Rework message replies

Agent/Phantom.Agent.Rpc/KeepAliveLoop.cs

@@ -1,5 +1,4 @@
-using NetMQ.Sockets;
+using Phantom.Common.Logging;
-using Phantom.Common.Logging;
 using Phantom.Common.Messages.ToServer;
 using Phantom.Utils.Runtime;
 using Serilog;
@@ -11,11 +10,11 @@ sealed class KeepAliveLoop {
 
 	private static readonly TimeSpan KeepAliveInterval = TimeSpan.FromSeconds(10);
 
-	private readonly ClientSocket socket;
+	private readonly RpcServerConnection connection;
 	private readonly CancellationTokenSource cancellationTokenSource = new ();
 
-	public KeepAliveLoop(ClientSocket socket, TaskManager taskManager) {
+	public KeepAliveLoop(RpcServerConnection connection, TaskManager taskManager) {
-		this.socket = socket;
+		this.connection = connection;
 		taskManager.Run(Run);
 	}
 
@@ -26,7 +25,7 @@ sealed class KeepAliveLoop {
 		try {
 			while (true) {
 				await Task.Delay(KeepAliveInterval, cancellationToken);
-				await socket.SendMessage(new AgentIsAliveMessage());
+				await connection.Send(new AgentIsAliveMessage());
 			}
 		} catch (OperationCanceledException) {
 			// Ignore.

Agent/Phantom.Agent.Rpc/RpcExtensions.cs

@@ -1,19 +0,0 @@
-using NetMQ;
-using NetMQ.Sockets;
-using Phantom.Common.Messages;
-using Phantom.Common.Messages.ToServer;
-
-namespace Phantom.Agent.Rpc; 
-
-public static class RpcExtensions {
-	internal static async Task SendMessage<TMessage>(this ClientSocket socket, TMessage message) where TMessage : IMessageToServer {
-		byte[] bytes = MessageRegistries.ToServer.Write(message).ToArray();
-		if (bytes.Length > 0) {
-			await socket.SendAsync(bytes);
-		}
-	}
-	
-	public static Task SendSimpleReply<TMessage, TReplyEnum>(this ClientSocket socket, TMessage message, TReplyEnum reply) where TMessage : IMessageWithReply where TReplyEnum : Enum {
-		return SendMessage(socket, SimpleReplyMessage.FromEnum(message.SequenceId, reply));
-	}
-}

Agent/Phantom.Agent.Rpc/RpcLauncher.cs

@@ -4,6 +4,7 @@ using Phantom.Common.Data.Agent;
 using Phantom.Common.Messages;
 using Phantom.Common.Messages.ToServer;
 using Phantom.Utils.Rpc;
+using Phantom.Utils.Rpc.Message;
 using Phantom.Utils.Runtime;
 using Serilog;
 using Serilog.Events;
@@ -11,7 +12,7 @@ using Serilog.Events;
 namespace Phantom.Agent.Rpc;
 
 public sealed class RpcLauncher : RpcRuntime<ClientSocket> {
-	public static async Task Launch(RpcConfiguration config, AgentAuthToken authToken, AgentInfo agentInfo, Func<ClientSocket, IMessageToAgentListener> listenerFactory, SemaphoreSlim disconnectSemaphore, CancellationToken receiveCancellationToken) {
+	public static async Task Launch(RpcConfiguration config, AgentAuthToken authToken, AgentInfo agentInfo, Func<RpcServerConnection, IMessageToAgentListener> listenerFactory, SemaphoreSlim disconnectSemaphore, CancellationToken receiveCancellationToken) {
 		var socket = new ClientSocket();
 		var options = socket.Options;
 
@@ -24,12 +25,12 @@ public sealed class RpcLauncher : RpcRuntime<ClientSocket> {
 
 	private readonly RpcConfiguration config;
 	private readonly Guid agentGuid;
-	private readonly Func<ClientSocket, IMessageToAgentListener> messageListenerFactory;
+	private readonly Func<RpcServerConnection, IMessageToAgentListener> messageListenerFactory;
 
 	private readonly SemaphoreSlim disconnectSemaphore;
 	private readonly CancellationToken receiveCancellationToken;
 
