WIP

Caused by 72ddaf91.

Agent/Phantom.Agent.Minecraft/Properties/MinecraftServerProperties.cs

@@ -18,4 +18,5 @@ static class MinecraftServerProperties {
 	public static readonly MinecraftServerProperty<ushort> ServerPort = new UnsignedShort("server-port");
 	public static readonly MinecraftServerProperty<ushort> RconPort = new UnsignedShort("rcon.port");
 	public static readonly MinecraftServerProperty<bool> EnableRcon = new Boolean("enable-rcon");
+	public static readonly MinecraftServerProperty<bool> SyncChunkWrites = new Boolean("sync-chunk-writes");
 }

Agent/Phantom.Agent.Minecraft/Properties/ServerProperties.cs

@@ -5,11 +5,13 @@ namespace Phantom.Agent.Minecraft.Properties;
 public sealed record ServerProperties(
 	ushort ServerPort,
 	ushort RconPort,
-	bool EnableRcon = true
+	bool EnableRcon = true,
+	bool SyncChunkWrites = false
 ) {
 	internal void SetTo(JavaPropertiesFileEditor properties) {
 		MinecraftServerProperties.ServerPort.Set(properties, ServerPort);
 		MinecraftServerProperties.RconPort.Set(properties, RconPort);
 		MinecraftServerProperties.EnableRcon.Set(properties, EnableRcon);
+		MinecraftServerProperties.SyncChunkWrites.Set(properties, SyncChunkWrites);
 	}
 }

Agent/Phantom.Agent.Minecraft/Server/ServerStatusProtocol.cs

@@ -3,28 +3,12 @@ using System.Buffers.Binary;
 using System.Net;
 using System.Net.Sockets;
 using System.Text;
-using Phantom.Utils.Logging;
+using Phantom.Common.Data.Instance;
-using Serilog;
 
 namespace Phantom.Agent.Minecraft.Server;
 
-public sealed class ServerStatusProtocol {
+public static class ServerStatusProtocol {
-	private readonly ILogger logger;
+	public static async Task<InstancePlayerCounts> GetPlayerCounts(ushort serverPort, CancellationToken cancellationToken) {
-
-	public ServerStatusProtocol(string loggerName) {
-		this.logger = PhantomLogger.Create<ServerStatusProtocol>(loggerName);
-	}
-
-	public async Task<int?> GetOnlinePlayerCount(int serverPort, CancellationToken cancellationToken) {
-		try {
-			return await GetOnlinePlayerCountOrThrow(serverPort, cancellationToken);
-		} catch (Exception e) {
-			logger.Error(e, "Caught exception while checking if players are online.");
-			return null;
-		}
-	}
-
-	private async Task<int?> GetOnlinePlayerCountOrThrow(int serverPort, CancellationToken cancellationToken) {
 		using var tcpClient = new TcpClient();
 		await tcpClient.ConnectAsync(IPAddress.Loopback, serverPort, cancellationToken);
 		var tcpStream = tcpClient.GetStream();
@@ -33,24 +17,22 @@ public sealed class ServerStatusProtocol {
 		tcpStream.WriteByte(0xFE);
 		await tcpStream.FlushAsync(cancellationToken);
 
-		short? messageLength = await ReadStreamHeader(tcpStream, cancellationToken);
+		short messageLength = await ReadStreamHeader(tcpStream, cancellationToken);
-		return messageLength == null ? null : await ReadOnlinePlayerCount(tcpStream, messageLength.Value * 2, cancellationToken);
+		return await ReadPlayerCounts(tcpStream, messageLength * 2, cancellationToken);
 	}
 
-	private async Task<short?> ReadStreamHeader(NetworkStream tcpStream, CancellationToken cancellationToken) {
+	private static async Task<short> ReadStreamHeader(NetworkStream tcpStream, CancellationToken cancellationToken) {
 		var headerBuffer = ArrayPool<byte>.Shared.Rent(3);
 		try {
 			await tcpStream.ReadExactlyAsync(headerBuffer, 0, 3, cancellationToken);
 
 			if (headerBuffer[0] != 0xFF) {
-				logger.Error("Unexpected first byte in response from server: {FirstByte}.", headerBuffer[0]);
+				throw new ProtocolException("Unexpected first byte in response from server: " + headerBuffer[0]);
-				return null;
 			}
 
 			short messageLength = BinaryPrimitives.ReadInt16BigEndian(headerBuffer.AsSpan(1));
 			if (messageLength <= 0) {
-				logger.Error("Unexpected message length in response from server: {MessageLength}.", messageLength);
+				throw new ProtocolException("Unexpected message length in response from server: " + messageLength);
-				return null;
 			}
 			
 			return messageLength;
@@ -59,35 +41,54 @@ public sealed class ServerStatusProtocol {
 		}
 	}
 
