From f327441089f2e58b30e3da91b2218daaf8420e75 Mon Sep 17 00:00:00 2001
From: Janis <janis-e@gmx.de>
Date: Sun, 26 Apr 2026 08:34:53 +0200
Subject: [PATCH] feat(coordinator): scaffold microservice for <300ms failover
 and load balancing
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

- Add coordinator module with gRPC stream-based instance health detection
- Implement InstanceHeartbeatService in core: bidirectional stream to coordinator every 200ms
- Track game subscriptions per core via Redis Sets (SADD/SREM)
- Add gRPC handlers for batch resubscribe/unsubscribe/evict/drain operations
- Implement coordinator services: InstanceRegistry, FailoverService, LoadBalancer, AutoScaler, CacheEvictionManager
- Add REST API for metrics and manual failover/rebalance/scaling
- Proto definition: coordinator_service.proto with HeartbeatStream + batch game operations
- Failover timeline: gRPC stream drop (50-200ms) → game migration (<300ms target)
- Support for Argo Rollouts auto-scaling (k8s CRD patching via Fabric8 client)

Note: Proto compilation issues documented in COORDINATOR_IMPLEMENTATION.md. Requires:
- Add task dependency: tasks.compileScala dependsOn tasks.compileJava
- Fix deprecated @Inject var = _ → = uninitialized syntax
- Implement remaining service methods (gRPC clients, FailoverService distribution)

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>
---
 .idea/gradle.xml                              |   2 +
 .idea/scala_compiler.xml                      |   2 +-
 COORDINATOR_IMPLEMENTATION.md                 | 316 ++++++++++++++++++
 modules/coordinator/build.gradle.kts          |  96 ++++++
 .../src/main/proto/coordinator_service.proto  |  57 ++++
 .../src/main/resources/application.yml        |  39 +++
 .../nowchess/coordinator/CoordinatorApp.scala |   7 +
 .../config/CoordinatorConfig.scala            |  55 +++
 .../coordinator/dto/InstanceMetadata.scala    |  23 ++
 .../grpc/CoordinatorGrpcServer.scala          | 110 ++++++
 .../resource/CoordinatorResource.scala        |  99 ++++++
 .../coordinator/service/AutoScaler.scala      |  58 ++++
 .../service/CacheEvictionManager.scala        |  53 +++
 .../coordinator/service/FailoverService.scala |  67 ++++
 .../coordinator/service/HealthMonitor.scala   |  68 ++++
 .../service/InstanceRegistry.scala            |  63 ++++
 .../coordinator/service/LoadBalancer.scala    |  44 +++
 .../src/main/proto/coordinator_service.proto  |  57 ++++
 .../core/src/main/resources/application.yml   |  20 ++
 .../grpc/CoordinatorServiceHandler.scala      |  67 ++++
 .../redis/GameRedisSubscriberManager.scala    |  37 ++
 .../service/InstanceHeartbeatService.scala    | 186 +++++++++++
 settings.gradle.kts                           |   1 +
 23 files changed, 1526 insertions(+), 1 deletion(-)
 create mode 100644 COORDINATOR_IMPLEMENTATION.md
 create mode 100644 modules/coordinator/build.gradle.kts
 create mode 100644 modules/coordinator/src/main/proto/coordinator_service.proto
 create mode 100644 modules/coordinator/src/main/resources/application.yml
 create mode 100644 modules/coordinator/src/main/scala/de/nowchess/coordinator/CoordinatorApp.scala
 create mode 100644 modules/coordinator/src/main/scala/de/nowchess/coordinator/config/CoordinatorConfig.scala
 create mode 100644 modules/coordinator/src/main/scala/de/nowchess/coordinator/dto/InstanceMetadata.scala
 create mode 100644 modules/coordinator/src/main/scala/de/nowchess/coordinator/grpc/CoordinatorGrpcServer.scala
 create mode 100644 modules/coordinator/src/main/scala/de/nowchess/coordinator/resource/CoordinatorResource.scala
 create mode 100644 modules/coordinator/src/main/scala/de/nowchess/coordinator/service/AutoScaler.scala
 create mode 100644 modules/coordinator/src/main/scala/de/nowchess/coordinator/service/CacheEvictionManager.scala
 create mode 100644 modules/coordinator/src/main/scala/de/nowchess/coordinator/service/FailoverService.scala
 create mode 100644 modules/coordinator/src/main/scala/de/nowchess/coordinator/service/HealthMonitor.scala
 create mode 100644 modules/coordinator/src/main/scala/de/nowchess/coordinator/service/InstanceRegistry.scala
 create mode 100644 modules/coordinator/src/main/scala/de/nowchess/coordinator/service/LoadBalancer.scala
 create mode 100644 modules/core/src/main/proto/coordinator_service.proto
 create mode 100644 modules/core/src/main/scala/de/nowchess/chess/grpc/CoordinatorServiceHandler.scala
 create mode 100644 modules/core/src/main/scala/de/nowchess/chess/service/InstanceHeartbeatService.scala

diff --git a/.idea/gradle.xml b/.idea/gradle.xml
index 92c218c..49efe00 100644
--- a/.idea/gradle.xml
+++ b/.idea/gradle.xml
@@ -13,10 +13,12 @@
             <option value="$PROJECT_DIR$/modules/account" />
             <option value="$PROJECT_DIR$/modules/api" />
             <option value="$PROJECT_DIR$/modules/bot" />
+            <option value="$PROJECT_DIR$/modules/coordinator" />
             <option value="$PROJECT_DIR$/modules/core" />
             <option value="$PROJECT_DIR$/modules/io" />
             <option value="$PROJECT_DIR$/modules/json" />
             <option value="$PROJECT_DIR$/modules/rule" />
+            <option value="$PROJECT_DIR$/modules/store" />
             <option value="$PROJECT_DIR$/modules/ws" />
           </set>
         </option>
diff --git a/.idea/scala_compiler.xml b/.idea/scala_compiler.xml
index 238131e..08120e7 100644
--- a/.idea/scala_compiler.xml
+++ b/.idea/scala_compiler.xml
@@ -5,7 +5,7 @@
       <option name="deprecationWarnings" value="true" />
       <option name="uncheckedWarnings" value="true" />
     </profile>
-    <profile name="Gradle 2" modules="NowChessSystems.modules.account.integrationTest,NowChessSystems.modules.account.main,NowChessSystems.modules.account.native-test,NowChessSystems.modules.account.quarkus-generated-sources,NowChessSystems.modules.account.quarkus-test-generated-sources,NowChessSystems.modules.account.scoverage,NowChessSystems.modules.account.test,NowChessSystems.modules.bot.main,NowChessSystems.modules.bot.scoverage,NowChessSystems.modules.bot.test,NowChessSystems.modules.core.integrationTest,NowChessSystems.modules.core.main,NowChessSystems.modules.core.native-test,NowChessSystems.modules.core.quarkus-generated-sources,NowChessSystems.modules.core.quarkus-test-generated-sources,NowChessSystems.modules.core.scoverage,NowChessSystems.modules.core.test,NowChessSystems.modules.io.integrationTest,NowChessSystems.modules.io.main,NowChessSystems.modules.io.native-test,NowChessSystems.modules.io.quarkus-generated-sources,NowChessSystems.modules.io.quarkus-test-generated-sources,NowChessSystems.modules.io.scoverage,NowChessSystems.modules.io.test,NowChessSystems.modules.json.main,NowChessSystems.modules.json.scoverage,NowChessSystems.modules.json.test,NowChessSystems.modules.rule.integrationTest,NowChessSystems.modules.rule.main,NowChessSystems.modules.rule.native-test,NowChessSystems.modules.rule.quarkus-generated-sources,NowChessSystems.modules.rule.quarkus-test-generated-sources,NowChessSystems.modules.rule.scoverage,NowChessSystems.modules.rule.test,NowChessSystems.modules.ui.main,NowChessSystems.modules.ui.scoverage,NowChessSystems.modules.ui.test,NowChessSystems.modules.ws.integrationTest,NowChessSystems.modules.ws.main,NowChessSystems.modules.ws.native-test,NowChessSystems.modules.ws.quarkus-generated-sources,NowChessSystems.modules.ws.quarkus-test-generated-sources,NowChessSystems.modules.ws.scoverage,NowChessSystems.modules.ws.test">
+    <profile name="Gradle 2" modules="NowChessSystems.modules.account.integrationTest,NowChessSystems.modules.account.main,NowChessSystems.modules.account.native-test,NowChessSystems.modules.account.quarkus-generated-sources,NowChessSystems.modules.account.quarkus-test-generated-sources,NowChessSystems.modules.account.scoverage,NowChessSystems.modules.account.test,NowChessSystems.modules.bot.main,NowChessSystems.modules.bot.scoverage,NowChessSystems.modules.bot.test,NowChessSystems.modules.coordinator.integrationTest,NowChessSystems.modules.coordinator.main,NowChessSystems.modules.coordinator.native-test,NowChessSystems.modules.coordinator.quarkus-generated-sources,NowChessSystems.modules.coordinator.quarkus-test-generated-sources,NowChessSystems.modules.coordinator.scoverage,NowChessSystems.modules.coordinator.test,NowChessSystems.modules.core.integrationTest,NowChessSystems.modules.core.main,NowChessSystems.modules.core.native-test,NowChessSystems.modules.core.quarkus-generated-sources,NowChessSystems.modules.core.quarkus-test-generated-sources,NowChessSystems.modules.core.scoverage,NowChessSystems.modules.core.test,NowChessSystems.modules.io.integrationTest,NowChessSystems.modules.io.main,NowChessSystems.modules.io.native-test,NowChessSystems.modules.io.quarkus-generated-sources,NowChessSystems.modules.io.quarkus-test-generated-sources,NowChessSystems.modules.io.scoverage,NowChessSystems.modules.io.test,NowChessSystems.modules.json.main,NowChessSystems.modules.json.scoverage,NowChessSystems.modules.json.test,NowChessSystems.modules.rule.integrationTest,NowChessSystems.modules.rule.main,NowChessSystems.modules.rule.native-test,NowChessSystems.modules.rule.quarkus-generated-sources,NowChessSystems.modules.rule.quarkus-test-generated-sources,NowChessSystems.modules.rule.scoverage,NowChessSystems.modules.rule.test,NowChessSystems.modules.store.integrationTest,NowChessSystems.modules.store.main,NowChessSystems.modules.store.native-test,NowChessSystems.modules.store.quarkus-generated-sources,NowChessSystems.modules.store.quarkus-test-generated-sources,NowChessSystems.modules.store.scoverage,NowChessSystems.modules.store.test,NowChessSystems.modules.ui.main,NowChessSystems.modules.ui.scoverage,NowChessSystems.modules.ui.test,NowChessSystems.modules.ws.integrationTest,NowChessSystems.modules.ws.main,NowChessSystems.modules.ws.native-test,NowChessSystems.modules.ws.quarkus-generated-sources,NowChessSystems.modules.ws.quarkus-test-generated-sources,NowChessSystems.modules.ws.scoverage,NowChessSystems.modules.ws.test">
       <option name="deprecationWarnings" value="true" />
       <option name="uncheckedWarnings" value="true" />
       <parameters>
diff --git a/COORDINATOR_IMPLEMENTATION.md b/COORDINATOR_IMPLEMENTATION.md
new file mode 100644
index 0000000..43069d4
--- /dev/null
+++ b/COORDINATOR_IMPLEMENTATION.md
@@ -0,0 +1,316 @@
+# Coordinator Microservice Implementation Guide
+
+## Status: Proto Compilation Blockers (Fixable)
+
+**Completed**: Module scaffold, InstanceHeartbeatService, GameRedisSubscriberManager updates, gRPC handlers, REST API stubs.
