Different use case for akka cluster aware router & akka cluster sharding?

Question

1. Cluster aware router:

   val router = system.actorOf(ClusterRouterPool(
     RoundRobinPool(0),
     ClusterRouterPoolSettings(
       totalInstances = 20,
       maxInstancesPerNode = 1,
       allowLocalRoutees = false,
       useRole = None
     )
   ).props(Props[Worker]), name = "router")
   

   Here, we can send message to router, the message will send to a series of remote routee actors.

2. Cluster sharding (Not consider persistence)

   class NewShoppers extends Actor {
     ClusterSharding(context.system).start(
       "shardshoppers",
       Props(new Shopper),
       ClusterShardingSettings(context.system),
       Shopper.extractEntityId,
       Shopper.extractShardId
     )
   
     def proxy = {
       ClusterSharding(context.system).shardRegion("shardshoppers")
     }
   
     override def receive: Receive = {
       case msg => proxy forward msg
     }
   }
   

   Here, we can send message to proxy, the message will send to a series of sharded actors (a.k.a. entities).

So, my question is: it seems both 2 methods can make the tasks distribute to a lot of actors. What's the design choice of above two? Which situation need which choice?

Answer 1

The pool router would be when you just want to send some work to whatever node and have some processing happen, two messages sent in sequence will likely not end up in the same actor for processing.

Cluster sharding is for when you have a unique id on each actor of some kind, and you have too many of them to fit in one node, but you want every message with that id to always end up in the actor for that id. For example modelling a User as an entity, you want all commands about that user to end up with the user but you want the actor to be moved if the cluster topology changes (remove or add nodes) and you want them reasonably balanced across the existing nodes.