implement cluster stubs: cross-node routing, rebalancing, and actor migration

- Fix ConsistentHashPlacement.PlaceActor() to use consistent hash ring
- Implement ConsistentHashPlacement.RebalanceShards() to redistribute shards
- Implement ClusterManager.handleRebalanceRequest() with actual rebalancing
- Implement ClusterManager.handleMigrationRequest() for actor state transfer
- Implement ClusterManager.triggerShardRebalancing() to compute and broadcast
- Implement DistributedVM.SendMessage() with cross-node NATS routing
- Implement DistributedVM.handleRebalanceRequest() to update shard map
- Fix route_message handler to check if actor is local before delivery
- Update ConsistentHashPlacement.RebalanceShards() test for new behavior
- Add handleShardMapUpdate() and broadcastShardMap() to ClusterManager

cluster/distributed.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"encoding/json"
 	"fmt"
+	"time"
 
 	"github.com/nats-io/nats.go"
 )
@@ -137,11 +138,30 @@ func (dvm *DistributedVM) LoadModel(model RuntimeModel) error {
 
 // SendMessage routes messages across the distributed cluster
 func (dvm *DistributedVM) SendMessage(message RuntimeMessage) error {
-	// This is a simplified implementation
-	// In practice, this would determine the target node based on sharding
-	// and route the message appropriately
+	actorID := message.GetTargetActorID()
+	targetNode := dvm.GetActorNode(actorID)
 
-	return dvm.localRuntime.SendMessage(message)
+	if targetNode == dvm.nodeID {
+		return dvm.localRuntime.SendMessage(message)
+	}
+
+	return dvm.routeMessageToNode(actorID, message)
+}
+
+// routeMessageToNode sends a message to another node for delivery to the target actor
+func (dvm *DistributedVM) routeMessageToNode(actorID string, message RuntimeMessage) error {
+	msg := ClusterMessage{
+		Type: "route_message",
+		From: dvm.nodeID,
+		To:   actorID,
+		Payload: MessagePayload{
+			TargetActorID: actorID,
+			Type:          message.GetType(),
+		},
+		Timestamp: time.Now(),
+	}
+
+	return dvm.publishClusterMessage(msg)
 }
 
 // GetActorNode determines which node should handle a specific actor
@@ -199,7 +219,14 @@ func (dvm *DistributedVM) handleClusterMessage(msg *nats.Msg) {
 		if err := json.Unmarshal(payloadBytes, &message); err != nil {
 			return
 		}
-		dvm.localRuntime.SendMessage(&message)
+
+		targetActor := message.TargetActorID
+		if dvm.IsLocalActor(targetActor) {
+			dvm.localRuntime.SendMessage(&message)
+		} else {
+			// Relay to the correct node
+			dvm.routeMessageToNode(targetActor, &message)
+		}
 
 	case "rebalance":
 		// Handle shard rebalancing requests
@@ -209,8 +236,27 @@ func (dvm *DistributedVM) handleClusterMessage(msg *nats.Msg) {
 
 // handleRebalanceRequest processes shard rebalancing requests
 func (dvm *DistributedVM) handleRebalanceRequest(msg ClusterMessage) {
-	// Simplified rebalancing logic
-	// In practice, this would implement complex actor migration
+	payloadBytes, err := json.Marshal(msg.Payload)
+	if err != nil {
+		dvm.cluster.logger.Printf("Failed to marshal rebalance payload: %v", err)
+		return
+	}
+
+	var newShardMap ShardMap
+	if err := json.Unmarshal(payloadBytes, &newShardMap); err != nil {
+		dvm.cluster.logger.Printf("Failed to unmarshal shard map: %v", err)
+		return
+	}
+
+	dvm.cluster.mutex.Lock()
+	if newShardMap.Version > dvm.cluster.shardMap.Version {
+		dvm.cluster.shardMap = &newShardMap
+		dvm.cluster.logger.Printf("Applied new shard map (version %d) from rebalance", newShardMap.Version)
+	} else {
+		dvm.cluster.logger.Printf("Ignoring stale shard map (got version %d, current %d)",
+			newShardMap.Version, dvm.cluster.shardMap.Version)
+	}
+	dvm.cluster.mutex.Unlock()
 }
 
 // publishClusterMessage sends a message to other cluster nodes