+
+**Blocking**: Proto file → Java stubs not resolving in Scala imports. Solution documented below.
+
+---
+
+## Architecture
+
+**Goal**: <300ms failover via gRPC bidirectional stream detection + sub-1s game migration.
+
+**Core Flow**:
+1. Core sends `HeartbeatFrame` every 200ms on stream to coordinator
+2. Core posts `{prefix}:instance:{id}:games` Redis Set (SADD on subscribe, SREM on unsubscribe)
+3. Core refreshes `{prefix}:instances:{id}` Redis key every 2s (5s TTL)
+4. Coordinator watches stream; on drop → immediate failover
+5. Failover: get `SMEMBERS {id}:games`, call `BatchResubscribeGames` on healthy cores
+
+**Key Insight**: Three detection signals (gRPC stream, Redis TTL, k8s watch), but **gRPC stream drop is primary** (50–200ms detection).
+
+---
+
+## Proto Compilation Fix
+
+### Problem
+Scala code imports `de.nowchess.coordinator.HeartbeatFrame` but proto plugin generates classes Gradle doesn't make visible.
+
+### Solution
+Quarkus gRPC plugin generates Java stubs in `build/generated/sources/protobuf/java/` during `quarkusGenerateCode` task. These are compiled to `.class` files but Scala compiler can't find them at compile time because they're not on Scala's classpath early enough.
+
+**Fix**: Add proto compilation order dependency in both modules:
+
+**modules/coordinator/build.gradle.kts** and **modules/core/build.gradle.kts**:
+```gradle
+tasks.compileScala {
+    dependsOn(tasks.named("compileJava")) // Ensures Java stubs compiled first
+}
+```
+
+Also ensure proto is on sourceSets:
+```gradle
+sourceSets {
+    main {
+        proto {
+            srcDir("src/main/proto")
+        }
+    }
+}
+```
+
+Quarkus v3.x should handle this automatically, but explicit dependency helps.
+
+### Alternative: Use Generated Java Classes Directly
+If proto stubs still not found, import **exactly as generated**:
+```scala
+// Don't try to import individual types
+import de.nowchess.coordinator.{
+  CoordinatorServiceGrpc,
+  HeartbeatFrame,
+  // ...
+}
+
+// Instead, use full paths or check actual generated names
+val frame = de.nowchess.coordinator.HeartbeatFrame.newBuilder()
+  .setInstanceId("...")
+  .build()
+```
+
+Run `./gradlew clean modules:coordinator:compileJava` to regenerate and inspect `build/generated/sources/protobuf/java/de/nowchess/coordinator/` to see actual class names.
+
+---
+
+## Code Quality Issues (Non-Blocking)
+
+**Fix in coordinator services** (already have `= _` deprecation warnings):
+
+```scala
+// OLD
+@Inject var redissonClient: RedissonClient = _
+
+// NEW
+import scala.compiletime.uninitialized
+@Inject var redissonClient: RedissonClient = uninitialized
+```
+
+**Jakarta optional injection**:
+```scala
+// Old (doesn't work)
+@Inject(optional = true) var kubeClient: KubernetesClient = _
+
+// Better (use null check)
+@Inject var kubeClient: KubernetesClient = null
+if (kubeClient != null) { ... }
+```
+
+**Method params in private helpers**: Remove unused params in `scaleUp()`, `scaleDown()`, `rebalance()`.
+
+---
+
+## Missing Implementation (Phase 2)
+
+### 1. **InstanceHeartbeatService** (DONE, needs testing)
+- [x] Startup: generate instanceId, open gRPC stream, schedule heartbeats
+- [x] Every 200ms: send `HeartbeatFrame` via stream
+- [x] Every 2s: refresh Redis TTL on `{prefix}:instances:{id}`
+- [x] `addGameSubscription(gameId)` → `SADD {id}:games {gameId}`
+- [x] `removeGameSubscription(gameId)` → `SREM {id}:games {gameId}`
+- [x] Shutdown: cleanup Redis + stream
+- [ ] **Test**: Kill core JVM, verify coordinator detects within 300ms
+
+### 2. **Coordinator HealthMonitor** (skeleton done)
+- [ ] Watch gRPC streams: on `onError()` or `onCompleted()`, mark instance DEAD
+- [ ] Fallback: poll Redis heartbeat TTL expiry every 5s
+- [ ] Fallback: k8s pod watch for label `app=nowchess-core`, detect NotReady status
+- [ ] Decision: if gRPC drop → immediate failover (no wait)
+
+### 3. **Coordinator FailoverService** (partial)
+```scala
+def onInstanceStreamDropped(instanceId: String): Unit =
+  val gameIds = SMEMBERS "{prefix}:instance:{id}:games"
+  val healthyInstances = getAllHealthyInstances()
+  
+  // Distribute games round-robin by load
+  gameIds.grouped(gameIds.size / healthyInstances.size).zipWithIndex.foreach {
+    case (batch, idx) =>
+      val target = healthyInstances(idx % healthyInstances.size)
+      call target.grpcStub.batchResubscribeGames(batch)
+  }
+  
+  DEL "{prefix}:instance:{id}:games"
+```
+
+### 4. **Coordinator gRPC Client Stubs** (need manual integration)
+Create **modules/coordinator/src/main/scala/de/nowchess/coordinator/grpc/CoreGrpcClient.scala**:
+```scala
+@ApplicationScoped
+class CoreGrpcClient:
+  @GrpcClient("core-grpc")
+  private var coreStub: CoordinatorServiceGrpc.CoordinatorServiceStub = uninitialized
+  
+  def batchResubscribeGames(host: String, port: Int, gameIds: List[String]): Int =
+    // Build request, call via dynamic stub to (host, port)
+    val response = coreStub.batchResubscribeGames(...)
+    response.getSubscribedCount
+```
+
+Need to add dynamic gRPC client (Quarkus doesn't support runtime host:port changing by default). **Workaround**: Use `io.grpc:grpc-netty-shaded` + `ManagedChannel` directly:
+```scala
+val channel = ManagedChannelBuilder.forAddress(host, port).usePlaintext().build()
+val stub = CoordinatorServiceGrpc.newStub(channel)
+```
+
+### 5. **LoadBalancer.rebalance()** (stub → full impl)
+```scala
+def rebalance(): Unit =
+  val instances = listInstancesFromRedis()
+  val loads = instances.map(_.subscriptionCount)
+  val mean = loads.sum / loads.size
+  
+  val overloaded = instances.filter(_.subscriptionCount > maxGamesPerCore)
+    .sortByDescending(_.subscriptionCount)
+  val underloaded = instances.filter(_.subscriptionCount < mean * 0.8)
+    .sortBy(_.subscriptionCount)
+  
+  overloaded.foreach { over =>
+    val excess = over.subscriptionCount - targetLoad
+    underloaded.headOption.foreach { under =>
+      val toMove = getGamesToMove(over.instanceId, excess)
+      call over.coreGrpc.unsubscribeGames(toMove)
+      call under.coreGrpc.batchResubscribeGames(toMove)
+      // Update Redis sets
+    }
+  }
+```
+
+### 6. **AutoScaler** (stub → k8s API calls)
+```scala
+def scaleUp(): Unit =
+  if (kubeClient != null && config.autoScaleEnabled) {
+    val rollout = kubeClient.resources(classOf[Rollout])
+      .inNamespace(config.k8sNamespace)
+      .withName(config.k8sRolloutName)
+      .get()
+    
+    val newReplicas = rollout.getSpec.getReplicas + 1
+    rollout.getSpec.setReplicas(newReplicas)
+    kubeClient.resources(classOf[Rollout])
+      .inNamespace(config.k8sNamespace)
+      .withName(config.k8sRolloutName)
+      .createOrReplace(rollout)
+  }
+```
+
+Requires: `io.fabric8:kubernetes-client:6.13.0` (already in build.gradle.kts).
+
+### 7. **CacheEvictionManager** (stub → full impl)
+```scala
+def evictStaleGames(): Unit =
+  val now = System.currentTimeMillis()
+  val keys = KEYS "{prefix}:game:entry:*"
+  
+  keys.foreach { key =>
+    val bucket = redissonClient.getBucket[String](key)
+    val json = bucket.get()
+    val lastUpdated = extractTimestamp(json) // Parse JSON
+    
+    if (now - lastUpdated > config.gameIdleThreshold.toMillis) {
+      val gameId = key.stripPrefix(...)
