modules/coordinator/src/main/proto/coordinator_service.proto

@@ -51,7 +51,9 @@ message EvictGamesResponse {
   int32 evictedCount = 1;
 }
 
-message DrainInstanceRequest {}
+message DrainInstanceRequest {
+  string instanceId = 1;
+}
 
 message DrainInstanceResponse {
   int32 gamesMigrated = 1;

modules/coordinator/src/main/scala/de/nowchess/coordinator/grpc/CoordinatorGrpcServer.scala

@@ -90,10 +90,10 @@ class CoordinatorGrpcServer extends CoordinatorServiceGrpc.CoordinatorServiceImp
       request: DrainInstanceRequest,
       responseObserver: StreamObserver[DrainInstanceResponse],
   ): Unit =
-    log.info("Drain instance request")
-    val response = DrainInstanceResponse
-      .newBuilder()
-      .setGamesMigrated(0)
-      .build()
+    val instanceId = request.getInstanceId
+    log.infof("Drain request for instance %s", instanceId)
+    val gamesBefore = instanceRegistry.getInstance(instanceId).map(_.subscriptionCount).getOrElse(0)
+    failoverService.onInstanceStreamDropped(instanceId)
+    val response = DrainInstanceResponse.newBuilder().setGamesMigrated(gamesBefore).build()
     responseObserver.onNext(response)
     responseObserver.onCompleted()

modules/coordinator/src/main/scala/de/nowchess/coordinator/grpc/CoreGrpcClient.scala

@@ -1,82 +1,65 @@
 package de.nowchess.coordinator.grpc
 
 import jakarta.enterprise.context.ApplicationScoped
+import jakarta.annotation.PreDestroy
 import org.jboss.logging.Logger
 import io.grpc.ManagedChannel
 import io.grpc.ManagedChannelBuilder
 import de.nowchess.coordinator.proto.{CoordinatorServiceGrpc, *}
 import scala.jdk.CollectionConverters.*
+import java.util.concurrent.ConcurrentHashMap
+import java.util.concurrent.TimeUnit
 
 @ApplicationScoped
 class CoreGrpcClient:
-  private val log = Logger.getLogger(classOf[CoreGrpcClient])
+  private val log      = Logger.getLogger(classOf[CoreGrpcClient])
+  private val channels = ConcurrentHashMap[String, ManagedChannel]()
+
+  private def getChannel(host: String, port: Int): ManagedChannel =
+    channels.computeIfAbsent(s"$host:$port", _ =>
+      ManagedChannelBuilder.forAddress(host, port).usePlaintext().build(),
+    )
+
+  private def evictStaleChannel(host: String, port: Int): Unit =
+    Option(channels.remove(s"$host:$port")).foreach(_.shutdownNow())
+
+  @PreDestroy
+  def shutdown(): Unit =
+    channels.values.asScala.foreach { ch =>
+      ch.shutdown()
+      if !ch.awaitTermination(5, TimeUnit.SECONDS) then ch.shutdownNow()
+    }
+    channels.clear()
 
   def batchResubscribeGames(host: String, port: Int, gameIds: List[String]): Int =
-    val channel = createChannel(host, port)
     try
-      val stub = CoordinatorServiceGrpc.newStub(channel)
-      val request = BatchResubscribeRequest
-        .newBuilder()
-        .addAllGameIds(gameIds.asJava)
-        .build()
-
-      val latch  = new java.util.concurrent.CountDownLatch(1)
-      var result = 0
-
-      stub.batchResubscribeGames(
-        request,
-        new io.grpc.stub.StreamObserver[BatchResubscribeResponse]:
-          override def onNext(response: BatchResubscribeResponse): Unit =
-            result = response.getSubscribedCount
-
-          override def onError(t: Throwable): Unit =
-            log.warnf(t, "batchResubscribeGames RPC failed for %s:%d", host, port)
-            latch.countDown()
-
-          override def onCompleted(): Unit =
-            latch.countDown(),
-      )
-
-      latch.await()
-      result
-    finally channel.shutdown()
+      val stub    = CoordinatorServiceGrpc.newBlockingStub(getChannel(host, port))
+      val request = BatchResubscribeRequest.newBuilder().addAllGameIds(gameIds.asJava).build()
+      stub.batchResubscribeGames(request).getSubscribedCount
+    catch
+      case ex: Exception =>
+        log.warnf(ex, "batchResubscribeGames RPC failed for %s:%d", host, port)
+        evictStaleChannel(host, port)
+        0
 