-	private async Task<int?> ReadOnlinePlayerCount(NetworkStream tcpStream, int messageLength, CancellationToken cancellationToken) {
+	private static async Task<InstancePlayerCounts> ReadPlayerCounts(NetworkStream tcpStream, int messageLength, CancellationToken cancellationToken) {
 		var messageBuffer = ArrayPool<byte>.Shared.Rent(messageLength);
 		try {
 			await tcpStream.ReadExactlyAsync(messageBuffer, 0, messageLength, cancellationToken);
-
+			return ReadPlayerCountsFromResponse(messageBuffer.AsSpan(0, messageLength));
-			// Valid response separator encoded in UTF-16BE is 0x00 0xA7 (§).
-			const byte SeparatorSecondByte = 0xA7;
-			
-			static bool IsValidSeparator(ReadOnlySpan<byte> buffer, int index) {
-				return index > 0 && buffer[index - 1] == 0x00;
-			}
-			
-			int separator2 = Array.LastIndexOf(messageBuffer, SeparatorSecondByte);
-			int separator1 = separator2 == -1 ? -1 : Array.LastIndexOf(messageBuffer, SeparatorSecondByte, separator2 - 1);
-			if (!IsValidSeparator(messageBuffer, separator1) || !IsValidSeparator(messageBuffer, separator2)) {
-				logger.Error("Could not find message separators in response from server.");
-				return null;
-			}
-
-			string onlinePlayerCountStr = Encoding.BigEndianUnicode.GetString(messageBuffer.AsSpan((separator1 + 1)..(separator2 - 1)));
-			if (!int.TryParse(onlinePlayerCountStr, out int onlinePlayerCount)) {
-				logger.Error("Could not parse online player count in response from server: {OnlinePlayerCount}.", onlinePlayerCountStr);
-				return null;
-			}
-			
-			logger.Debug("Detected {OnlinePlayerCount} online player(s).", onlinePlayerCount);
-			return onlinePlayerCount;
 		} finally {
 			ArrayPool<byte>.Shared.Return(messageBuffer);
 		}
 	}
+	
+	/// <summary>
+	/// Legacy query protocol uses the paragraph symbol (§) as separator encoded in UTF-16BE.
+	/// </summary>
+	private static readonly byte[] Separator = { 0x00, 0xA7 };
+	
+	private static InstancePlayerCounts ReadPlayerCountsFromResponse(ReadOnlySpan<byte> messageBuffer) {
+		int lastSeparator = messageBuffer.LastIndexOf(Separator);
+		int middleSeparator = messageBuffer[..lastSeparator].LastIndexOf(Separator);
+		
+		if (lastSeparator == -1 || middleSeparator == -1) {
+			throw new ProtocolException("Could not find message separators in response from server.");
+		}
+
+		var onlinePlayerCountBuffer = messageBuffer[(middleSeparator + Separator.Length)..lastSeparator];
+		var maximumPlayerCountBuffer = messageBuffer[(lastSeparator + Separator.Length)..];
+		
+		// Player counts are integers, whose maximum string length is 10 characters.
+		Span<char> integerStringBuffer = stackalloc char[10];
+		
+		return new InstancePlayerCounts(
+			DecodeAndParsePlayerCount(onlinePlayerCountBuffer, integerStringBuffer, "online"),
+			DecodeAndParsePlayerCount(maximumPlayerCountBuffer, integerStringBuffer, "maximum")
+		);
+	}
+	
+	private static int DecodeAndParsePlayerCount(ReadOnlySpan<byte> inputBuffer, Span<char> tempCharBuffer, string countType) {
+		if (!Encoding.BigEndianUnicode.TryGetChars(inputBuffer, tempCharBuffer, out int charCount)) {
+			throw new ProtocolException("Could not decode " + countType + " player count in response from server.");
+		}
+		
+		if (!int.TryParse(tempCharBuffer, out int playerCount)) {
+			throw new ProtocolException("Could not parse " + countType + " player count in response from server: " + tempCharBuffer[..charCount].ToString());
+		}
+		
+		return playerCount;
+	}
+	
+	public sealed class ProtocolException : Exception {
+		internal ProtocolException(string message) : base(message) {}
+	}
 }

