feat: add forced-host label for Velocity forced hosts support

Add a new 'forced-host' label/annotation that can be set on Pods and
ReplicaSets to configure Velocity's forced hosts functionality. Players
connecting via a specific hostname will be routed to the server with
the matching forced-host label.

See: https://docs.papermc.io/velocity/configuration/\#forced-hosts-section

Author: ChipWolf

.gitignore

@@ -109,4 +109,6 @@ buildNumber.properties
 # JDT-specific (Eclipse Java Development Tools)
 .classpath
 
-# End of https://www.toptal.com/developers/gitignore/api/maven,intellij+all
+# End of https://www.toptal.com/developers/gitignore/api/maven,intellij+all
+
+.vscode/

src/main/java/net/azisaba/kuvel/KuvelServiceHandler.java

@@ -10,8 +10,11 @@
 import java.util.ArrayList;
 import java.util.HashMap;
 import java.util.List;
+import java.util.Map;
 import java.util.Map.Entry;
 import java.util.Optional;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.CopyOnWriteArrayList;
 import java.util.concurrent.atomic.AtomicReference;
 import javax.annotation.Nullable;
 
@@ -38,6 +41,7 @@ public class KuvelServiceHandler {
   private final UidAndServerNameMap replicaSetUidAndServerNameMap = new UidAndServerNameMap();
 
   private final List<String> initialServerNames = new ArrayList<>();
+  private final Map<String, List<String>> forcedHosts = new ConcurrentHashMap<>();
 
   private final AtomicReference<ServerDiscovery> serverDiscovery = new AtomicReference<>();
   private final AtomicReference<LoadBalancerDiscovery> loadBalancerDiscovery =
@@ -59,6 +63,10 @@ public void registerLoadBalancer(LoadBalancer loadBalancer) {
       initialServerNames.add(serverName);
     }
 
+    if (loadBalancer.getForcedHost() != null) {
+      addForcedHost(loadBalancer.getForcedHost(), serverName);
+    }
+
     plugin
         .getLogger()
         .info(
@@ -101,6 +109,7 @@ public void unregisterLoadBalancer(LoadBalancer loadBalancer) {
     replicaSetUidAndServerNameMap.unregister(loadBalancer.getReplicaSetUid());
 
     initialServerNames.remove(serverName);
+    removeForcedHost(serverName);
 
     plugin
         .getLogger()
@@ -244,13 +253,26 @@ public boolean registerPod(Pod pod, String serverName) {
       }
     }
 
+    String labelKeyPrefix = plugin.getKuvelConfig().getLabelKeyPrefix();
     String initialServerStr =
         pod.getMetadata().getLabels().getOrDefault(
-                LabelKeys.INITIAL_SERVER.getKey(plugin.getKuvelConfig().getLabelKeyPrefix()), "false");
+                LabelKeys.INITIAL_SERVER.getKey(labelKeyPrefix), "false");
     if (Boolean.parseBoolean(initialServerStr)) {
       initialServerNames.add(serverName);
     }
 
+    String forcedHost =
+        pod.getMetadata().getLabels().getOrDefault(
+                LabelKeys.FORCED_HOST.getKey(labelKeyPrefix), null);
+    if (forcedHost == null) {
+      forcedHost =
+          pod.getMetadata().getAnnotations().getOrDefault(
+                  LabelKeys.FORCED_HOST.getKey(labelKeyPrefix), null);
+    }
+    if (forcedHost != null) {
+      addForcedHost(forcedHost, serverName);
+    }
+
     plugin
         .getLogger()
         .info("Registered server: " + serverName + " (" + pod.getMetadata().getUid() + ")");
@@ -315,6 +337,7 @@ public void unregisterPod(String podUid) {
     }
 
     initialServerNames.remove(serverName);
+    removeForcedHost(serverName);
 
     plugin.getLogger().info("Unregistered server: " + serverName + " (" + podUid + ")");
   }
@@ -337,4 +360,17 @@ public void unregisterPod(Pod pod) {
   public boolean isPodRegistered(String podId) {
     return podUidAndServerNameMap.getServerNameFromUid(podId) != null;
   }
+
+  private void addForcedHost(String hostname, String serverName) {
+    forcedHosts.computeIfAbsent(hostname, k -> new CopyOnWriteArrayList<>()).add(serverName);
+  }
+
+  private void removeForcedHost(String serverName) {
+    forcedHosts.values().forEach(list -> list.remove(serverName));
+    forcedHosts.entrySet().removeIf(entry -> entry.getValue().isEmpty());
+  }
+
+  public Map<String, List<String>> getForcedHosts() {
+    return forcedHosts;
+  }
 }

src/main/java/net/azisaba/kuvel/discovery/impl/redis/RedisLoadBalancerDiscovery.java

