
    ... on SystemSnapshot { someSystemField }
    ... on DeviceSnapshot { someDeviceField }
  }
}

Thank you, this was very helpful.