Agent/Phantom.Agent.Services/Backups/BackupScheduler.cs

@@ -1,10 +1,8 @@
-using Phantom.Agent.Minecraft.Instance;
+using Phantom.Agent.Services.Instances;
-using Phantom.Agent.Minecraft.Server;
+using Phantom.Agent.Services.Instances.State;
-using Phantom.Agent.Services.Instances;
 using Phantom.Common.Data.Backups;
 using Phantom.Utils.Logging;
 using Phantom.Utils.Tasks;
-using Phantom.Utils.Threading;
 
 namespace Phantom.Agent.Services.Backups;
 
@@ -16,20 +14,16 @@ sealed class BackupScheduler : CancellableBackgroundTask {
 
 	private readonly BackupManager backupManager;
 	private readonly InstanceContext context;
-	private readonly InstanceProcess process;
 	private readonly SemaphoreSlim backupSemaphore = new (1, 1);
-	private readonly int serverPort;
-	private readonly ServerStatusProtocol serverStatusProtocol;
 	private readonly ManualResetEventSlim serverOutputWhileWaitingForOnlinePlayers = new ();
+	private readonly InstancePlayerCountTracker playerCountTracker;
 	
 	public event EventHandler<BackupCreationResult>? BackupCompleted;
 
-	public BackupScheduler(InstanceContext context, InstanceProcess process, int serverPort) : base(PhantomLogger.Create<BackupScheduler>(context.ShortName)) {
+	public BackupScheduler(InstanceContext context, InstancePlayerCountTracker playerCountTracker) : base(PhantomLogger.Create<BackupScheduler>(context.ShortName)) {
 		this.backupManager = context.Services.BackupManager;
 		this.context = context;
-		this.process = process;
+		this.playerCountTracker = playerCountTracker;
-		this.serverPort = serverPort;
-		this.serverStatusProtocol = new ServerStatusProtocol(context.ShortName);
 		Start();
 	}
 
@@ -69,43 +63,18 @@ sealed class BackupScheduler : CancellableBackgroundTask {
 	}
 
 	private async Task WaitForOnlinePlayers() {
-		bool needsToLogOfflinePlayersMessage = true;
+		var task = playerCountTracker.WaitForOnlinePlayers(CancellationToken);
+		if (!task.IsCompleted) {
+			Logger.Information("Waiting for someone to join before starting a new backup.");
+		}
 		
-		process.AddOutputListener(ServerOutputListener, maxLinesToReadFromHistory: 0);
 		try {
-			while (!CancellationToken.IsCancellationRequested) {
+			await task;
-				serverOutputWhileWaitingForOnlinePlayers.Reset();
-				
-				var onlinePlayerCount = await serverStatusProtocol.GetOnlinePlayerCount(serverPort, CancellationToken);
-				if (onlinePlayerCount == null) {
-					Logger.Warning("Could not detect whether any players are online, starting a new backup.");
-					break;
-				}
-
-				if (onlinePlayerCount > 0) {
 			Logger.Information("Players are online, starting a new backup.");
-					break;
+		} catch (OperationCanceledException) {
-				}
+			throw;
-
+		} catch (Exception) {
-				if (needsToLogOfflinePlayersMessage) {
+			Logger.Warning("Could not detect whether any players are online, starting a new backup.");
-					needsToLogOfflinePlayersMessage = false;
-					Logger.Information("No players are online, waiting for someone to join before starting a new backup.");
-				}
-
-				await Task.Delay(TimeSpan.FromSeconds(10), CancellationToken);
-				
-				Logger.Debug("Waiting for server output before checking for online players again...");
-				await serverOutputWhileWaitingForOnlinePlayers.WaitHandle.WaitOneAsync(CancellationToken);
-			}
-		} finally {
-			process.RemoveOutputListener(ServerOutputListener);
-		}
-	}
-
-	private void ServerOutputListener(object? sender, string line) {
-		if (!serverOutputWhileWaitingForOnlinePlayers.IsSet) {
-			serverOutputWhileWaitingForOnlinePlayers.Set();
-			Logger.Debug("Detected server output, signalling to check for online players again.");
 		}
 	}
 