@@ -136,6 +136,16 @@ private void registerOrIgnore(ReplicaSet replicaSet, boolean isFetchedFromRedis)
             .getLabels()
             .getOrDefault(LabelKeys.INITIAL_SERVER.getKey(labelKeyPrefix), "false")
             .equalsIgnoreCase("true");
+    String forcedHost =
+        metadata
+            .getLabels()
+            .getOrDefault(LabelKeys.FORCED_HOST.getKey(labelKeyPrefix), null);
+    if (forcedHost == null) {
+      forcedHost =
+          metadata
+              .getAnnotations()
+              .getOrDefault(LabelKeys.FORCED_HOST.getKey(labelKeyPrefix), null);
+    }
     if (isDisableLoadBalancer(metadata)) {
       return;
     }
@@ -169,7 +179,7 @@ private void registerOrIgnore(ReplicaSet replicaSet, boolean isFetchedFromRedis)
       kuvelServiceHandler.getReplicaSetUidAndServerNameMap().register(uid, serverName);
       jedis.hset(RedisKeys.LOAD_BALANCERS_PREFIX.getKey() + groupName, uid, serverName);
 
-      redisConnectionLeader.publishNewLoadBalancer(uid, serverName, initialServer);
+      redisConnectionLeader.publishNewLoadBalancer(uid, serverName, initialServer, forcedHost);
 
       RegisteredServer server =
           plugin
@@ -182,7 +192,8 @@ private void registerOrIgnore(ReplicaSet replicaSet, boolean isFetchedFromRedis)
               server,
               new RoundRobinLoadBalancingStrategy(),
               uid,
-              initialServer));
+              initialServer,
+              forcedHost));
     }
   }
 

src/main/java/net/azisaba/kuvel/listener/ChooseInitialServerListener.java

@@ -4,6 +4,7 @@
 import com.velocitypowered.api.event.player.PlayerChooseInitialServerEvent;
 import com.velocitypowered.api.proxy.ProxyServer;
 import com.velocitypowered.api.proxy.server.RegisteredServer;
+import java.net.InetSocketAddress;
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.List;
@@ -19,6 +20,21 @@ public class ChooseInitialServerListener {
 
   @Subscribe
   public void onInitialServerChoose(PlayerChooseInitialServerEvent event) {
+    Optional<InetSocketAddress> virtualHost = event.getPlayer().getVirtualHost();
+    if (virtualHost.isPresent()) {
+      String hostname = virtualHost.get().getHostString();
+      List<String> forcedServerNames = handler.getForcedHosts().get(hostname);
+      if (forcedServerNames != null && !forcedServerNames.isEmpty()) {
+        for (String serverName : forcedServerNames) {
+          Optional<RegisteredServer> optionalServer = proxy.getServer(serverName);
+          if (optionalServer.isPresent()) {
+            event.setInitialServer(optionalServer.get());
+            return;
+          }
+        }
+      }
+    }
+
     if (handler.getInitialServerNames().isEmpty()) {
       return;
     }

src/main/java/net/azisaba/kuvel/loadbalancer/LoadBalancer.java

@@ -19,6 +19,7 @@ public class LoadBalancer {
   private final String replicaSetUid;
 
   private final boolean isInitialServer;
+  private final String forcedHost;
   private final List<String> endpointServers = new ArrayList<>();
 
   public void addEndpoint(String serverName) {

src/main/java/net/azisaba/kuvel/redis/RedisConnectionLeader.java

@@ -80,11 +80,11 @@ public void leaveLeader() {
   }
 
   public void publishNewLoadBalancer(
-      String replicaSetUid, String serverName, boolean initialServer) {
+      String replicaSetUid, String serverName, boolean initialServer, String forcedHost) {
     try (Jedis jedis = jedisPool.getResource()) {
       jedis.publish(
           RedisKeys.LOAD_BALANCER_ADDED_NOTIFY_PREFIX.getKey() + groupName,
-          replicaSetUid + ":" + serverName + ":" + initialServer);
+          replicaSetUid + ":" + serverName + ":" + initialServer + ":" + (forcedHost != null ? forcedHost : ""));
     }
   }
 

src/main/java/net/azisaba/kuvel/redis/RedisSubscriber.java

@@ -45,6 +45,10 @@ public void onPMessage(String pattern, String channel, String message) {
       String replicaSetUid = message.split(":")[0];
       String serverName = message.split(":")[1];
       boolean initialServer = Boolean.parseBoolean(message.split(":")[2]);
+      String forcedHost = message.split(":").length > 3 ? message.split(":")[3] : null;
+      if (forcedHost != null && forcedHost.isEmpty()) {
+        forcedHost = null;
+      }
 
       RegisteredServer server =
           plugin
@@ -56,7 +60,8 @@ public void onPMessage(String pattern, String channel, String message) {
               server,
               new RoundRobinLoadBalancingStrategy(),
               replicaSetUid,
-              initialServer);
+              initialServer,
+              forcedHost);
       kuvelServiceHandler.registerLoadBalancer(loadBalancer);
     } else if (channel.startsWith(RedisKeys.POD_DELETED_NOTIFY_PREFIX.getKey())) {
       kuvelServiceHandler.unregisterPod(message);

src/main/java/net/azisaba/kuvel/util/LabelKeys.java

@@ -9,6 +9,7 @@ public enum LabelKeys {
   INITIAL_SERVER("initial-server"),
   DISABLE_NAME_SUFFIX("disable-name-suffix"),
   DISABLE_LOAD_BALANCER("disable-load-balancer"),
+  FORCED_HOST("forced-host"),
   ;
 
   private final String key;