+      val instance = findInstanceWithGame(gameId)
+      
+      instance.foreach { inst =>
+        call inst.coreGrpc.evictGames(List(gameId))
+      }
+      bucket.delete()
+    }
+  }
+```
+
+### 8. **CoordinatorGrpcServer HeartbeatStream** (stub → full impl)
+```scala
+override def heartbeatStream(
+  responseObserver: StreamObserver[CoordinatorCommand]
+): StreamObserver[HeartbeatFrame] =
+  new StreamObserver[HeartbeatFrame]:
+    private var lastInstanceId = ""
+    
+    override def onNext(frame: HeartbeatFrame): Unit =
+      lastInstanceId = frame.getInstanceId
+      instanceRegistry.updateInstanceFromRedis(lastInstanceId)
+      
+    override def onError(t: Throwable): Unit =
+      log.warnf(t, "Stream error for %s", lastInstanceId)
+      failoverService.onInstanceStreamDropped(lastInstanceId)
+    
+    override def onCompleted(): Unit =
+      log.infof("Stream completed for %s", lastInstanceId)
+```
+
+---
+
+## Testing Checklist
+
+- [ ] Compile with proto fix
+- [ ] Start core + coordinator
+- [ ] Create game, subscribe core
+- [ ] Watch `redis-cli SMEMBERS nowchess:instance:{id}:games` → game appears
+- [ ] Kill core JVM via `kill -9`
+- [ ] Verify coordinator log shows "stream error" within 200ms
+- [ ] Verify second core receives `batchResubscribeGames` call within 300ms total
+- [ ] Create second core, rebalance load, verify games migrate
+- [ ] Scale up: verify Argo Rollout replica count increases
+- [ ] 45min idle game: verify coordinator calls `evictGames`
+
+---
+
+## File Checklist
+
+✅ **Created**:
+- `modules/coordinator/build.gradle.kts`
+- `modules/coordinator/src/main/proto/coordinator_service.proto`
+- `modules/coordinator/src/main/resources/application.yml`
+- `modules/coordinator/src/main/scala/de/nowchess/coordinator/config/CoordinatorConfig.scala`
+- `modules/coordinator/src/main/scala/de/nowchess/coordinator/dto/InstanceMetadata.scala`
+- `modules/coordinator/src/main/scala/de/nowchess/coordinator/service/InstanceRegistry.scala`
+- `modules/coordinator/src/main/scala/de/nowchess/coordinator/service/FailoverService.scala`
+- `modules/coordinator/src/main/scala/de/nowchess/coordinator/service/LoadBalancer.scala`
+- `modules/coordinator/src/main/scala/de/nowchess/coordinator/service/AutoScaler.scala`
+- `modules/coordinator/src/main/scala/de/nowchess/coordinator/service/HealthMonitor.scala`
+- `modules/coordinator/src/main/scala/de/nowchess/coordinator/service/CacheEvictionManager.scala`
+- `modules/coordinator/src/main/scala/de/nowchess/coordinator/grpc/CoordinatorGrpcServer.scala`
+- `modules/coordinator/src/main/scala/de/nowchess/coordinator/resource/CoordinatorResource.scala`
+- `modules/coordinator/src/main/scala/de/nowchess/coordinator/CoordinatorApp.scala`
+- `modules/core/src/main/proto/coordinator_service.proto`
+- `modules/core/src/main/scala/de/nowchess/chess/service/InstanceHeartbeatService.scala`
+- `modules/core/src/main/scala/de/nowchess/chess/grpc/CoordinatorServiceHandler.scala`
+
+✅ **Modified**:
+- `settings.gradle.kts` → added `modules:coordinator`
+- `modules/core/src/main/resources/application.yml` → added coordinator gRPC client + heartbeat config
+- `modules/core/build.gradle.kts` → (no changes, proto handled by quarkus-grpc)
+- `modules/core/src/main/scala/de/nowchess/chess/redis/GameRedisSubscriberManager.scala` → added InstanceHeartbeatService injection, SADD/SREM, batch ops
+
+---
+
+## Next Steps (New Session)
+
+1. Run `./gradlew clean modules:coordinator:compileScala` with proto fix
+2. Finish gRPC client stubs (Rollout, managed channels)
+3. Implement `FailoverService.distributeGames()` with actual core gRPC calls
+4. Implement `LoadBalancer.rebalance()` with game migration
+5. Implement `AutoScaler` with k8s API
+6. Implement `CacheEvictionManager` with timestamp parsing
+7. Run integration tests (manual or `@QuarkusTest`)
+8. Benchmark: create 5000 games, kill 1 core, measure failover time
+
+---
+
+## Design Decisions (Record for Future)
+
+- **GRPC stream as primary**: TCP-level detection <200ms vs polling/TTL 5-30s trade-off
+- **Redis game sets**: SADD/SREM for O(1) lookup vs scanning Redis per failover
+- **Argo Rollouts not StatefulSet**: Respects canary/blue-green; patch via Fabric8 `GenericKubernetesResource`
+- **Batch gRPC calls**: One call per target core vs 1:1 calls per game (saves RPC overhead)
+- **No persistent subscriptions**: On coordinator restart, gRPC reconnects auto-trigger resubscribe; best-effort is OK
+
+---
+
+## Known Gaps
+
+- Error handling: what if `batchResubscribeGames` fails? Retry? Partial migration? (Add circuit breaker)
+- Coordinator HA: single instance. Add Quorum or K8s deployment with multiple replicas + leader election if needed
+- Metrics: no Prometheus exports yet (add via `quarkus-micrometer`)
+- Monitoring: logs only, no alerts on failover latency SLA violation
diff --git a/modules/coordinator/build.gradle.kts b/modules/coordinator/build.gradle.kts
new file mode 100644
index 0000000..11aee8d
--- /dev/null
+++ b/modules/coordinator/build.gradle.kts
@@ -0,0 +1,96 @@
+plugins {
+    id("scala")
+    id("org.scoverage") version "8.1"
+    id("io.quarkus")
+}
+
+group = "de.nowchess"
+version = "1.0-SNAPSHOT"
+
+@Suppress("UNCHECKED_CAST")
+val versions = rootProject.extra["VERSIONS"] as Map<String, String>
+@Suppress("UNCHECKED_CAST")
+val scoverageExcluded = rootProject.extra["SCOVERAGE_EXCLUDED"] as List<String>
+
+repositories {
+    mavenCentral()
+}
+
+scala {
+    scalaVersion = versions["SCALA3"]!!
+}
+
+scoverage {
+    scoverageVersion.set(versions["SCOVERAGE"]!!)
+    excludedFiles.set(scoverageExcluded)
+}
+
+tasks.withType<ScalaCompile> {
+    scalaCompileOptions.additionalParameters = listOf("-encoding", "UTF-8")
+}
+
+val quarkusPlatformGroupId: String by project
+val quarkusPlatformArtifactId: String by project
+val quarkusPlatformVersion: String by project
+
+dependencies {
+
+    compileOnly("org.scala-lang:scala3-compiler_3") {
+        version {
+            strictly(versions["SCALA3"]!!)
+        }
+    }
+    implementation("org.scala-lang:scala3-library_3") {
+        version {
+            strictly(versions["SCALA3"]!!)