Agent/Phantom.Agent.Services/Backups/BackupServerCommandDispatcher.cs

@@ -55,7 +55,6 @@ sealed partial class BackupServerCommandDispatcher : IDisposable {
 	}
 
 	public async Task SaveAllChunks() {
-		// TODO Try if not flushing and waiting a few seconds before flushing reduces lag.
 		await process.SendCommand(MinecraftCommand.SaveAll(flush: true), cancellationToken);
 		await savedTheGame.Task.WaitAsync(TimeSpan.FromMinutes(1), cancellationToken);
 	}

Agent/Phantom.Agent.Services/Instances/State/InstancePlayerCountTracker.cs

@@ -0,0 +1,140 @@
+using Phantom.Agent.Minecraft.Instance;
+using Phantom.Agent.Minecraft.Server;
+using Phantom.Agent.Rpc;
+using Phantom.Common.Data.Instance;
+using Phantom.Common.Messages.Agent.ToController;
+using Phantom.Utils.Logging;
+using Phantom.Utils.Tasks;
+using Phantom.Utils.Threading;
+
+namespace Phantom.Agent.Services.Instances.State;
+
+sealed class InstancePlayerCountTracker : CancellableBackgroundTask {
+	private readonly ControllerConnection controllerConnection;
+	private readonly Guid instanceGuid;
+	private readonly ushort serverPort;
+	private readonly InstanceProcess process;
+
+	private readonly TaskCompletionSource firstDetection = AsyncTasks.CreateCompletionSource();
+	private readonly ManualResetEventSlim serverOutputEvent = new ();
+
+	private InstancePlayerCounts? playerCounts;
+
+	public InstancePlayerCounts? PlayerCounts {
+		get {
+			lock (this) {
+				return playerCounts;
+			}
+		}
+		private set {
+			EventHandler<int?>? onlinePlayerCountChanged;
+			lock (this) {
+				if (playerCounts == value) {
+					return;
+				}
+				
+				playerCounts = value;
+				onlinePlayerCountChanged = OnlinePlayerCountChanged;
+			}
+
+			onlinePlayerCountChanged?.Invoke(this, value?.Online);
+			controllerConnection.Send(new ReportInstancePlayerCountsMessage(instanceGuid, value));
+		}
+	}
+
+	private event EventHandler<int?>? OnlinePlayerCountChanged;
+
+	private bool isDisposed = false;
+
+	public InstancePlayerCountTracker(InstanceContext context, InstanceProcess process, ushort serverPort) : base(PhantomLogger.Create<InstancePlayerCountTracker>(context.ShortName)) {
+		this.controllerConnection = context.Services.ControllerConnection;
+		this.instanceGuid = context.InstanceGuid;
+		this.process = process;
+		this.serverPort = serverPort;
+		Start();
+	}
+
+	protected override async Task RunTask() {
+		// Give the server time to start accepting connections.
+		await Task.Delay(TimeSpan.FromSeconds(10), CancellationToken);
+
+		serverOutputEvent.Set();
+		process.AddOutputListener(OnOutput, maxLinesToReadFromHistory: 0);
+		
+		while (!CancellationToken.IsCancellationRequested) {
+			serverOutputEvent.Reset();
+
+			PlayerCounts = await TryGetPlayerCounts();
+			
+			if (!firstDetection.Task.IsCompleted) {
+				firstDetection.SetResult();
+			}
+
+			await Task.Delay(TimeSpan.FromSeconds(10), CancellationToken);
+			await serverOutputEvent.WaitHandle.WaitOneAsync(CancellationToken);
+			await Task.Delay(TimeSpan.FromSeconds(1), CancellationToken);
+		}
+	}
+
+	private async Task<InstancePlayerCounts?> TryGetPlayerCounts() {
+		try {
+			var result = await ServerStatusProtocol.GetPlayerCounts(serverPort, CancellationToken);
+			Logger.Debug("Detected {OnlinePlayerCount} / {MaximumPlayerCount} online player(s).", result.Online, result.Maximum);
+			return result;
+		} catch (ServerStatusProtocol.ProtocolException e) {
+			Logger.Error(e.Message);
+			return null;
+		} catch (Exception e) {
+			Logger.Error(e, "Caught exception while checking online player count.");
+			return null;
+		}
+	}
+
+	public async Task WaitForOnlinePlayers(CancellationToken cancellationToken) {
+		await firstDetection.Task.WaitAsync(cancellationToken);
+
+		var onlinePlayersDetected = AsyncTasks.CreateCompletionSource();
+
+		lock (this) {
+			if (playerCounts is { Online: > 0 }) {
+				return;
+			}
+			else if (playerCounts == null) {
+				throw new InvalidOperationException();
+			}
+
+			OnlinePlayerCountChanged += OnOnlinePlayerCountChanged;
+
+			void OnOnlinePlayerCountChanged(object? sender, int? newPlayerCount) {
+				if (newPlayerCount == null) {
+					onlinePlayersDetected.TrySetException(new InvalidOperationException());
+					OnlinePlayerCountChanged -= OnOnlinePlayerCountChanged;
+				}
+				else if (newPlayerCount > 0) {
+					onlinePlayersDetected.TrySetResult();
+					OnlinePlayerCountChanged -= OnOnlinePlayerCountChanged;
+				}
+			}
+		}
+
+		await onlinePlayersDetected.Task;
+	}
+
+	private void OnOutput(object? sender, string? line) {
+		lock (this) {
+			if (!isDisposed) {
+				serverOutputEvent.Set();
+			}
+		}
+	}
+
+	protected override void Dispose() {
+		lock (this) {
+			isDisposed = true;
+			playerCounts = null;
+		}
+
+		process.RemoveOutputListener(OnOutput);
+		serverOutputEvent.Dispose();
+	}
+}