   def unsubscribeGames(host: String, port: Int, gameIds: List[String]): Int =
-    val channel = createChannel(host, port)
     try
-      val stub = CoordinatorServiceGrpc.newBlockingStub(channel)
-      val request = UnsubscribeGamesRequest
-        .newBuilder()
-        .addAllGameIds(gameIds.asJava)
-        .build()
-
-      val response = stub.unsubscribeGames(request)
-      response.getUnsubscribedCount
+      val stub    = CoordinatorServiceGrpc.newBlockingStub(getChannel(host, port))
+      val request = UnsubscribeGamesRequest.newBuilder().addAllGameIds(gameIds.asJava).build()
+      stub.unsubscribeGames(request).getUnsubscribedCount
     catch
       case ex: Exception =>
         log.warnf(ex, "unsubscribeGames RPC failed for %s:%d", host, port)
+        evictStaleChannel(host, port)
         0
-    finally channel.shutdown()
 
   def evictGames(host: String, port: Int, gameIds: List[String]): Int =
-    val channel = createChannel(host, port)
     try
-      val stub = CoordinatorServiceGrpc.newBlockingStub(channel)
-      val request = EvictGamesRequest
-        .newBuilder()
-        .addAllGameIds(gameIds.asJava)
-        .build()
-
-      val response = stub.evictGames(request)
-      response.getEvictedCount
+      val stub    = CoordinatorServiceGrpc.newBlockingStub(getChannel(host, port))
+      val request = EvictGamesRequest.newBuilder().addAllGameIds(gameIds.asJava).build()
+      stub.evictGames(request).getEvictedCount
     catch
       case ex: Exception =>
         log.warnf(ex, "evictGames RPC failed for %s:%d", host, port)
+        evictStaleChannel(host, port)
         0
-    finally channel.shutdown()
-
-  private def createChannel(host: String, port: Int): ManagedChannel =
-    ManagedChannelBuilder
-      .forAddress(host, port)
-      .usePlaintext()
-      .build()

modules/coordinator/src/main/scala/de/nowchess/coordinator/resource/CoordinatorResource.scala

@@ -78,6 +78,7 @@ class CoordinatorResource:
   @Produces(Array(MediaType.APPLICATION_JSON))
   def triggerScaleUp: scala.collection.Map[String, String] =
     log.info("Manual scale up triggered")
+    autoScaler.scaleUp()
     Map("status" -> "scale_up_started")
 
   @POST
@@ -85,6 +86,7 @@ class CoordinatorResource:
   @Produces(Array(MediaType.APPLICATION_JSON))
   def triggerScaleDown: scala.collection.Map[String, String] =
     log.info("Manual scale down triggered")
+    autoScaler.scaleDown()
     Map("status" -> "scale_down_started")
 
 case class MetricsDto(

modules/coordinator/src/main/scala/de/nowchess/coordinator/service/AutoScaler.scala

@@ -3,8 +3,10 @@ package de.nowchess.coordinator.service
 import jakarta.enterprise.context.ApplicationScoped
 import jakarta.inject.Inject
 import de.nowchess.coordinator.config.CoordinatorConfig
+import io.fabric8.kubernetes.api.model.GenericKubernetesResource
 import io.fabric8.kubernetes.client.KubernetesClient
 import org.jboss.logging.Logger
+
 import scala.compiletime.uninitialized
 