+        }
+    }
+
+    implementation(enforcedPlatform("${quarkusPlatformGroupId}:${quarkusPlatformArtifactId}:${quarkusPlatformVersion}"))
+    implementation("io.quarkus:quarkus-rest")
+    implementation("io.quarkus:quarkus-grpc")
+    implementation("io.quarkus:quarkus-arc")
+    implementation("io.quarkus:quarkus-config-yaml")
+    implementation("io.quarkus:quarkus-smallrye-health")
+    implementation("io.quarkus:quarkus-rest-client")
+    implementation("io.quarkus:quarkus-rest-client-jackson")
+    implementation("org.redisson:redisson:${versions["REDISSON"]!!}")
+    implementation("io.fabric8:kubernetes-client:6.13.0")
+
+    testImplementation(platform("org.junit:junit-bom:${versions["JUNIT_BOM"]!!}"))
+    testImplementation("org.junit.jupiter:junit-jupiter")
+    testImplementation("org.scalatest:scalatest_3:${versions["SCALATEST"]!!}")
+    testImplementation("co.helmethair:scalatest-junit-runner:${versions["SCALATEST_JUNIT"]!!}")
+    testImplementation("io.quarkus:quarkus-junit5")
+    testImplementation("io.quarkus:quarkus-junit5-mockito")
+
+    testRuntimeOnly("org.junit.platform:junit-platform-launcher")
+    testRuntimeOnly("org.junit.jupiter:junit-jupiter-engine")
+}
+
+configurations.matching { !it.name.startsWith("scoverage") }.configureEach {
+    resolutionStrategy.force("org.scala-lang:scala-library:${versions["SCALA_LIBRARY"]!!}")
+}
+configurations.scoverage {
+    resolutionStrategy.eachDependency {
+        if (requested.group == "org.scoverage" && requested.name.startsWith("scalac-scoverage-plugin_")) {
+            useTarget("${requested.group}:scalac-scoverage-plugin_2.13.16:2.3.0")
+        }
+    }
+}
+
+tasks.withType<JavaCompile> {
+    options.encoding = "UTF-8"
+    options.compilerArgs.add("-parameters")
+}
+tasks.withType<Jar>().configureEach { duplicatesStrategy = DuplicatesStrategy.EXCLUDE }
+
+tasks.test {
+    useJUnitPlatform {
+        includeEngines("scalatest", "junit-jupiter")
+        testLogging { events("passed", "skipped", "failed") }
+    }
+    finalizedBy(tasks.reportScoverage)
+}
+tasks.reportScoverage { dependsOn(tasks.test) }
+tasks.jar { duplicatesStrategy = DuplicatesStrategy.EXCLUDE }
diff --git a/modules/coordinator/src/main/proto/coordinator_service.proto b/modules/coordinator/src/main/proto/coordinator_service.proto
new file mode 100644
index 0000000..b233606
--- /dev/null
+++ b/modules/coordinator/src/main/proto/coordinator_service.proto
@@ -0,0 +1,57 @@
+syntax = "proto3";
+
+package de.nowchess.coordinator;
+
+service CoordinatorService {
+  rpc HeartbeatStream(stream HeartbeatFrame) returns (stream CoordinatorCommand);
+  rpc BatchResubscribeGames(BatchResubscribeRequest) returns (BatchResubscribeResponse);
+  rpc UnsubscribeGames(UnsubscribeGamesRequest) returns (UnsubscribeGamesResponse);
+  rpc EvictGames(EvictGamesRequest) returns (EvictGamesResponse);
+  rpc DrainInstance(DrainInstanceRequest) returns (DrainInstanceResponse);
+}
+
+message HeartbeatFrame {
+  string instanceId = 1;
+  string hostname = 2;
+  int32 httpPort = 3;
+  int32 grpcPort = 4;
+  int32 subscriptionCount = 5;
+  int32 localCacheSize = 6;
+  int64 timestampMillis = 7;
+}
+
+message CoordinatorCommand {
+  string type = 1;
+  string payload = 2;
+}
+
+message BatchResubscribeRequest {
+  repeated string gameIds = 1;
+}
+
+message BatchResubscribeResponse {
+  int32 subscribedCount = 1;
+  repeated string failedGameIds = 2;
+}
+
+message UnsubscribeGamesRequest {
+  repeated string gameIds = 1;
+}
+
+message UnsubscribeGamesResponse {
+  int32 unsubscribedCount = 1;
+}
+
+message EvictGamesRequest {
+  repeated string gameIds = 1;
+}
+
+message EvictGamesResponse {
+  int32 evictedCount = 1;
+}
+
+message DrainInstanceRequest {}
+
+message DrainInstanceResponse {
+  int32 gamesMigrated = 1;
+}
diff --git a/modules/coordinator/src/main/resources/application.yml b/modules/coordinator/src/main/resources/application.yml
new file mode 100644
index 0000000..c07cdc0
--- /dev/null
+++ b/modules/coordinator/src/main/resources/application.yml
@@ -0,0 +1,39 @@
+quarkus:
+  application.name: nowchess-coordinator
+  http.port: 8086
+  grpc.server.port: 9086
+  config.yaml.enabled: true
+  rest-client.connection-timeout: 5000
+  rest-client.read-timeout: 10000
+
+nowchess:
+  redis:
+    host: ${REDIS_HOST:localhost}
+    port: ${REDIS_PORT:6379}
+    prefix: ${REDIS_PREFIX:nowchess}
+
+  coordinator:
+    max-games-per-core: 500
+    max-deviation-percent: 20
+    rebalance-interval: 30s
+    rebalance-min-interval: 60s
+    heartbeat-ttl: 5s
+    stream-heartbeat-interval: 200ms
+    cache-eviction-interval: 10m
+    game-idle-threshold: 45m
+    auto-scale-enabled: false
+    scale-up-threshold: 0.8
+    scale-down-threshold: 0.3
+    scale-min-replicas: 2
+    scale-max-replicas: 10
+    k8s-namespace: default
+    k8s-rollout-name: nowchess-core
+    k8s-rollout-label-selector: "app=nowchess-core"
+
+---
+# dev profile
+"%dev":
+  quarkus:
+    log.level: DEBUG
+    log.category:
+      "de.nowchess": DEBUG
diff --git a/modules/coordinator/src/main/scala/de/nowchess/coordinator/CoordinatorApp.scala b/modules/coordinator/src/main/scala/de/nowchess/coordinator/CoordinatorApp.scala
new file mode 100644
index 0000000..dddef0f
--- /dev/null
+++ b/modules/coordinator/src/main/scala/de/nowchess/coordinator/CoordinatorApp.scala
@@ -0,0 +1,7 @@
+package de.nowchess.coordinator
+
+import jakarta.ws.rs.core.Application
+import jakarta.enterprise.context.ApplicationScoped
+
+@ApplicationScoped
+class CoordinatorApp extends Application
diff --git a/modules/coordinator/src/main/scala/de/nowchess/coordinator/config/CoordinatorConfig.scala b/modules/coordinator/src/main/scala/de/nowchess/coordinator/config/CoordinatorConfig.scala
new file mode 100644
index 0000000..45fdd92
--- /dev/null
+++ b/modules/coordinator/src/main/scala/de/nowchess/coordinator/config/CoordinatorConfig.scala
@@ -0,0 +1,55 @@
+package de.nowchess.coordinator.config
+
+import io.smallrye.config.ConfigMapping
+import io.smallrye.config.WithName
+import java.time.Duration
+
+@ConfigMapping(prefix = "nowchess.coordinator")
+class CoordinatorConfig:
+  @WithName("max-games-per-core")
+  def maxGamesPerCore: Int = ???
+
+  @WithName("max-deviation-percent")
+  def maxDeviationPercent: Int = ???
+
+  @WithName("rebalance-interval")
+  def rebalanceInterval: Duration = ???
+
+  @WithName("rebalance-min-interval")
+  def rebalanceMinInterval: Duration = ???
+
+  @WithName("heartbeat-ttl")
+  def heartbeatTtl: Duration = ???
+
+  @WithName("stream-heartbeat-interval")
+  def streamHeartbeatInterval: Duration = ???
+
+  @WithName("cache-eviction-interval")
+  def cacheEvictionInterval: Duration = ???
+
+  @WithName("game-idle-threshold")
+  def gameIdleThreshold: Duration = ???
+
+  @WithName("auto-scale-enabled")
+  def autoScaleEnabled: Boolean = ???
+
+  @WithName("scale-up-threshold")
+  def scaleUpThreshold: Double = ???
+
+  @WithName("scale-down-threshold")
+  def scaleDownThreshold: Double = ???
+
+  @WithName("scale-min-replicas")
+  def scaleMinReplicas: Int = ???
+
+  @WithName("scale-max-replicas")
+  def scaleMaxReplicas: Int = ???
+
+  @WithName("k8s-namespace")
+  def k8sNamespace: String = ???
+
+  @WithName("k8s-rollout-name")
+  def k8sRolloutName: String = ???
+
+  @WithName("k8s-rollout-label-selector")
+  def k8sRolloutLabelSelector: String = ???
diff --git a/modules/coordinator/src/main/scala/de/nowchess/coordinator/dto/InstanceMetadata.scala b/modules/coordinator/src/main/scala/de/nowchess/coordinator/dto/InstanceMetadata.scala
new file mode 100644
index 0000000..b0313e0
--- /dev/null
+++ b/modules/coordinator/src/main/scala/de/nowchess/coordinator/dto/InstanceMetadata.scala
@@ -0,0 +1,23 @@
+package de.nowchess.coordinator.dto
+
+import com.fasterxml.jackson.annotation.JsonProperty
+import java.time.Instant
+
+case class InstanceMetadata(
+  @JsonProperty("instanceId")
+  instanceId: String,
+  @JsonProperty("hostname")
+  hostname: String,
+  @JsonProperty("httpPort")
+  httpPort: Int,
+  @JsonProperty("grpcPort")
+  grpcPort: Int,
+  @JsonProperty("subscriptionCount")
+  subscriptionCount: Int,
+  @JsonProperty("localCacheSize")
+  localCacheSize: Int,
+  @JsonProperty("lastHeartbeat")
+  lastHeartbeat: String,
+  @JsonProperty("state")
+  state: String = "HEALTHY"
+)
diff --git a/modules/coordinator/src/main/scala/de/nowchess/coordinator/grpc/CoordinatorGrpcServer.scala b/modules/coordinator/src/main/scala/de/nowchess/coordinator/grpc/CoordinatorGrpcServer.scala
new file mode 100644
index 0000000..f5c8ebf
--- /dev/null
+++ b/modules/coordinator/src/main/scala/de/nowchess/coordinator/grpc/CoordinatorGrpcServer.scala
@@ -0,0 +1,110 @@
+package de.nowchess.coordinator.grpc
+
+import jakarta.enterprise.context.ApplicationScoped
+import jakarta.inject.Inject
+import scala.compiletime.uninitialized
+import de.nowchess.coordinator.service.{FailoverService, InstanceRegistry}
+import de.nowchess.coordinator.CoordinatorServiceGrpc
+import de.nowchess.coordinator.{
+  HeartbeatFrame,
+  CoordinatorCommand,
+  BatchResubscribeRequest,
+  BatchResubscribeResponse,
+  UnsubscribeGamesRequest,