Agent/Phantom.Agent.Services/Instances/State/InstanceRunningState.cs

@@ -19,6 +19,7 @@ sealed class InstanceRunningState : IDisposable {
 	private readonly CancellationToken cancellationToken;
 
 	private readonly InstanceLogSender logSender;
+	private readonly InstancePlayerCountTracker playerCountTracker;
 	private readonly BackupScheduler backupScheduler;
 
 	private bool isDisposed;
@@ -32,8 +33,9 @@ sealed class InstanceRunningState : IDisposable {
 		this.cancellationToken = cancellationToken;
 
 		this.logSender = new InstanceLogSender(context.Services.ControllerConnection, context.InstanceGuid, context.ShortName);
+		this.playerCountTracker = new InstancePlayerCountTracker(context, process, configuration.ServerPort);
 
-		this.backupScheduler = new BackupScheduler(context, process, configuration.ServerPort);
+		this.backupScheduler = new BackupScheduler(context, playerCountTracker);
 		this.backupScheduler.BackupCompleted += OnScheduledBackupCompleted;
 	}
 
@@ -93,6 +95,11 @@ sealed class InstanceRunningState : IDisposable {
 		}
 	}
 
+	public void OnStopInitiated() {
+		backupScheduler.Stop();
+		playerCountTracker.Stop();
+	}
+	
 	private bool TryDispose() {
 		lock (this) {
 			if (isDisposed) {
@@ -102,8 +109,8 @@ sealed class InstanceRunningState : IDisposable {
 			isDisposed = true;
 		}
 
+		OnStopInitiated();
 		logSender.Stop();
-		backupScheduler.Stop();
 		
 		Process.Dispose();
 		

Agent/Phantom.Agent.Services/Instances/State/InstanceStopProcedure.cs

@@ -25,6 +25,8 @@ static class InstanceStopProcedure {
 
 		try {
 			// Too late to cancel the stop procedure now.
+			runningState.OnStopInitiated();
+			
 			if (!process.HasEnded) {
 				context.Logger.Information("Session stopping now.");
 				await DoStop(context, process);
@@ -85,7 +87,7 @@ static class InstanceStopProcedure {
 	private static async Task WaitForSessionToEnd(InstanceContext context, InstanceProcess process) {
 		try {
 			await process.WaitForExit(TimeSpan.FromSeconds(55));
-		} catch (OperationCanceledException) {
+		} catch (TimeoutException) {
 			try {
 				context.Logger.Warning("Waiting timed out, killing session...");
 				process.Kill();