 @ApplicationScoped
@@ -19,30 +21,26 @@ class AutoScaler:
   private var instanceRegistry: InstanceRegistry = uninitialized
 
   private val log           = Logger.getLogger(classOf[AutoScaler])
-  private var lastScaleTime = 0L
+  private val lastScaleTime = new java.util.concurrent.atomic.AtomicLong(0L)
 
   def checkAndScale: Unit =
     if !config.autoScaleEnabled then return
 
-    val now = System.currentTimeMillis()
-    if now - lastScaleTime < 120000 then // 2 minute backoff
-      return
+    val now  = System.currentTimeMillis()
+    val last = lastScaleTime.get()
+    if now - last < 120000 then return
+    if !lastScaleTime.compareAndSet(last, now) then return
 
     val instances = instanceRegistry.getAllInstances.filter(_.state == "HEALTHY")
     if instances.isEmpty then return
 
-    val avgLoad     = instances.map(_.subscriptionCount).sum.toDouble / instances.size
-    val maxCapacity = config.maxGamesPerCore * instances.size
+    val avgLoad = instances.map(_.subscriptionCount).sum.toDouble / instances.size
 
-    if avgLoad > config.scaleUpThreshold * config.maxGamesPerCore then
-      scaleUp()
-      lastScaleTime = now
+    if avgLoad > config.scaleUpThreshold * config.maxGamesPerCore then scaleUp()
     else if avgLoad < config.scaleDownThreshold * config.maxGamesPerCore && instances.size > config.scaleMinReplicas
-    then
-      scaleDown()
-      lastScaleTime = now
+    then scaleDown()
 
-  private def scaleUp(): Unit =
+  def scaleUp(): Unit =
     log.info("Scaling up Argo Rollout")
     if kubeClient == null then
       log.warn("Kubernetes client not available, cannot scale")
@@ -50,7 +48,7 @@ class AutoScaler:
 
     try
       val rollout = kubeClient
-        .resources(classOf[io.fabric8.kubernetes.api.model.GenericKubernetesResource])
+        .resources(classOf[GenericKubernetesResource])
         .inNamespace(config.k8sNamespace)
         .withName(config.k8sRolloutName)
         .get()
@@ -63,7 +61,7 @@ class AutoScaler:
         if currentReplicas < maxReplicas then
           spec.put("replicas", currentReplicas + 1)
           kubeClient
-            .resources(classOf[io.fabric8.kubernetes.api.model.GenericKubernetesResource])
+            .resources(classOf[GenericKubernetesResource])
             .inNamespace(config.k8sNamespace)
             .withName(config.k8sRolloutName)
             .createOrReplace(rollout)
@@ -73,7 +71,7 @@ class AutoScaler:
       case ex: Exception =>
         log.warnf(ex, "Failed to scale up %s", config.k8sRolloutName)
 
-  private def scaleDown(): Unit =
+  def scaleDown(): Unit =
     log.info("Scaling down Argo Rollout")
     if kubeClient == null then
       log.warn("Kubernetes client not available, cannot scale")
@@ -81,7 +79,7 @@ class AutoScaler:
 
     try
       val rollout = kubeClient
-        .resources(classOf[io.fabric8.kubernetes.api.model.GenericKubernetesResource])
+        .resources(classOf[GenericKubernetesResource])
         .inNamespace(config.k8sNamespace)
         .withName(config.k8sRolloutName)
         .get()
@@ -94,7 +92,7 @@ class AutoScaler:
         if currentReplicas > minReplicas then
           spec.put("replicas", currentReplicas - 1)
           kubeClient
-            .resources(classOf[io.fabric8.kubernetes.api.model.GenericKubernetesResource])
+            .resources(classOf[GenericKubernetesResource])
             .inNamespace(config.k8sNamespace)
             .withName(config.k8sRolloutName)
             .createOrReplace(rollout)

modules/coordinator/src/main/scala/de/nowchess/coordinator/service/FailoverService.scala