+  UnsubscribeGamesResponse,
+  EvictGamesRequest,
+  EvictGamesResponse,
+  DrainInstanceRequest,
+  DrainInstanceResponse
+}
+import io.grpc.stub.StreamObserver
+import com.fasterxml.jackson.databind.ObjectMapper
+import org.jboss.logging.Logger
+
+@ApplicationScoped
+class CoordinatorGrpcServer extends CoordinatorServiceGrpc.CoordinatorServiceImplBase:
+  // scalafix:off DisableSyntax.var
+  @Inject
+  private var instanceRegistry: InstanceRegistry = uninitialized
+
+  @Inject
+  private var failoverService: FailoverService = uninitialized
+  // scalafix:on DisableSyntax.var
+
+  private val mapper = ObjectMapper()
+  private val log = Logger.getLogger(classOf[CoordinatorGrpcServer])
+
+  override def heartbeatStream(
+    responseObserver: StreamObserver[CoordinatorCommand]
+  ): StreamObserver[HeartbeatFrame] =
+    new StreamObserver[HeartbeatFrame]:
+      private var lastInstanceId = ""
+
+      override def onNext(frame: HeartbeatFrame): Unit =
+        lastInstanceId = frame.getInstanceId
+        // Update instance registry with heartbeat data
+        val metadata = de.nowchess.coordinator.dto.InstanceMetadata(
+          instanceId = frame.getInstanceId,
+          hostname = frame.getHostname,
+          httpPort = frame.getHttpPort,
+          grpcPort = frame.getGrpcPort,
+          subscriptionCount = frame.getSubscriptionCount,
+          localCacheSize = frame.getLocalCacheSize,
+          lastHeartbeat = java.time.Instant.ofEpochMilli(frame.getTimestampMillis).toString,
+          state = "HEALTHY"
+        )
+        // Store in registry (placeholder for actual storage)
+        log.debugf("Received heartbeat from %s with %d subscriptions",
+          frame.getInstanceId, frame.getSubscriptionCount)
+
+      override def onError(t: Throwable): Unit =
+        log.warnf(t, "Heartbeat stream error for instance %s", lastInstanceId)
+        if lastInstanceId.nonEmpty then
+          failoverService.onInstanceStreamDropped(lastInstanceId)
+
+      override def onCompleted: Unit =
+        log.infof("Heartbeat stream completed for instance %s", lastInstanceId)
+
+  override def batchResubscribeGames(
+    request: BatchResubscribeRequest,
+    responseObserver: StreamObserver[BatchResubscribeResponse]
+  ): Unit =
+    log.infof("Batch resubscribe request for %d games", request.getGameIdsList.size())
+    val response = BatchResubscribeResponse.newBuilder()
+      .setSubscribedCount(request.getGameIdsList.size())
+      .build()
+    responseObserver.onNext(response)
+    responseObserver.onCompleted()
+
+  override def unsubscribeGames(
+    request: UnsubscribeGamesRequest,
+    responseObserver: StreamObserver[UnsubscribeGamesResponse]
+  ): Unit =
+    log.infof("Unsubscribe request for %d games", request.getGameIdsList.size())
+    val response = UnsubscribeGamesResponse.newBuilder()
+      .setUnsubscribedCount(request.getGameIdsList.size())
+      .build()
+    responseObserver.onNext(response)
+    responseObserver.onCompleted()
+
+  override def evictGames(
+    request: EvictGamesRequest,
+    responseObserver: StreamObserver[EvictGamesResponse]
+  ): Unit =
+    log.infof("Evict request for %d games", request.getGameIdsList.size())
+    val response = EvictGamesResponse.newBuilder()
+      .setEvictedCount(request.getGameIdsList.size())
+      .build()
+    responseObserver.onNext(response)
+    responseObserver.onCompleted()
+
+  override def drainInstance(
+    request: DrainInstanceRequest,
+    responseObserver: StreamObserver[DrainInstanceResponse]
+  ): Unit =
+    log.info("Drain instance request")
+    val response = DrainInstanceResponse.newBuilder()
+      .setGamesMigrated(0)
+      .build()
+    responseObserver.onNext(response)
+    responseObserver.onCompleted()
diff --git a/modules/coordinator/src/main/scala/de/nowchess/coordinator/resource/CoordinatorResource.scala b/modules/coordinator/src/main/scala/de/nowchess/coordinator/resource/CoordinatorResource.scala
new file mode 100644
index 0000000..6cbc0b4
--- /dev/null
+++ b/modules/coordinator/src/main/scala/de/nowchess/coordinator/resource/CoordinatorResource.scala
@@ -0,0 +1,99 @@
+package de.nowchess.coordinator.resource
+
+import jakarta.enterprise.context.ApplicationScoped
+import jakarta.inject.Inject
+import jakarta.ws.rs.*
+import jakarta.ws.rs.core.MediaType
+import scala.compiletime.uninitialized
+import scala.jdk.CollectionConverters.*
+import de.nowchess.coordinator.service.{InstanceRegistry, LoadBalancer, AutoScaler, FailoverService}
+import de.nowchess.coordinator.dto.InstanceMetadata
+import org.jboss.logging.Logger
+
+@Path("/api/coordinator")
+@ApplicationScoped
+class CoordinatorResource:
+  // scalafix:off DisableSyntax.var
+  @Inject
+  private var instanceRegistry: InstanceRegistry = uninitialized
+
+  @Inject
+  private var loadBalancer: LoadBalancer = uninitialized
+
+  @Inject
+  private var autoScaler: AutoScaler = uninitialized
+
+  @Inject
+  private var failoverService: FailoverService = uninitialized
+  // scalafix:on DisableSyntax.var
+
+  private val log = Logger.getLogger(classOf[CoordinatorResource])
+
+  @GET
+  @Path("/instances")
+  @Produces(Array(MediaType.APPLICATION_JSON))
+  def listInstances: java.util.List[InstanceMetadata] =
+    instanceRegistry.getAllInstances.asJava.asInstanceOf[java.util.List[InstanceMetadata]]
+
+  @GET
+  @Path("/metrics")
+  @Produces(Array(MediaType.APPLICATION_JSON))
+  def getMetrics: MetricsDto =
+    val instances = instanceRegistry.getAllInstances
+    val loads = instances.map(_.subscriptionCount)
+    val totalGames = loads.sum
+    val avgLoad = if instances.nonEmpty then loads.sum.toDouble / instances.size else 0.0
+    val maxLoad = if loads.nonEmpty then loads.max else 0
+    val minLoad = if loads.nonEmpty then loads.min else 0
+
+    MetricsDto(
+      totalInstances = instances.size,
+      healthyInstances = instances.count(_.state == "HEALTHY"),
+      deadInstances = instances.count(_.state == "DEAD"),
+      totalGames = totalGames,
+      avgGamesPerCore = avgLoad,
+      maxGamesPerCore = maxLoad,
+      minGamesPerCore = minLoad,
+      instances = instances
+    )
+
+  @POST
+  @Path("/rebalance")
+  @Produces(Array(MediaType.APPLICATION_JSON))
+  def triggerRebalance: scala.collection.Map[String, String] =
+    log.info("Manual rebalance triggered")
+    loadBalancer.rebalance()
+    Map("status" -> "rebalance_started")
+
+  @POST
+  @Path("/failover/{instanceId}")
+  @Produces(Array(MediaType.APPLICATION_JSON))
+  def triggerFailover(@PathParam("instanceId") instanceId: String): scala.collection.Map[String, String] =
+    log.infof("Manual failover triggered for instance %s", instanceId)
+    failoverService.onInstanceStreamDropped(instanceId)
+    Map("status" -> "failover_started", "instanceId" -> instanceId)
+
+  @POST
+  @Path("/scale-up")
+  @Produces(Array(MediaType.APPLICATION_JSON))
+  def triggerScaleUp: scala.collection.Map[String, String] =
+    log.info("Manual scale up triggered")
+    Map("status" -> "scale_up_started")
+
+  @POST
+  @Path("/scale-down")
+  @Produces(Array(MediaType.APPLICATION_JSON))
+  def triggerScaleDown: scala.collection.Map[String, String] =
+    log.info("Manual scale down triggered")
+    Map("status" -> "scale_down_started")
+
+case class MetricsDto(
+  totalInstances: Int,
+  healthyInstances: Int,
+  deadInstances: Int,
+  totalGames: Int,
+  avgGamesPerCore: Double,
+  maxGamesPerCore: Int,
+  minGamesPerCore: Int,
+  instances: List[InstanceMetadata]
+)
diff --git a/modules/coordinator/src/main/scala/de/nowchess/coordinator/service/AutoScaler.scala b/modules/coordinator/src/main/scala/de/nowchess/coordinator/service/AutoScaler.scala
new file mode 100644
index 0000000..8b6e285
--- /dev/null
+++ b/modules/coordinator/src/main/scala/de/nowchess/coordinator/service/AutoScaler.scala
@@ -0,0 +1,58 @@
+package de.nowchess.coordinator.service
+
+import jakarta.enterprise.context.ApplicationScoped
+import jakarta.inject.Inject
+import de.nowchess.coordinator.config.CoordinatorConfig
+import io.fabric8.kubernetes.client.KubernetesClient
+import org.jboss.logging.Logger
+
+@ApplicationScoped
+class AutoScaler:
+  @Inject(optional = true)
+  private var kubeClient: KubernetesClient = _
+
+  @Inject
+  private var config: CoordinatorConfig = _
+
+  @Inject
+  private var instanceRegistry: InstanceRegistry = _
+
+  private val log = Logger.getLogger(classOf[AutoScaler])
+  private var lastScaleTime = 0L
+
+  def checkAndScale: Unit =
+    if !config.autoScaleEnabled then
+      return
+
+    val now = System.currentTimeMillis()
+    if now - lastScaleTime < 120000 then // 2 minute backoff
+      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
+
+    if avgLoad > config.scaleUpThreshold * config.maxGamesPerCore then
+      scaleUp()
+      lastScaleTime = now
+    else if avgLoad < config.scaleDownThreshold * config.maxGamesPerCore && instances.size > config.scaleMinReplicas then
+      scaleDown()
+      lastScaleTime = now
+
+  private def scaleUp: Unit =
+    log.info("Scaling up Argo Rollout")
+    if kubeClient != null then
+      // Placeholder: will patch Rollout replicas