Common/Phantom.Common.Data.Web/Instance/Instance.cs

@@ -8,9 +8,10 @@ public sealed partial record Instance(
 	[property: MemoryPackOrder(0)] Guid InstanceGuid,
 	[property: MemoryPackOrder(1)] InstanceConfiguration Configuration,
 	[property: MemoryPackOrder(2)] IInstanceStatus Status,
-	[property: MemoryPackOrder(3)] bool LaunchAutomatically
+	[property: MemoryPackOrder(3)] InstancePlayerCounts? PlayerCounts,
+	[property: MemoryPackOrder(4)] bool LaunchAutomatically
 ) {
 	public static Instance Offline(Guid instanceGuid, InstanceConfiguration configuration, bool launchAutomatically = false) {
-		return new Instance(instanceGuid, configuration, InstanceStatus.Offline, launchAutomatically);
+		return new Instance(instanceGuid, configuration, InstanceStatus.Offline, PlayerCounts: null, launchAutomatically);
 	}
 }

Common/Phantom.Common.Data/Instance/InstancePlayerCounts.cs

@@ -0,0 +1,9 @@
+using MemoryPack;
+
+namespace Phantom.Common.Data.Instance;
+
+[MemoryPackable(GenerateType.VersionTolerant)]
+public readonly partial record struct InstancePlayerCounts(
+	[property: MemoryPackOrder(0)] int Online,
+	[property: MemoryPackOrder(1)] int Maximum
+);

Common/Phantom.Common.Messages.Agent/AgentMessageRegistries.cs

@@ -31,6 +31,7 @@ public static class AgentMessageRegistries {
 		ToController.Add<InstanceOutputMessage>(5);
 		ToController.Add<ReportAgentStatusMessage>(6);
 		ToController.Add<ReportInstanceEventMessage>(7);
+		ToController.Add<ReportInstancePlayerCountsMessage>(8);
 		ToController.Add<ReplyMessage>(127);
 	}
 

Common/Phantom.Common.Messages.Agent/ToController/ReportInstancePlayerCountsMessage.cs

@@ -0,0 +1,10 @@
+using MemoryPack;
+using Phantom.Common.Data.Instance;
+
+namespace Phantom.Common.Messages.Agent.ToController;
+
+[MemoryPackable(GenerateType.VersionTolerant)]
+public sealed partial record ReportInstancePlayerCountsMessage(
+	[property: MemoryPackOrder(0)] Guid InstanceGuid,
+	[property: MemoryPackOrder(1)] InstancePlayerCounts? PlayerCounts
+) : IMessageToController;

Controller/Phantom.Controller.Services/Agents/AgentActor.cs

@@ -96,6 +96,7 @@ sealed class AgentActor : ReceiveActor<AgentActor.ICommand> {
 		Receive<UpdateJavaRuntimesCommand>(UpdateJavaRuntimes);
 		ReceiveAndReplyLater<CreateOrUpdateInstanceCommand, Result<CreateOrUpdateInstanceResult, InstanceActionFailure>>(CreateOrUpdateInstance);
 		Receive<UpdateInstanceStatusCommand>(UpdateInstanceStatus);
+		Receive<UpdateInstancePlayerCountsCommand>(UpdateInstancePlayerCounts);
 		ReceiveAndReplyLater<LaunchInstanceCommand, Result<LaunchInstanceResult, InstanceActionFailure>>(LaunchInstance);
 		ReceiveAndReplyLater<StopInstanceCommand, Result<StopInstanceResult, InstanceActionFailure>>(StopInstance);
 		ReceiveAndReplyLater<SendCommandToInstanceCommand, Result<SendCommandToInstanceResult, InstanceActionFailure>>(SendMinecraftCommand);
@@ -159,7 +160,7 @@ sealed class AgentActor : ReceiveActor<AgentActor.ICommand> {
 	private async Task<ImmutableArray<ConfigureInstanceMessage>> PrepareInitialConfigurationMessages() {
 		var configurationMessages = ImmutableArray.CreateBuilder<ConfigureInstanceMessage>();
 		