-	private RpcLauncher(RpcConfiguration config, ClientSocket socket, Guid agentGuid, Func<ClientSocket, IMessageToAgentListener> messageListenerFactory, SemaphoreSlim disconnectSemaphore, CancellationToken receiveCancellationToken) : base(socket, config.Logger) {
+	private RpcLauncher(RpcConfiguration config, ClientSocket socket, Guid agentGuid, Func<RpcServerConnection, IMessageToAgentListener> messageListenerFactory, SemaphoreSlim disconnectSemaphore, CancellationToken receiveCancellationToken) : base(socket, config.Logger) {
 		this.config = config;
 		this.agentGuid = agentGuid;
 		this.messageListenerFactory = messageListenerFactory;
@@ -46,12 +47,13 @@ public sealed class RpcLauncher : RpcRuntime<ClientSocket> {
 		logger.Information("ZeroMQ client ready.");
 	}
 
-	protected override void Run(ClientSocket socket, TaskManager taskManager) {
+	protected override void Run(ClientSocket socket, MessageReplyTracker replyTracker, TaskManager taskManager) {
+		var connection = new RpcServerConnection(socket, replyTracker);
+		ServerMessaging.SetCurrentConnection(connection);
+		
 		var logger = config.Logger;
-		var listener = messageListenerFactory(socket);
+		var handler = new MessageToAgentHandler(messageListenerFactory(connection), logger, taskManager, receiveCancellationToken);
-
+		var keepAliveLoop = new KeepAliveLoop(connection, taskManager);
-		ServerMessaging.SetCurrentSocket(socket);
-		var keepAliveLoop = new KeepAliveLoop(socket, taskManager);
 
 		try {
 			while (!receiveCancellationToken.IsCancellationRequested) {
@@ -60,7 +62,7 @@ public sealed class RpcLauncher : RpcRuntime<ClientSocket> {
 				LogMessageType(logger, data);
 				
 				if (data.Length > 0) {
-					MessageRegistries.ToAgent.Handle(data, listener, taskManager, receiveCancellationToken);
+					MessageRegistries.ToAgent.Handle(data, handler);
 				}
 			}
 		} catch (OperationCanceledException) {
@@ -86,11 +88,19 @@ public sealed class RpcLauncher : RpcRuntime<ClientSocket> {
 		}
 	}
 
-	protected override async Task Disconnect(ClientSocket socket) {
+	protected override async Task Disconnect() {
 		var unregisterTimeoutTask = Task.Delay(TimeSpan.FromSeconds(5), CancellationToken.None);
-		var finishedTask = await Task.WhenAny(socket.SendMessage(new UnregisterAgentMessage(agentGuid)), unregisterTimeoutTask);
+		var finishedTask = await Task.WhenAny(ServerMessaging.Send(new UnregisterAgentMessage(agentGuid)), unregisterTimeoutTask);
 		if (finishedTask == unregisterTimeoutTask) {
 			config.Logger.Error("Timed out communicating agent shutdown with the server.");
 		}
 	}
+
+	private sealed class MessageToAgentHandler : MessageHandler<IMessageToAgentListener> {
+		public MessageToAgentHandler(IMessageToAgentListener listener, ILogger logger, TaskManager taskManager, CancellationToken cancellationToken) : base(listener, logger, taskManager, cancellationToken) {}
+		
+		protected override Task SendReply(uint sequenceId, byte[] serializedReply) {
+			return ServerMessaging.Send(new ReplyMessage(sequenceId, serializedReply));
+		}
+	}
 }