+      log.infof("Would scale up %s in namespace %s", config.k8sRolloutName, config.k8sNamespace)
+    else
+      log.warn("Kubernetes client not available, cannot scale")
+
+  private def scaleDown: Unit =
+    log.info("Scaling down Argo Rollout")
+    if kubeClient != null then
+      // Placeholder: will patch Rollout replicas
+      log.infof("Would scale down %s in namespace %s", config.k8sRolloutName, config.k8sNamespace)
+    else
+      log.warn("Kubernetes client not available, cannot scale")
diff --git a/modules/coordinator/src/main/scala/de/nowchess/coordinator/service/CacheEvictionManager.scala b/modules/coordinator/src/main/scala/de/nowchess/coordinator/service/CacheEvictionManager.scala
new file mode 100644
index 0000000..53b7741
--- /dev/null
+++ b/modules/coordinator/src/main/scala/de/nowchess/coordinator/service/CacheEvictionManager.scala
@@ -0,0 +1,53 @@
+package de.nowchess.coordinator.service
+
+import jakarta.enterprise.context.ApplicationScoped
+import jakarta.inject.Inject
+import org.redisson.api.RedissonClient
+import de.nowchess.coordinator.config.CoordinatorConfig
+import scala.jdk.CollectionConverters.*
+import org.jboss.logging.Logger
+import java.time.Instant
+
+@ApplicationScoped
+class CacheEvictionManager:
+  @Inject
+  private var redissonClient: RedissonClient = _
+
+  @Inject
+  private var config: CoordinatorConfig = _
+
+  @Inject
+  private var instanceRegistry: InstanceRegistry = _
+
+  private val log = Logger.getLogger(classOf[CacheEvictionManager])
+  private var redisPrefix = "nowchess"
+
+  def setRedisPrefix(prefix: String): Unit =
+    redisPrefix = prefix
+
+  def evictStaleGames: Unit =
+    log.info("Starting cache eviction scan")
+
+    val pattern = s"$redisPrefix:game:entry:*"
+    val keys = redissonClient.getKeys.getKeysByPattern(pattern, 100)
+
+    val now = System.currentTimeMillis()
+    val idleThresholdMs = config.gameIdleThreshold.toMillis
+
+    var evictedCount = 0
+    keys.asScala.foreach { key =>
+      try
+        val bucket = redissonClient.getBucket[String](key)
+        val value = bucket.get()
+        if value != null then
+          // Parse JSON to extract lastUpdated timestamp
+          // Placeholder: actual parsing will be implemented
+          val gameId = key.stripPrefix(s"$redisPrefix:game:entry:")
+          // Check if game should be evicted and call core
+          ()
+      catch
+        case ex: Exception =>
+          log.warnf(ex, "Error processing game key %s", key)
+    }
+
+    log.infof("Cache eviction scan completed, evicted %d games", evictedCount)
diff --git a/modules/coordinator/src/main/scala/de/nowchess/coordinator/service/FailoverService.scala b/modules/coordinator/src/main/scala/de/nowchess/coordinator/service/FailoverService.scala
new file mode 100644
index 0000000..a5a084f
--- /dev/null
+++ b/modules/coordinator/src/main/scala/de/nowchess/coordinator/service/FailoverService.scala
@@ -0,0 +1,67 @@
+package de.nowchess.coordinator.service
+
+import jakarta.enterprise.context.ApplicationScoped
+import jakarta.inject.Inject
+import org.redisson.api.RedissonClient
+import scala.jdk.CollectionConverters.*
+import scala.concurrent.duration.*
+import java.time.Instant
+import org.jboss.logging.Logger
+
+@ApplicationScoped
+class FailoverService:
+  @Inject
+  private var redissonClient: RedissonClient = _
+
+  @Inject
+  private var instanceRegistry: InstanceRegistry = _
+
+  private val log = Logger.getLogger(classOf[FailoverService])
+  private var redisPrefix = "nowchess"
+
+  def setRedisPrefix(prefix: String): Unit =
+    redisPrefix = prefix
+
+  def onInstanceStreamDropped(instanceId: String): Unit =
+    log.infof("Instance %s stream dropped, triggering failover", instanceId)
+
+    val startTime = System.currentTimeMillis()
+    instanceRegistry.markInstanceDead(instanceId)
+
+    val gameIds = getOrphanedGames(instanceId)
+    log.infof("Found %d orphaned games for instance %s", gameIds.size, instanceId)
+
+    if gameIds.nonEmpty then
+      val healthyInstances = instanceRegistry.getAllInstances
+        .filter(_.state == "HEALTHY")
+        .sortBy(_.subscriptionCount)
+
+      if healthyInstances.nonEmpty then
+        distributeGames(gameIds, healthyInstances, instanceId)
+
+        val elapsed = System.currentTimeMillis() - startTime
+        log.infof("Failover completed in %dms for instance %s", elapsed, instanceId)
+      else
+        log.warnf("No healthy instances available for failover of %s", instanceId)
+
+    cleanupDeadInstance(instanceId)
+
+  private def getOrphanedGames(instanceId: String): List[String] =
+    val setKey = s"$redisPrefix:instance:$instanceId:games"
+    val gameSet = redissonClient.getSet[String](setKey)
+    gameSet.readAll.asScala.toList
+
+  private def distributeGames(
+    gameIds: List[String],
+    healthyInstances: List[_],
+    deadInstanceId: String
+  ): Unit =
+    // Placeholder: will be replaced with actual gRPC calls
+    log.infof("Would distribute %d games from %s to %d healthy instances",
+      gameIds.size, deadInstanceId, healthyInstances.size)
+
+  private def cleanupDeadInstance(instanceId: String): Unit =
+    val setKey = s"$redisPrefix:instance:$instanceId:games"
+    val gameSet = redissonClient.getSet[String](setKey)
+    gameSet.delete()
+    log.infof("Cleaned up games set for instance %s", instanceId)
diff --git a/modules/coordinator/src/main/scala/de/nowchess/coordinator/service/HealthMonitor.scala b/modules/coordinator/src/main/scala/de/nowchess/coordinator/service/HealthMonitor.scala
new file mode 100644
index 0000000..77a063b
--- /dev/null
+++ b/modules/coordinator/src/main/scala/de/nowchess/coordinator/service/HealthMonitor.scala
@@ -0,0 +1,68 @@
+package de.nowchess.coordinator.service
+
+import jakarta.enterprise.context.ApplicationScoped
+import jakarta.inject.Inject
+import de.nowchess.coordinator.config.CoordinatorConfig
+import io.fabric8.kubernetes.client.KubernetesClient
+import io.fabric8.kubernetes.api.model.Pod
+import scala.jdk.CollectionConverters.*
+import org.jboss.logging.Logger
+import java.time.Instant
+
+@ApplicationScoped
+class HealthMonitor:
+  @Inject(optional = true)
+  private var kubeClient: KubernetesClient = _
+
+  @Inject
+  private var config: CoordinatorConfig = _
+
+  @Inject
+  private var instanceRegistry: InstanceRegistry = _
+
+  private val log = Logger.getLogger(classOf[HealthMonitor])
+
+  def checkInstanceHealth: Unit =
+    val instances = instanceRegistry.getAllInstances
+    instances.foreach { inst =>
+      val isHealthy = checkHealth(inst.instanceId)
+      if !isHealthy && inst.state == "HEALTHY" then
+        log.warnf("Instance %s marked unhealthy", inst.instanceId)
+        instanceRegistry.markInstanceDead(inst.instanceId)
+    }
+
+  private def checkHealth(instanceId: String): Boolean =
+    // Placeholder: will check Redis heartbeat, k8s pod status, and HTTP health endpoint
+    true
+
+  def watchK8sPods: Unit =
+    if kubeClient != null then
+      try
+        val pods = kubeClient.pods()
+          .inNamespace(config.k8sNamespace)
+          .withLabel(config.k8sRolloutLabelSelector)
+          .list()
+          .getItems
+          .asScala
+
+        pods.foreach { pod =>
+          val podName = pod.getMetadata.getName
+          val isReady = isPodReady(pod)
+          log.debugf("Pod %s ready: %b", podName, isReady)
+        }
+      catch
+        case ex: Exception =>
+          log.warnf(ex, "Failed to watch k8s pods")
+    else
+      log.debug("Kubernetes client not available for pod watch")
+
+  private def isPodReady(pod: Pod): Boolean =
+    val status = pod.getStatus
+    if status == null then false
+    else
+      val conditions = status.getConditions
+      if conditions == null then false
+      else
+        conditions.asScala.exists { cond =>
+          cond.getType == "Ready" && cond.getStatus == "True"
+        }
diff --git a/modules/coordinator/src/main/scala/de/nowchess/coordinator/service/InstanceRegistry.scala b/modules/coordinator/src/main/scala/de/nowchess/coordinator/service/InstanceRegistry.scala
new file mode 100644
index 0000000..7b2c7f7
--- /dev/null
+++ b/modules/coordinator/src/main/scala/de/nowchess/coordinator/service/InstanceRegistry.scala
@@ -0,0 +1,63 @@
+package de.nowchess.coordinator.service
+
+import jakarta.enterprise.context.ApplicationScoped
+import jakarta.inject.Inject
+import org.redisson.api.RedissonClient
+import scala.jdk.CollectionConverters.*
+import scala.collection.mutable
+import com.fasterxml.jackson.databind.ObjectMapper
+import de.nowchess.coordinator.dto.InstanceMetadata
+import java.time.Instant
+
+@ApplicationScoped
+class InstanceRegistry:
+  @Inject
+  private var redissonClient: RedissonClient = _
+
+  private val mapper = ObjectMapper()
+  private val instances = mutable.Map[String, InstanceMetadata]()
+  private var redisPrefix = "nowchess"
+
+  def setRedisPrefix(prefix: String): Unit =
+    redisPrefix = prefix
+
+  def getInstance(instanceId: String): Option[InstanceMetadata] =
+    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
+
+  def updateInstanceFromRedis(instanceId: String): Unit =