-		foreach (var (instanceGuid, instanceConfiguration, _, launchAutomatically) in instanceDataByGuid.Values.ToImmutableArray()) {
+		foreach (var (instanceGuid, instanceConfiguration, _, _, launchAutomatically) in instanceDataByGuid.Values.ToImmutableArray()) {
 			var serverExecutableInfo = await minecraftVersions.GetServerExecutableInfo(instanceConfiguration.MinecraftVersion, cancellationToken);
 			configurationMessages.Add(new ConfigureInstanceMessage(instanceGuid, instanceConfiguration, new InstanceLaunchProperties(serverExecutableInfo), launchAutomatically));
 		}
@@ -187,6 +188,8 @@ sealed class AgentActor : ReceiveActor<AgentActor.ICommand> {
 	
 	public sealed record UpdateInstanceStatusCommand(Guid InstanceGuid, IInstanceStatus Status) : ICommand;
 	
+	public sealed record UpdateInstancePlayerCountsCommand(Guid InstanceGuid, InstancePlayerCounts? PlayerCounts) : ICommand;
+
 	public sealed record LaunchInstanceCommand(Guid LoggedInUserGuid, Guid InstanceGuid) : ICommand, ICanReply<Result<LaunchInstanceResult, InstanceActionFailure>>;
 	
 	public sealed record StopInstanceCommand(Guid LoggedInUserGuid, Guid InstanceGuid, MinecraftStopStrategy StopStrategy) : ICommand, ICanReply<Result<StopInstanceResult, InstanceActionFailure>>;
@@ -342,6 +345,10 @@ sealed class AgentActor : ReceiveActor<AgentActor.ICommand> {
 		TellInstance(command.InstanceGuid, new InstanceActor.SetStatusCommand(command.Status));
 	}
 	
+	private void UpdateInstancePlayerCounts(UpdateInstancePlayerCountsCommand command) {
+		TellInstance(command.InstanceGuid, new InstanceActor.SetPlayerCountsCommand(command.PlayerCounts));
+	}
+
 	private Task<Result<LaunchInstanceResult, InstanceActionFailure>> LaunchInstance(LaunchInstanceCommand command) {
 		return RequestInstance<InstanceActor.LaunchInstanceCommand, LaunchInstanceResult>(command.InstanceGuid, new InstanceActor.LaunchInstanceCommand(command.LoggedInUserGuid));
 	}

Controller/Phantom.Controller.Services/Instances/InstanceActor.cs

@@ -26,6 +26,7 @@ sealed class InstanceActor : ReceiveActor<InstanceActor.ICommand> {
 	
 	private InstanceConfiguration configuration;
 	private IInstanceStatus status;
+	private InstancePlayerCounts? playerCounts;
 	private bool launchAutomatically;
 
 	private readonly ActorRef<InstanceDatabaseStorageActor.ICommand> databaseStorageActor;
@@ -35,11 +36,12 @@ sealed class InstanceActor : ReceiveActor<InstanceActor.ICommand> {
 		this.agentConnection = init.AgentConnection;
 		this.cancellationToken = init.CancellationToken;
 		
-		(this.instanceGuid, this.configuration, this.status, this.launchAutomatically) = init.Instance;
+		(this.instanceGuid, this.configuration, this.status, this.playerCounts, this.launchAutomatically) = init.Instance;
 
 		this.databaseStorageActor = Context.ActorOf(InstanceDatabaseStorageActor.Factory(new InstanceDatabaseStorageActor.Init(instanceGuid, init.DbProvider, init.CancellationToken)), "DatabaseStorage");
 
 		Receive<SetStatusCommand>(SetStatus);
+		Receive<SetPlayerCountsCommand>(SetPlayerCounts);
 		ReceiveAsyncAndReply<ConfigureInstanceCommand, Result<ConfigureInstanceResult, InstanceActionFailure>>(ConfigureInstance);
 		ReceiveAsyncAndReply<LaunchInstanceCommand, Result<LaunchInstanceResult, InstanceActionFailure>>(LaunchInstance);
 		ReceiveAsyncAndReply<StopInstanceCommand, Result<StopInstanceResult, InstanceActionFailure>>(StopInstance);
@@ -47,7 +49,7 @@ sealed class InstanceActor : ReceiveActor<InstanceActor.ICommand> {
 	}
 
 	private void NotifyInstanceUpdated() {
-		agentActor.Tell(new AgentActor.ReceiveInstanceDataCommand(new Instance(instanceGuid, configuration, status, launchAutomatically)));
+		agentActor.Tell(new AgentActor.ReceiveInstanceDataCommand(new Instance(instanceGuid, configuration, status, playerCounts, launchAutomatically)));
 	}
 
 	private void SetLaunchAutomatically(bool newValue) {
@@ -66,6 +68,8 @@ sealed class InstanceActor : ReceiveActor<InstanceActor.ICommand> {
 
 	public sealed record SetStatusCommand(IInstanceStatus Status) : ICommand;
 	
