Filter AppSync Subscription - amazon-web-services

Question

I'm building a simple multiplayer game using AppSync (to avoid having to manage WebSocket from zero) and Amplify (specifically on Android, using Kotlin).
The problem is that when players are connected to a game, should listen for updates for the other players (consider that I have a table on dynamo with Primary Key a UUID and the corresponding game-id and position of the player).
So what I've done, is that after the player joins his game session register itself to onUpdate of Player... however now he receives the notification of the update of all the player... and even though I can easily filter this incoming events client side, I think it's a terrible idea, since AppSync have to notify to all registered users, all the updates.
Instead I would like to specify a filter on the subscription, like when you perform a query, something like:
Amplify.API.subscribe(
ModelSubscription.onUpdate(
Player::class.java,
Player.GAMEROOM.eq(currentPlayer.gameroom.id) // only players in the same gameroom
),
{ Log.i("ApiQuickStart", "Subscription established") },
{ onCreated ->
...
},
{ onFailure -> Log.e("ApiQuickStart", "Subscription failed", onFailure) },
{ Log.i("ApiQuickStart", "Subscription completed") }
)
Are there ways to obtain this?
Since this is the only tipe of subscription that I will do to Player Update, maybe with a custom resolver something might be done?

Answer 1

You need to add one more param into your subscription, and its subscribed mutations. I guess your GraphQL schema looks like
type Subscription {
onUpdate(gameRoomId: ID!, playerId: ID): UpdateUserResult
#aws_subscribe(mutations: ["leaveCurrentGameRoom", <other_interested_mutation>])
}
type Mutation {
leaveCurrentGameRoom: UpdateUserResult
}
type UpdateUserResult {
gameRoomId: ID!
playerId: ID!
// other fields
}
You notice Subscription.onUpdate has an optional param, playerId. The subscription can be used as follows
onUpdate("room-1") // AppSync gives updates of all players in "room-1"
onUpdate("room-1", "player-A") // AppSync gives updates of "player-A" in "room-1"
In order for AppSync to filter out irrelevant updates, the subscribed mutations, such as leaveCurrentGameRoom, have to return a type which includes the two fields, gameRoomId, and playerId. Please note that you may also update the resolvers of the mutations to resolve data for the fields
Here is another example of subscription arguments
https://docs.aws.amazon.com/appsync/latest/devguide/aws-appsync-real-time-data.html#using-subscription-arguments