+    val key = s"$redisPrefix:instances:$instanceId"
+    val bucket = redissonClient.getBucket[String](key)
+    val value = bucket.get()
+    if value != null then
+      try
+        val metadata = mapper.readValue(value, classOf[InstanceMetadata])
+        instances(instanceId) = metadata
+      catch
+        case _: Exception => ()
+
+  def markInstanceDead(instanceId: String): Unit =
+    instances.get(instanceId).foreach { inst =>
+      val dead = inst.copy(state = "DEAD")
+      instances(instanceId) = dead
+    }
+
+  def removeInstance(instanceId: String): Unit =
+    instances.remove(instanceId)
diff --git a/modules/coordinator/src/main/scala/de/nowchess/coordinator/service/LoadBalancer.scala b/modules/coordinator/src/main/scala/de/nowchess/coordinator/service/LoadBalancer.scala
new file mode 100644
index 0000000..1bd2e86
--- /dev/null
+++ b/modules/coordinator/src/main/scala/de/nowchess/coordinator/service/LoadBalancer.scala
@@ -0,0 +1,44 @@
+package de.nowchess.coordinator.service
+
+import jakarta.enterprise.context.ApplicationScoped
+import jakarta.inject.Inject
+import de.nowchess.coordinator.config.CoordinatorConfig
+import org.jboss.logging.Logger
+import scala.concurrent.duration.*
+
+@ApplicationScoped
+class LoadBalancer:
+  @Inject
+  private var config: CoordinatorConfig = _
+
+  @Inject
+  private var instanceRegistry: InstanceRegistry = _
+
+  private val log = Logger.getLogger(classOf[LoadBalancer])
+  private var lastRebalanceTime = 0L
+
+  def shouldRebalance: Boolean =
+    val now = System.currentTimeMillis()
+    val minInterval = config.rebalanceMinInterval.toMillis
+    if now - lastRebalanceTime < minInterval then
+      return false
+
+    val instances = instanceRegistry.getAllInstances
+    if instances.isEmpty then return false
+
+    val loads = instances.map(_.subscriptionCount)
+    val maxLoad = loads.max
+    val minLoad = loads.min
+    val avgLoad = loads.sum.toDouble / loads.size
+
+    val exceededMax = maxLoad > config.maxGamesPerCore
+    val deviationPercent = 100.0 * (maxLoad - avgLoad) / avgLoad
+    val exceededDeviation = maxLoad > avgLoad && deviationPercent > config.maxDeviationPercent && (maxLoad - minLoad) > 50
+
+    exceededMax || exceededDeviation
+
+  def rebalance: Unit =
+    log.info("Starting rebalance")
+    lastRebalanceTime = System.currentTimeMillis()
+    // Placeholder: actual rebalance logic will be implemented
+    log.info("Rebalance completed")
diff --git a/modules/core/src/main/proto/coordinator_service.proto b/modules/core/src/main/proto/coordinator_service.proto
new file mode 100644
index 0000000..b233606
--- /dev/null
+++ b/modules/core/src/main/proto/coordinator_service.proto
@@ -0,0 +1,57 @@
+syntax = "proto3";
+
+package de.nowchess.coordinator;
+
+service CoordinatorService {
+  rpc HeartbeatStream(stream HeartbeatFrame) returns (stream CoordinatorCommand);
+  rpc BatchResubscribeGames(BatchResubscribeRequest) returns (BatchResubscribeResponse);
+  rpc UnsubscribeGames(UnsubscribeGamesRequest) returns (UnsubscribeGamesResponse);
+  rpc EvictGames(EvictGamesRequest) returns (EvictGamesResponse);
+  rpc DrainInstance(DrainInstanceRequest) returns (DrainInstanceResponse);
+}
+
+message HeartbeatFrame {
+  string instanceId = 1;
+  string hostname = 2;
+  int32 httpPort = 3;
+  int32 grpcPort = 4;
+  int32 subscriptionCount = 5;
+  int32 localCacheSize = 6;
+  int64 timestampMillis = 7;
+}
+
+message CoordinatorCommand {
+  string type = 1;
+  string payload = 2;
+}
+
+message BatchResubscribeRequest {
+  repeated string gameIds = 1;
+}
+
+message BatchResubscribeResponse {
+  int32 subscribedCount = 1;
+  repeated string failedGameIds = 2;
+}
+
+message UnsubscribeGamesRequest {
+  repeated string gameIds = 1;
+}
+
+message UnsubscribeGamesResponse {
+  int32 unsubscribedCount = 1;
+}
+
+message EvictGamesRequest {
+  repeated string gameIds = 1;
+}
+
+message EvictGamesResponse {
+  int32 evictedCount = 1;
+}
+
+message DrainInstanceRequest {}
+
+message DrainInstanceResponse {
+  int32 gamesMigrated = 1;
+}
diff --git a/modules/core/src/main/resources/application.yml b/modules/core/src/main/resources/application.yml
index 383bbf9..31a2130 100644
--- a/modules/core/src/main/resources/application.yml
+++ b/modules/core/src/main/resources/application.yml
@@ -11,6 +11,9 @@ quarkus:
       io-grpc:
         host: localhost
         port: 8081
+      coordinator-grpc:
+        host: localhost
+        port: 9086
     server:
       use-separate-server: false
 
@@ -20,6 +23,13 @@ nowchess:
     port: 6379
     prefix: nowchess
 
+  coordinator:
+    host: localhost
+    grpc-port: 9086
+    stream-heartbeat-interval: 200ms
+    redis-heartbeat-interval: 2s
+    instance-id: ${HOSTNAME:local}-${quarkus.uuid}
+
 "%dev":
   mp:
     jwt:
@@ -72,6 +82,9 @@ nowchess:
         io-grpc:
           host: ${IO_SERVICE_HOST}
           port: ${IO_SERVICE_GRPC_PORT:9081}
+        coordinator-grpc:
+          host: ${COORDINATOR_SERVICE_HOST:localhost}
+          port: ${COORDINATOR_SERVICE_GRPC_PORT:9086}
     rest-client:
       io-service:
         url: ${IO_SERVICE_URL}
@@ -84,3 +97,10 @@ nowchess:
       host: ${REDIS_HOST}
       port: ${REDIS_PORT:6379}
       prefix: ${REDIS_PREFIX:nowchess}
+
+    coordinator:
+      host: ${COORDINATOR_SERVICE_HOST:localhost}
+      grpc-port: ${COORDINATOR_SERVICE_GRPC_PORT:9086}
+      stream-heartbeat-interval: 200ms
+      redis-heartbeat-interval: 2s
+      instance-id: ${HOSTNAME:local}-${quarkus.uuid}
diff --git a/modules/core/src/main/scala/de/nowchess/chess/grpc/CoordinatorServiceHandler.scala b/modules/core/src/main/scala/de/nowchess/chess/grpc/CoordinatorServiceHandler.scala
new file mode 100644
index 0000000..67981ae
--- /dev/null
+++ b/modules/core/src/main/scala/de/nowchess/chess/grpc/CoordinatorServiceHandler.scala
@@ -0,0 +1,67 @@
+package de.nowchess.chess.grpc
+
+import jakarta.enterprise.context.ApplicationScoped
+import jakarta.inject.Inject
+import de.nowchess.coordinator.CoordinatorServiceGrpc
+import de.nowchess.coordinator.{
+  BatchResubscribeRequest,
+  BatchResubscribeResponse,
+  UnsubscribeGamesRequest,
+  UnsubscribeGamesResponse,
+  EvictGamesRequest,
+  EvictGamesResponse,
+  DrainInstanceRequest,
+  DrainInstanceResponse
+}
+import de.nowchess.chess.redis.GameRedisSubscriberManager
+import io.grpc.stub.StreamObserver
+import scala.jdk.CollectionConverters.*
+
+@ApplicationScoped
+class CoordinatorServiceHandler extends CoordinatorServiceGrpc.CoordinatorServiceImplBase:
+  @Inject
+  private var gameSubscriberManager: GameRedisSubscriberManager = _
+
+  override def batchResubscribeGames(
+    request: BatchResubscribeRequest,
+    responseObserver: StreamObserver[BatchResubscribeResponse]
+  ): Unit =
+    val count = gameSubscriberManager.batchResubscribeGames(request.getGameIdsList)
+    val response = BatchResubscribeResponse.newBuilder()
+      .setSubscribedCount(count)
+      .build()
+    responseObserver.onNext(response)
+    responseObserver.onCompleted()
+
+  override def unsubscribeGames(
+    request: UnsubscribeGamesRequest,
+    responseObserver: StreamObserver[UnsubscribeGamesResponse]
+  ): Unit =
+    val count = gameSubscriberManager.unsubscribeGames(request.getGameIdsList)
+    val response = UnsubscribeGamesResponse.newBuilder()
+      .setUnsubscribedCount(count)
+      .build()
+    responseObserver.onNext(response)
+    responseObserver.onCompleted()
+
+  override def evictGames(
+    request: EvictGamesRequest,
+    responseObserver: StreamObserver[EvictGamesResponse]
+  ): Unit =
+    val count = gameSubscriberManager.evictGames(request.getGameIdsList)
+    val response = EvictGamesResponse.newBuilder()
+      .setEvictedCount(count)
+      .build()
+    responseObserver.onNext(response)
+    responseObserver.onCompleted()
+
+  override def drainInstance(
+    request: DrainInstanceRequest,
+    responseObserver: StreamObserver[DrainInstanceResponse]
+  ): Unit =
+    gameSubscriberManager.drainInstance()
+    val response = DrainInstanceResponse.newBuilder()
+      .setGamesMigrated(0)
+      .build()
+    responseObserver.onNext(response)
+    responseObserver.onCompleted()
diff --git a/modules/core/src/main/scala/de/nowchess/chess/redis/GameRedisSubscriberManager.scala b/modules/core/src/main/scala/de/nowchess/chess/redis/GameRedisSubscriberManager.scala
index bbb8705..e6d75ea 100644
--- a/modules/core/src/main/scala/de/nowchess/chess/redis/GameRedisSubscriberManager.scala
+++ b/modules/core/src/main/scala/de/nowchess/chess/redis/GameRedisSubscriberManager.scala
@@ -7,6 +7,7 @@ import de.nowchess.chess.grpc.IoGrpcClientWrapper
 import de.nowchess.chess.observer.Observer
 import de.nowchess.chess.registry.GameRegistry
 import de.nowchess.chess.resource.GameDtoMapper
+import de.nowchess.chess.service.InstanceHeartbeatService
 import jakarta.annotation.PreDestroy
 import jakarta.enterprise.context.ApplicationScoped
 import jakarta.inject.Inject
@@ -25,6 +26,7 @@ class GameRedisSubscriberManager:
   @Inject var objectMapper: ObjectMapper = uninitialized
   @Inject var redisConfig: RedisConfig = uninitialized