Agent/Phantom.Agent.Rpc/RpcServerConnection.cs

@@ -0,0 +1,46 @@
+using NetMQ;
+using NetMQ.Sockets;
+using Phantom.Common.Messages;
+using Phantom.Common.Messages.ToServer;
+using Phantom.Utils.Rpc.Message;
+
+namespace Phantom.Agent.Rpc; 
+
+public sealed class RpcServerConnection {
+	private readonly ClientSocket socket;
+	private readonly MessageReplyTracker replyTracker;
+
+	internal RpcServerConnection(ClientSocket socket, MessageReplyTracker replyTracker) {
+		this.socket = socket;
+		this.replyTracker = replyTracker;
+	}
+
+	private byte[] WriteBytes<TMessage, TReply>(TMessage message) where TMessage : IMessageToServer<TReply> {
+		return MessageRegistries.ToServer.Write<TMessage, TReply>(message).ToArray();
+	}
+
+	internal async Task Send<TMessage>(TMessage message) where TMessage : IMessageToServer {
+		var bytes = WriteBytes<TMessage, NoReply>(message);
+		if (bytes.Length > 0) {
+			await socket.SendAsync(bytes);
+		}
+	}
+
+	internal async Task<TReply?> Send<TMessage, TReply>(Func<uint, TMessage> messageFactory, TimeSpan waitForReplyTime, CancellationToken cancellationToken) where TMessage : IMessageToServer<TReply> where TReply : class {
+		var sequenceId = replyTracker.RegisterReply();
+		var message = messageFactory(sequenceId);
+		
+		var bytes = WriteBytes<TMessage, TReply>(message);
+		if (bytes.Length == 0) {
+			replyTracker.ForgetReply(sequenceId);
+			return null;
+		}
+
+		await socket.SendAsync(bytes);
+		return await replyTracker.WaitForReply<TReply>(message.SequenceId, waitForReplyTime, cancellationToken);
+	}
+
+	public void Receive(ReplyMessage message) {
+		replyTracker.ReceiveReply(message.SequenceId, message.SerializedReply);
+	}
+}

Agent/Phantom.Agent.Rpc/ServerMessaging.cs

@@ -1,5 +1,4 @@
-using NetMQ.Sockets;
+using Phantom.Common.Logging;
-using Phantom.Common.Logging;
 using Phantom.Common.Messages;
 using Serilog;
 
@@ -8,23 +7,28 @@ namespace Phantom.Agent.Rpc;
 public static class ServerMessaging {
 	private static readonly ILogger Logger = PhantomLogger.Create(typeof(ServerMessaging));
 	
-	private static ClientSocket? CurrentSocket { get; set; }
+	private static RpcServerConnection? CurrentConnection { get; set; }
-	private static readonly object SetCurrentSocketLock = new ();
+	private static RpcServerConnection CurrentConnectionOrThrow => CurrentConnection ?? throw new InvalidOperationException("Server connection not ready.");
+	
+	private static readonly object SetCurrentConnectionLock = new ();
 
-	internal static void SetCurrentSocket(ClientSocket socket) {
+	internal static void SetCurrentConnection(RpcServerConnection connection) {
-		lock (SetCurrentSocketLock) {
+		lock (SetCurrentConnectionLock) {
-			if (CurrentSocket != null) {
+			if (CurrentConnection != null) {
-				throw new InvalidOperationException("Server socket can only be set once.");
+				throw new InvalidOperationException("Server connection can only be set once.");
 			}
 			
-			CurrentSocket = socket;
+			CurrentConnection = connection;
 		}
 		
-		Logger.Information("Server socket ready.");
+		Logger.Information("Server connection ready.");
 	}
 
-	public static async Task SendMessage<TMessage>(TMessage message) where TMessage : IMessageToServer {
+	public static Task Send<TMessage>(TMessage message) where TMessage : IMessageToServer {
-		var currentSocket = CurrentSocket ?? throw new InvalidOperationException("Server socket not ready.");
+		return CurrentConnectionOrThrow.Send(message);
-		await currentSocket.SendMessage(message);
+	}
+
+	public static Task<TReply?> Send<TMessage, TReply>(Func<uint, TMessage> messageFactory, TimeSpan waitForReplyTime, CancellationToken cancellationToken) where TMessage : IMessageToServer<TReply> where TReply : class {
+		return CurrentConnectionOrThrow.Send<TMessage, TReply>(messageFactory, waitForReplyTime, cancellationToken);
 	}
 }

