Difference between tell and ask in a shard

@alan.brewster73 wrote:

Hello.

Could someone please explain in the below snippet why the tellShard call always work as expected in a multi-node cluster. But the askShard call only works if an external call hits the node that hosts the shard and times out on all other nodes? Many thanks.

  val sharding = ClusterSharding(system)

  val messageExtractor =
    new HashCodeNoEnvelopeMessageExtractor[ShoppingCartActor.CartCommand](numberOfShards = 30) {
      override def entityId(message: ShoppingCartActor.CartCommand): String = "Cart"
    }

  val shardRegion: ActorRef[ShoppingCartActor.CartCommand] = sharding.init(
      Entity(ShoppingCartActor.TypeKey) { context =>
        ShoppingCartActor()
      }.withMessageExtractor(messageExtractor)
  )


  def tellShard() = {
    shardRegion ! ShoppingCartActor.Ping
  }

  def askShard(id: String, quantity: Int): Future[CartState] = {
    shardRegion ? (ShoppingCartActor.AddItem(id, quantity, _))
  }

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