@@ -4,9 +4,7 @@ import jakarta.enterprise.context.ApplicationScoped
 import jakarta.inject.Inject
 import org.redisson.api.RedissonClient
 import scala.jdk.CollectionConverters.*
-import scala.concurrent.duration.*
 import scala.compiletime.uninitialized
-import java.time.Instant
 import org.jboss.logging.Logger
 import de.nowchess.coordinator.dto.InstanceMetadata
 import de.nowchess.coordinator.grpc.CoreGrpcClient
@@ -67,17 +65,25 @@ class FailoverService:
     val batches   = gameIds.grouped(batchSize).toList
 
     batches.zipWithIndex.foreach { case (batch, idx) =>
-      val targetInstance = healthyInstances(idx % healthyInstances.size)
-      try
-        val subscribed = coreGrpcClient.batchResubscribeGames(
-          targetInstance.hostname,
-          targetInstance.grpcPort,
-          batch,
+      var migrated = false
+      var attempt  = 0
+      while !migrated && attempt < healthyInstances.size do
+        val target = healthyInstances((idx + attempt) % healthyInstances.size)
+        attempt += 1
+        try
+          val subscribed = coreGrpcClient.batchResubscribeGames(target.hostname, target.grpcPort, batch)
+          if subscribed > 0 then
+            log.infof("Migrated %d games from %s to %s", subscribed, deadInstanceId, target.instanceId)
+            migrated = true
+        catch
+          case ex: Exception =>
+            log.warnf(ex, "Failed to migrate batch to %s, trying next", target.instanceId)
+      if !migrated then
+        log.errorf(
+          "Failed to migrate batch of %d games from %s to any healthy instance",
+          batch.size,
+          deadInstanceId,
         )
-        log.infof("Migrated %d games from %s to %s", subscribed, deadInstanceId, targetInstance.instanceId)
-      catch
-        case ex: Exception =>
-          log.warnf(ex, "Failed to migrate batch to %s, will retry", targetInstance.instanceId)
     }
 
   private def cleanupDeadInstance(instanceId: String): Unit =

modules/coordinator/src/main/scala/de/nowchess/coordinator/service/InstanceRegistry.scala

@@ -4,11 +4,10 @@ import jakarta.enterprise.context.ApplicationScoped
 import jakarta.inject.Inject
 import org.redisson.api.RedissonClient
 import scala.jdk.CollectionConverters.*
-import scala.collection.mutable
 import scala.compiletime.uninitialized
 import com.fasterxml.jackson.databind.ObjectMapper
 import de.nowchess.coordinator.dto.InstanceMetadata
-import java.time.Instant
+import java.util.concurrent.ConcurrentHashMap
 
 @ApplicationScoped
 class InstanceRegistry:
@@ -16,29 +15,17 @@ class InstanceRegistry:
   private var redissonClient: RedissonClient = uninitialized
 
   private val mapper      = ObjectMapper()
-  private val instances   = mutable.Map[String, InstanceMetadata]()
+  private val instances   = ConcurrentHashMap[String, InstanceMetadata]()
   private var redisPrefix = "nowchess"
 
   def setRedisPrefix(prefix: String): Unit =
     redisPrefix = prefix
 
   def getInstance(instanceId: String): Option[InstanceMetadata] =
-    instances.get(instanceId)
+    Option(instances.get(instanceId))
 
   def getAllInstances: List[InstanceMetadata] =
-    instances.values.toList
-
-  def listInstancesFromRedis: List[InstanceMetadata] =
-    val pattern = s"$redisPrefix:instances:*"
-    val keys    = redissonClient.getKeys.getKeysByPattern(pattern, 100)
-    keys.asScala.flatMap { key =>
-      val bucket = redissonClient.getBucket[String](key)
-      val value  = bucket.getAndDelete()
-      if value != null then
-        try Some(mapper.readValue(value, classOf[InstanceMetadata]))
-        catch case _: Exception => None
-      else None
-    }.toList
+    instances.values.asScala.toList
 
   def updateInstanceFromRedis(instanceId: String): Unit =
     val key    = s"$redisPrefix:instances:$instanceId"
@@ -47,14 +34,13 @@ class InstanceRegistry:
     if value != null then
       try
         val metadata = mapper.readValue(value, classOf[InstanceMetadata])
-        instances(instanceId) = metadata
+        instances.put(instanceId, metadata)
       catch case _: Exception => ()
 
   def markInstanceDead(instanceId: String): Unit =
-    instances.get(instanceId).foreach { inst =>
-      val dead = inst.copy(state = "DEAD")
-      instances(instanceId) = dead
-    }
+    instances.computeIfPresent(instanceId, (_, inst) => inst.copy(state = "DEAD"))
+    ()
 
   def removeInstance(instanceId: String): Unit =
     instances.remove(instanceId)
+    ()

modules/coordinator/src/main/scala/de/nowchess/coordinator/service/LoadBalancer.scala