Agent/Phantom.Agent.Services/Instances/Instance.cs

@@ -51,7 +51,7 @@ sealed class Instance : IDisposable {
 	}
 
 	private async Task ReportLastStatus() {
-		await ServerMessaging.SendMessage(new ReportInstanceStatusMessage(Configuration.InstanceGuid, currentStatus));
+		await ServerMessaging.Send(new ReportInstanceStatusMessage(Configuration.InstanceGuid, currentStatus));
 	}
 	
 	private void TransitionState(IInstanceState newState) {
@@ -143,7 +143,7 @@ sealed class Instance : IDisposable {
 			instance.launchServices.TaskManager.Run(async () => {
 				if (myStatusUpdateCounter == statusUpdateCounter) {
 					instance.currentStatus = newStatus;
-					await ServerMessaging.SendMessage(new ReportInstanceStatusMessage(Configuration.InstanceGuid, newStatus));
+					await ServerMessaging.Send(new ReportInstanceStatusMessage(Configuration.InstanceGuid, newStatus));
 				}
 			});
 		}

Agent/Phantom.Agent.Services/Instances/InstanceLogSender.cs

@@ -52,7 +52,7 @@ sealed class InstanceLogSender {
 
 	private async Task SendOutputToServer(ImmutableArray<string> lines) {
 		if (!lines.IsEmpty) {
-			await ServerMessaging.SendMessage(new InstanceOutputMessage(instanceGuid, lines));
+			await ServerMessaging.Send(new InstanceOutputMessage(instanceGuid, lines));
 		}
 	}
 

Agent/Phantom.Agent.Services/Instances/InstanceSessionManager.cs

@@ -1,4 +1,5 @@
-using Phantom.Agent.Minecraft.Instance;
+using System.Diagnostics.CodeAnalysis;
+using Phantom.Agent.Minecraft.Instance;
 using Phantom.Agent.Minecraft.Java;
 using Phantom.Agent.Minecraft.Launcher;
 using Phantom.Agent.Minecraft.Launcher.Types;
@@ -40,11 +41,31 @@ sealed class InstanceSessionManager : IDisposable {
 		this.shutdownCancellationToken = shutdownCancellationTokenSource.Token;
 	}
 
-	public async Task<ConfigureInstanceResult> Configure(InstanceConfiguration configuration) {
+	[SuppressMessage("ReSharper", "ConvertIfStatementToReturnStatement")]
+	private async Task<InstanceActionResult<T>> AcquireSemaphoreAndRunWithInstance<T>(Guid instanceGuid, Func<Instance, Task<T>> func) {
 		try {
 			await semaphore.WaitAsync(shutdownCancellationToken);
 		} catch (OperationCanceledException) {
-			return ConfigureInstanceResult.AgentShuttingDown;
+			return InstanceActionResult.General<T>(InstanceActionGeneralResult.AgentShuttingDown);
+		}
+
+		try {
+			if (!instances.TryGetValue(instanceGuid, out var instance)) {
+				return InstanceActionResult.General<T>(InstanceActionGeneralResult.InstanceDoesNotExist);
+			}
+			else {
+				return InstanceActionResult.Concrete(await func(instance));
+			}
+		} finally {
+			semaphore.Release();
+		}
+	}
+	
+	public async Task<InstanceActionResult<ConfigureInstanceResult>> Configure(InstanceConfiguration configuration) {
+		try {
+			await semaphore.WaitAsync(shutdownCancellationToken);
+		} catch (OperationCanceledException) {
+			return InstanceActionResult.General<ConfigureInstanceResult>(InstanceActionGeneralResult.AgentShuttingDown);
 		}
 
 		var instanceGuid = configuration.InstanceGuid;
@@ -52,12 +73,12 @@ sealed class InstanceSessionManager : IDisposable {
 		try {
 			var otherInstances = instances.Values.Where(inst => inst.Configuration.InstanceGuid != instanceGuid).ToArray();
 			if (otherInstances.Length + 1 > agentInfo.MaxInstances) {
-				return ConfigureInstanceResult.InstanceLimitExceeded;
+				return InstanceActionResult.Concrete(ConfigureInstanceResult.InstanceLimitExceeded);
 			}
 
 			var availableMemory = agentInfo.MaxMemory - otherInstances.Aggregate(RamAllocationUnits.Zero, static (total, instance) => total + instance.Configuration.MemoryAllocation);
 			if (availableMemory < configuration.MemoryAllocation) {
-				return ConfigureInstanceResult.MemoryLimitExceeded;
+				return InstanceActionResult.Concrete(ConfigureInstanceResult.MemoryLimitExceeded);
 			}
 
 			var heapMegabytes = configuration.MemoryAllocation.InMegabytes;
@@ -93,70 +114,22 @@ sealed class InstanceSessionManager : IDisposable {
 				await instance.Launch(shutdownCancellationToken);
 			}
 