   @Inject var ioClient: IoGrpcClientWrapper = uninitialized
+  @Inject(optional = true) var heartbeatService: InstanceHeartbeatService = uninitialized
   // scalafix:on DisableSyntax.var
 
   private val c2sListeners = new ConcurrentHashMap[String, Int]()
@@ -50,6 +52,9 @@ class GameRedisSubscriberManager:
       val obs = new GameRedisPublisher(gameId, registry, redisson, objectMapper, s2cTopicName(gameId), writebackFn, ioClient, unsubscribeGame)
       s2cObservers.put(gameId, obs)
       registry.get(gameId).foreach(_.engine.subscribe(obs))
+
+      if heartbeatService != null then
+        heartbeatService.addGameSubscription(gameId)
     catch
       case e: Exception =>
         System.err.println(s"Warning: Redis subscription failed for game $gameId: ${e.getMessage}")
@@ -63,6 +68,9 @@ class GameRedisSubscriberManager:
       registry.get(gameId).foreach(_.engine.unsubscribe(obs))
     }
 
+    if heartbeatService != null then
+      heartbeatService.removeGameSubscription(gameId)
+
   private def handleC2sMessage(gameId: String, msg: String): Unit =
     parseC2sMessage(msg) match
       case Some(C2sMessage.Connected)   => handleConnected(gameId)
@@ -92,6 +100,35 @@ class GameRedisSubscriberManager:
       }
     }
 
+  def batchResubscribeGames(gameIds: java.util.List[String]): Int =
+    var count = 0
+    gameIds.forEach { gameId =>
+      subscribeGame(gameId)
+      count += 1
+    }
+    count
+
+  def unsubscribeGames(gameIds: java.util.List[String]): Int =
+    var count = 0
+    gameIds.forEach { gameId =>
+      unsubscribeGame(gameId)
+      count += 1
+    }
+    count
+
+  def evictGames(gameIds: java.util.List[String]): Int =
+    var count = 0
+    gameIds.forEach { gameId =>
+      unsubscribeGame(gameId)
+      registry.remove(gameId)
+      count += 1
+    }
+    count
+
+  def drainInstance(): Unit =
+    val gameIds = new java.util.ArrayList(c2sListeners.keySet())
+    gameIds.forEach(unsubscribeGame)
+
   @PreDestroy
   def cleanup(): Unit =
     c2sListeners.forEach((gameId, listenerId) =>
diff --git a/modules/core/src/main/scala/de/nowchess/chess/service/InstanceHeartbeatService.scala b/modules/core/src/main/scala/de/nowchess/chess/service/InstanceHeartbeatService.scala
new file mode 100644
index 0000000..058ac45
--- /dev/null
+++ b/modules/core/src/main/scala/de/nowchess/chess/service/InstanceHeartbeatService.scala
@@ -0,0 +1,186 @@
+package de.nowchess.chess.service
+
+import jakarta.enterprise.context.ApplicationScoped
+import jakarta.enterprise.event.Observes
+import jakarta.inject.Inject
+import io.quarkus.runtime.StartupEvent
+import io.quarkus.runtime.ShutdownEvent
+import io.quarkus.grpc.GrpcClient
+import org.redisson.api.RedissonClient
+import scala.concurrent.duration.*
+import java.util.concurrent.{Executors, TimeUnit}
+import java.net.InetAddress
+import com.fasterxml.jackson.databind.ObjectMapper
+import org.jboss.logging.Logger
+import de.nowchess.coordinator.{HeartbeatFrame, CoordinatorServiceGrpc}
+import de.nowchess.coordinator.CoordinatorServiceGrpc.CoordinatorServiceStub
+import io.grpc.stub.StreamObserver
+import scala.jdk.FutureConverters.*
+
+@ApplicationScoped
+class InstanceHeartbeatService:
+  @Inject
+  private var redissonClient: RedissonClient = _
+
+  @GrpcClient("coordinator-grpc")
+  private var coordinatorStub: CoordinatorServiceStub = _
+
+  private val log = Logger.getLogger(classOf[InstanceHeartbeatService])
+  private val mapper = ObjectMapper()
+
+  private var instanceId = ""
+  private var redisPrefix = "nowchess"
+  private var streamObserver: Option[StreamObserver[HeartbeatFrame]] = None
+  private var heartbeatExecutor = Executors.newScheduledThreadPool(1)
+  private var redisHeartbeatExecutor = Executors.newScheduledThreadPool(1)
+  private var subscriptionCount = 0
+  private var localCacheSize = 0
+
+  def onStart(@Observes event: StartupEvent): Unit =
+    try
+      generateInstanceId()
+      initializeHeartbeatStream()
+      scheduleHeartbeats()
+      log.infof("Instance heartbeat service started with ID: %s", instanceId)
+    catch
+      case ex: Exception =>
+        log.errorf(ex, "Failed to start instance heartbeat service")
+
+  def onShutdown(@Observes event: ShutdownEvent): Unit =
+    try
+      cleanup()
+      log.info("Instance heartbeat service stopped")
+    catch
+      case ex: Exception =>
+        log.errorf(ex, "Error during heartbeat service shutdown")
+
+  def setRedisPrefix(prefix: String): Unit =
+    redisPrefix = prefix
+
+  def setSubscriptionCount(count: Int): Unit =
+    subscriptionCount = count
+
+  def setLocalCacheSize(count: Int): Unit =
+    localCacheSize = count
+
+  def addGameSubscription(gameId: String): Unit =
+    val setKey = s"$redisPrefix:instance:$instanceId:games"
+    val gameSet = redissonClient.getSet[String](setKey)
+    gameSet.add(gameId)
+    subscriptionCount += 1
+
+  def removeGameSubscription(gameId: String): Unit =
+    val setKey = s"$redisPrefix:instance:$instanceId:games"
+    val gameSet = redissonClient.getSet[String](setKey)
+    gameSet.remove(gameId)
+    subscriptionCount = Math.max(0, subscriptionCount - 1)
+
+  private def generateInstanceId(): Unit =
+    val hostname = try
+      InetAddress.getLocalHost.getHostName
+    catch
+      case _: Exception => "unknown"
+
+    val uuid = java.util.UUID.randomUUID().toString.take(8)
+    instanceId = s"$hostname-$uuid"
+
+  private def initializeHeartbeatStream(): Unit =
+    try
+      val responseObserver = new StreamObserver[de.nowchess.coordinator.CoordinatorCommand]:
+        override def onNext(value: de.nowchess.coordinator.CoordinatorCommand): Unit =
+          log.debugf("Received coordinator command: %s", value.getType)
+
+        override def onError(t: Throwable): Unit =
+          log.warnf(t, "Heartbeat stream error")
+          // Reconnect on error
+          () // Placeholder for reconnect logic
+
+        override def onCompleted: Unit =
+          log.info("Heartbeat stream completed")
+
+      streamObserver = Some(coordinatorStub.heartbeatStream(responseObserver))
+      log.info("Connected to coordinator heartbeat stream")
+    catch
+      case ex: Exception =>
+        log.warnf(ex, "Failed to connect to coordinator")
+        streamObserver = None
+
+  private def scheduleHeartbeats(): Unit =
+    // Send heartbeat every 200ms
+    heartbeatExecutor.scheduleAtFixedRate(
+      () => sendHeartbeat(),
+      0,
+      200,
+      TimeUnit.MILLISECONDS
+    )
+
+    // Refresh Redis TTL every 2s
+    redisHeartbeatExecutor.scheduleAtFixedRate(
+      () => refreshRedisHeartbeat(),
+      0,
+      2,
+      TimeUnit.SECONDS
+    )
+
+  private def sendHeartbeat(): Unit =
+    streamObserver.foreach { observer =>
+      try
+        val frame = HeartbeatFrame.newBuilder()
+          .setInstanceId(instanceId)
+          .setHostname(getHostname)
+          .setHttpPort(8080) // Placeholder, should be configurable
+          .setGrpcPort(9080) // Placeholder
+          .setSubscriptionCount(subscriptionCount)
+          .setLocalCacheSize(localCacheSize)
+          .setTimestampMillis(System.currentTimeMillis())
+          .build()
+        observer.onNext(frame)
+      catch
+        case ex: Exception =>
+          log.warnf(ex, "Failed to send heartbeat frame")
+    }
+
+  private def refreshRedisHeartbeat(): Unit =
+    try
+      val key = s"$redisPrefix:instances:$instanceId"
+      val bucket = redissonClient.getBucket[String](key)
+
+      val metadata = Map(
+        "instanceId" -> instanceId,
+        "hostname" -> getHostname,
+        "httpPort" -> 8080,
+        "grpcPort" -> 9080,
+        "subscriptionCount" -> subscriptionCount,
+        "localCacheSize" -> localCacheSize,
+        "lastHeartbeat" -> java.time.Instant.now().toString,
+        "state" -> "HEALTHY"
+      )
+
+      val json = mapper.writeValueAsString(metadata)
+      bucket.set(json, 5, TimeUnit.SECONDS) // 5-second TTL, refreshed every 2s
+    catch
+      case ex: Exception =>
+        log.warnf(ex, "Failed to refresh Redis heartbeat")
+
+  private def getHostname: String =
+    try
+      InetAddress.getLocalHost.getHostName
+    catch
+      case _: Exception => "unknown"
+
+  private def cleanup(): Unit =
+    streamObserver.foreach(_.onCompleted())
+    streamObserver = None
+
+    val key = s"$redisPrefix:instances:$instanceId"
+    redissonClient.getBucket[String](key).delete()
+
+    val setKey = s"$redisPrefix:instance:$instanceId:games"
+    redissonClient.getSet[String](setKey).delete()
+
+    heartbeatExecutor.shutdown()
+    redisHeartbeatExecutor.shutdown()
+    if !heartbeatExecutor.awaitTermination(5, TimeUnit.SECONDS) then
+      heartbeatExecutor.shutdownNow()
+    if !redisHeartbeatExecutor.awaitTermination(5, TimeUnit.SECONDS) then
+      redisHeartbeatExecutor.shutdownNow()
diff --git a/settings.gradle.kts b/settings.gradle.kts
index 4d39f7a..37e1ae2 100644
--- a/settings.gradle.kts
+++ b/settings.gradle.kts
@@ -23,4 +23,5 @@ include(
     "modules:account",
     "modules:ws",
     "modules:store",
+    "modules:coordinator",
 )
\ No newline at end of file