@@ -25,7 +25,7 @@ class LoadBalancer:
   private var coreGrpcClient: CoreGrpcClient = uninitialized
 
   private val log               = Logger.getLogger(classOf[LoadBalancer])
-  private var lastRebalanceTime = 0L
+  private val lastRebalanceTime = new java.util.concurrent.atomic.AtomicLong(0L)
   private var redisPrefix       = "nowchess"
 
   def setRedisPrefix(prefix: String): Unit =
@@ -34,7 +34,7 @@ class LoadBalancer:
   def shouldRebalance: Boolean =
     val now         = System.currentTimeMillis()
     val minInterval = config.rebalanceMinInterval.toMillis
-    if now - lastRebalanceTime < minInterval then return false
+    if now - lastRebalanceTime.get() < minInterval then return false
 
     val instances = instanceRegistry.getAllInstances
     if instances.isEmpty then return false
@@ -54,11 +54,10 @@ class LoadBalancer:
   def rebalance: Unit =
     log.info("Starting rebalance")
     val startTime = System.currentTimeMillis()
-    lastRebalanceTime = startTime
+    lastRebalanceTime.set(startTime)
 
     try
-      val instances = instanceRegistry.getAllInstances
-        .filter(_.state == "HEALTHY")
+      val instances = instanceRegistry.getAllInstances.filter(_.state == "HEALTHY")
 
       if instances.size < 2 then
         log.info("Not enough healthy instances for rebalance")
@@ -75,23 +74,29 @@ class LoadBalancer:
         .filter(_.subscriptionCount < avgLoad * 0.8)
         .sortBy(_.subscriptionCount)
 
-      overloaded.foreach { over =>
-        val excess = over.subscriptionCount - avgLoad.toInt
-        if excess > 0 && underloaded.nonEmpty then
-          val gamesToMove = getGamesToMove(over.instanceId, excess)
-          if gamesToMove.nonEmpty then
-            underloaded.headOption.foreach { under =>
-              try
-                val unsubscribed = coreGrpcClient.unsubscribeGames(over.hostname, over.grpcPort, gamesToMove)
-                val subscribed   = coreGrpcClient.batchResubscribeGames(under.hostname, under.grpcPort, gamesToMove)
+      if underloaded.isEmpty then
+        log.info("No underloaded instances available for rebalance")
+        return
 
-                if subscribed > 0 then
-                  updateRedisGameSets(over.instanceId, under.instanceId, gamesToMove)
-                  log.infof("Moved %d games from %s to %s", subscribed, over.instanceId, under.instanceId)
-              catch
-                case ex: Exception =>
-                  log.warnf(ex, "Failed to move games from %s to %s", over.instanceId, under.instanceId)
-            }
+      var targetIdx = 0
+      overloaded.foreach { over =>
+        val excess      = math.max(0, over.subscriptionCount - avgLoad.toInt)
+        val gamesToMove = getGamesToMove(over.instanceId, excess)
+        if gamesToMove.nonEmpty then
+          val batchSize = math.max(1, (gamesToMove.size + underloaded.size - 1) / underloaded.size)
+          gamesToMove.grouped(batchSize).foreach { batch =>
+            val target = underloaded(targetIdx % underloaded.size)
+            targetIdx += 1
+            try
+              coreGrpcClient.unsubscribeGames(over.hostname, over.grpcPort, batch)
+              val subscribed = coreGrpcClient.batchResubscribeGames(target.hostname, target.grpcPort, batch)
+              if subscribed > 0 then
+                updateRedisGameSets(over.instanceId, target.instanceId, batch)
+                log.infof("Moved %d games from %s to %s", subscribed, over.instanceId, target.instanceId)
+            catch
+              case ex: Exception =>
+                log.warnf(ex, "Failed to move games from %s to %s", over.instanceId, target.instanceId)
+          }
       }
 
       val elapsed = System.currentTimeMillis() - startTime