+	public sealed record SetPlayerCountsCommand(InstancePlayerCounts? PlayerCounts) : ICommand;
+
 	public sealed record ConfigureInstanceCommand(Guid AuditLogUserGuid, Guid InstanceGuid, InstanceConfiguration Configuration, InstanceLaunchProperties LaunchProperties, bool IsCreatingInstance) : ICommand, ICanReply<Result<ConfigureInstanceResult, InstanceActionFailure>>;
 
 	public sealed record LaunchInstanceCommand(Guid AuditLogUserGuid) : ICommand, ICanReply<Result<LaunchInstanceResult, InstanceActionFailure>>;
@@ -76,6 +80,16 @@ sealed class InstanceActor : ReceiveActor<InstanceActor.ICommand> {
 
 	private void SetStatus(SetStatusCommand command) {
 		status = command.Status;
+		
+		if (!status.IsRunning() && status != InstanceStatus.Offline /* Guard against temporary disconnects */) {
+			playerCounts = null;
+		}
+		
+		NotifyInstanceUpdated();
+	}
+	
+	private void SetPlayerCounts(SetPlayerCountsCommand command) {
+		playerCounts = command.PlayerCounts;
 		NotifyInstanceUpdated();
 	}
 

Controller/Phantom.Controller.Services/Rpc/AgentMessageHandlerActor.cs

@@ -39,6 +39,7 @@ sealed class AgentMessageHandlerActor : ReceiveActor<IMessageToController> {
 		Receive<AdvertiseJavaRuntimesMessage>(HandleAdvertiseJavaRuntimes);
 		Receive<ReportAgentStatusMessage>(HandleReportAgentStatus);
 		Receive<ReportInstanceStatusMessage>(HandleReportInstanceStatus);
+		Receive<ReportInstancePlayerCountsMessage>(HandleReportInstancePlayerCounts);
 		Receive<ReportInstanceEventMessage>(HandleReportInstanceEvent);
 		Receive<InstanceOutputMessage>(HandleInstanceOutput);
 		Receive<ReplyMessage>(HandleReply);
@@ -74,6 +75,10 @@ sealed class AgentMessageHandlerActor : ReceiveActor<IMessageToController> {
 		agentManager.TellAgent(agentGuid, new AgentActor.UpdateInstanceStatusCommand(message.InstanceGuid, message.InstanceStatus));
 	}
 
+	private void HandleReportInstancePlayerCounts(ReportInstancePlayerCountsMessage message) {
+		agentManager.TellAgent(agentGuid, new AgentActor.UpdateInstancePlayerCountsCommand(message.InstanceGuid, message.PlayerCounts));
+	}
+
 	private void HandleReportInstanceEvent(ReportInstanceEventMessage message) {
 		message.Event.Accept(eventLogManager.CreateInstanceEventVisitor(message.EventGuid, message.UtcTime, agentGuid, message.InstanceGuid));
 	}

Web/Phantom.Web/Pages/Instances.razor

@@ -21,6 +21,7 @@
     <Column Width="40%">Agent</Column>
     <Column Width="40%">Name</Column>
     <Column MinWidth="215px">Status</Column>
+    <Column Class="text-center" MinWidth="120px">Players</Column>
     <Column Width="20%">Version</Column>
     <Column Class="text-center" MinWidth="110px">Server Port</Column>
     <Column Class="text-center" MinWidth="110px">Rcon Port</Column>
@@ -40,6 +41,14 @@
     <Cell>
       <InstanceStatusText Status="instance.Status" />
     </Cell>
+    <Cell class="text-center">
+      @if (instance.PlayerCounts is var (online, maximum)) {
+	      <p class="font-monospace">@online.ToString() / @maximum.ToString()</p>
+      }
+      else {
+	      <p class="font-monospace">-</p>
+      }
+    </Cell>
     <Cell>@configuration.MinecraftServerKind @configuration.MinecraftVersion</Cell>
     <Cell class="text-center">
       <p class="font-monospace">@configuration.ServerPort.ToString()</p>