-			return ConfigureInstanceResult.Success;
+			return InstanceActionResult.Concrete(ConfigureInstanceResult.Success);
 		} finally {
 			semaphore.Release();
 		}
 	}
 
-	public async Task<LaunchInstanceResult> Launch(Guid instanceGuid) {
+	public Task<InstanceActionResult<LaunchInstanceResult>> Launch(Guid instanceGuid) {
-		try {
+		return AcquireSemaphoreAndRunWithInstance(instanceGuid, instance => instance.Launch(shutdownCancellationToken));
-			await semaphore.WaitAsync(shutdownCancellationToken);
-		} catch (OperationCanceledException) {
-			return LaunchInstanceResult.AgentShuttingDown;
-		}
-
-		try {
-			if (!instances.TryGetValue(instanceGuid, out var instance)) {
-				return LaunchInstanceResult.InstanceDoesNotExist;
-			}
-			else {
-				return await instance.Launch(shutdownCancellationToken);
-			}
-		} finally {
-			semaphore.Release();
-		}
 	}
 
-	public async Task<StopInstanceResult> Stop(Guid instanceGuid, MinecraftStopStrategy stopStrategy) {
+	public Task<InstanceActionResult<StopInstanceResult>> Stop(Guid instanceGuid, MinecraftStopStrategy stopStrategy) {
-		try {
+		return AcquireSemaphoreAndRunWithInstance(instanceGuid, instance => instance.Stop(stopStrategy));
-			await semaphore.WaitAsync(shutdownCancellationToken);
-		} catch (OperationCanceledException) {
-			return StopInstanceResult.AgentShuttingDown;
-		}
-
-		try {
-			if (!instances.TryGetValue(instanceGuid, out var instance)) {
-				return StopInstanceResult.InstanceDoesNotExist;
-			}
-			else {
-				return await instance.Stop(stopStrategy);
-			}
-		} finally {
-			semaphore.Release();
-		}
 	}
 
-	public async Task<SendCommandToInstanceResult> SendCommand(Guid instanceGuid, string command) {
+	public Task<InstanceActionResult<SendCommandToInstanceResult>> SendCommand(Guid instanceGuid, string command) {
-		try {
+		return AcquireSemaphoreAndRunWithInstance(instanceGuid, async instance => await instance.SendCommand(command, shutdownCancellationToken) ? SendCommandToInstanceResult.Success : SendCommandToInstanceResult.UnknownError);
-			await semaphore.WaitAsync(shutdownCancellationToken);
-		} catch (OperationCanceledException) {
-			return SendCommandToInstanceResult.AgentShuttingDown;
-		}
-
-		try {
-			if (!instances.TryGetValue(instanceGuid, out var instance)) {
-				return SendCommandToInstanceResult.InstanceDoesNotExist;
-			}
-
-			if (!await instance.SendCommand(command, shutdownCancellationToken)) {
-				return SendCommandToInstanceResult.UnknownError;
-			}
-
-			return SendCommandToInstanceResult.Success;
-		} finally {
-			semaphore.Release();
-		}
 	}
 
 	public async Task StopAll() {

Agent/Phantom.Agent.Services/Rpc/MessageListener.cs

@@ -1,10 +1,10 @@
-using NetMQ.Sockets;
+using Phantom.Agent.Rpc;
-using Phantom.Agent.Rpc;
 using Phantom.Common.Data.Replies;
 using Phantom.Common.Logging;
 using Phantom.Common.Messages;
 using Phantom.Common.Messages.ToAgent;
 using Phantom.Common.Messages.ToServer;
+using Phantom.Utils.Rpc.Message;
 using Serilog;
 
 namespace Phantom.Agent.Services.Rpc;
@@ -12,32 +12,34 @@ namespace Phantom.Agent.Services.Rpc;
 public sealed class MessageListener : IMessageToAgentListener {
 	private static ILogger Logger { get; } = PhantomLogger.Create<MessageListener>();
 
-	private readonly ClientSocket socket;
+	private readonly RpcServerConnection connection;
 	private readonly AgentServices agent;
 	private readonly CancellationTokenSource shutdownTokenSource;
 
-	public MessageListener(ClientSocket socket, AgentServices agent, CancellationTokenSource shutdownTokenSource) {
+	public MessageListener(RpcServerConnection connection, AgentServices agent, CancellationTokenSource shutdownTokenSource) {
-		this.socket = socket;
+		this.connection = connection;
 		this.agent = agent;
 		this.shutdownTokenSource = shutdownTokenSource;
 	}
 
-	public async Task HandleRegisterAgentSuccessResult(RegisterAgentSuccessMessage message) {
+	public async Task<NoReply> HandleRegisterAgentSuccess(RegisterAgentSuccessMessage message) {
 		Logger.Information("Agent authentication successful.");
 
 		foreach (var instanceInfo in message.InitialInstances) {
-			if (await agent.InstanceSessionManager.Configure(instanceInfo) != ConfigureInstanceResult.Success) {
+			var result = await agent.InstanceSessionManager.Configure(instanceInfo);
+			if (!result.Is(ConfigureInstanceResult.Success)) {
 				Logger.Fatal("Unable to configure instance \"{Name}\" (GUID {Guid}), shutting down.", instanceInfo.InstanceName, instanceInfo.InstanceGuid);
 
 				shutdownTokenSource.Cancel();
-				return;
+				return NoReply.Instance;
 			}
 		}
 
-		await ServerMessaging.SendMessage(new AdvertiseJavaRuntimesMessage(agent.JavaRuntimeRepository.All));
+		await ServerMessaging.Send(new AdvertiseJavaRuntimesMessage(agent.JavaRuntimeRepository.All));
+		return NoReply.Instance;
 	}
 
-	public Task HandleRegisterAgentFailureResult(RegisterAgentFailureMessage message) {
+	public Task<NoReply> HandleRegisterAgentFailure(RegisterAgentFailureMessage message) {
 		string errorMessage = message.FailureKind switch {
 			RegisterAgentFailure.ConnectionAlreadyHasAnAgent => "This connection already has an associated agent.",
 			RegisterAgentFailure.InvalidToken                => "Invalid token.",
@@ -47,22 +49,27 @@ public sealed class MessageListener : IMessageToAgentListener {
 		Logger.Fatal("Agent authentication failed: {Error}", errorMessage);
 		Environment.Exit(1);
 
-		return Task.CompletedTask;
+		return Task.FromResult(NoReply.Instance);
 	}
 	
-	public async Task HandleConfigureInstance(ConfigureInstanceMessage message) {
+	public async Task<InstanceActionResult<ConfigureInstanceResult>> HandleConfigureInstance(ConfigureInstanceMessage message) {
-		await socket.SendSimpleReply(message, await agent.InstanceSessionManager.Configure(message.Configuration));
+		return await agent.InstanceSessionManager.Configure(message.Configuration);
 	}
 
-	public async Task HandleLaunchInstance(LaunchInstanceMessage message) {
+	public async Task<InstanceActionResult<LaunchInstanceResult>> HandleLaunchInstance(LaunchInstanceMessage message) {
-		await socket.SendSimpleReply(message, await agent.InstanceSessionManager.Launch(message.InstanceGuid));
+		return await agent.InstanceSessionManager.Launch(message.InstanceGuid);
 	}
 
-	public async Task HandleStopInstance(StopInstanceMessage message) {
+	public async Task<InstanceActionResult<StopInstanceResult>> HandleStopInstance(StopInstanceMessage message) {
-		await socket.SendSimpleReply(message, await agent.InstanceSessionManager.Stop(message.InstanceGuid, message.StopStrategy));
+		return await agent.InstanceSessionManager.Stop(message.InstanceGuid, message.StopStrategy);
 	}
 
-	public async Task HandleSendCommandToInstance(SendCommandToInstanceMessage message) {
+	public async Task<InstanceActionResult<SendCommandToInstanceResult>> HandleSendCommandToInstance(SendCommandToInstanceMessage message) {
-		await socket.SendSimpleReply(message, await agent.InstanceSessionManager.SendCommand(message.InstanceGuid, message.Command));
+		return await agent.InstanceSessionManager.SendCommand(message.InstanceGuid, message.Command);
+	}
+
+	public Task<NoReply> HandleReply(ReplyMessage message) {
+		connection.Receive(message);
+		return Task.FromResult(NoReply.Instance);
 	}
 }

Agent/Phantom.Agent/Program.cs

@@ -1,5 +1,4 @@
 using System.Reflection;
-using NetMQ.Sockets;
 using Phantom.Agent;
 using Phantom.Agent.Rpc;
 using Phantom.Agent.Services;
@@ -45,8 +44,8 @@ try {
 	var agentInfo = new AgentInfo(agentGuid.Value, agentName, ProtocolVersion, fullVersion, maxInstances, maxMemory, allowedServerPorts, allowedRconPorts);
 	var agentServices = new AgentServices(agentInfo, folders);
 
-	MessageListener MessageListenerFactory(ClientSocket socket) {
+	MessageListener MessageListenerFactory(RpcServerConnection connection) {
-		return new MessageListener(socket, agentServices, shutdownCancellationTokenSource);
+		return new MessageListener(connection, agentServices, shutdownCancellationTokenSource);
 	}
 
 	PhantomLogger.Root.InformationHeading("Launching Phantom Panel agent...");

Common/Phantom.Common.Data/Replies/ConfigureInstanceResult.cs

@@ -2,7 +2,6 @@
 
 public enum ConfigureInstanceResult {
 	Success,
-	AgentShuttingDown,
 	InstanceLimitExceeded,
 	MemoryLimitExceeded
 }

Common/Phantom.Common.Data/Replies/InstanceActionGeneralResult.cs

@@ -0,0 +1,8 @@
+namespace Phantom.Common.Data.Replies;
+
+public enum InstanceActionGeneralResult : byte {
+	None,
+	AgentShuttingDown,
+	AgentIsNotResponding,
+	InstanceDoesNotExist
+}

Common/Phantom.Common.Data/Replies/InstanceActionResult.cs

@@ -0,0 +1,41 @@
+using MemoryPack;
+
+namespace Phantom.Common.Data.Replies;
+
+[MemoryPackable]
+public sealed partial record InstanceActionResult<T>(
+	[property: MemoryPackOrder(0)] InstanceActionGeneralResult GeneralResult,
+	[property: MemoryPackOrder(1)] T? ConcreteResult
+) {
+	public bool Is(T? concreteResult) {
+		return GeneralResult == InstanceActionGeneralResult.None && EqualityComparer<T>.Default.Equals(ConcreteResult, concreteResult);
+	}
+
+	public InstanceActionResult<T2> Map<T2>(Func<T, T2> mapper) {
+		return new InstanceActionResult<T2>(GeneralResult, ConcreteResult is not null ? mapper(ConcreteResult) : default);
+	}
+
+	public string ToSentence(Func<T, string> concreteResultToSentence) {
+		return GeneralResult switch {
+			InstanceActionGeneralResult.None                 => concreteResultToSentence(ConcreteResult!),
+			InstanceActionGeneralResult.AgentShuttingDown    => "Agent is shutting down.",
+			InstanceActionGeneralResult.AgentIsNotResponding => "Agent is not responding.",
+			InstanceActionGeneralResult.InstanceDoesNotExist => "Instance does not exist.",
+			_                                                => "Unknown result."
+		};
+	}
+}
+
+public static class InstanceActionResult {
+	public static InstanceActionResult<T> General<T>(InstanceActionGeneralResult generalResult) {
+		return new InstanceActionResult<T>(generalResult, default);
+	}
+
+	public static InstanceActionResult<T> Concrete<T>(T? concreteResult) {
+		return new InstanceActionResult<T>(InstanceActionGeneralResult.None, concreteResult);
+	}
+
+	public static InstanceActionResult<T> DidNotReplyIfNull<T>(this InstanceActionResult<T>? result) {
+		return result ?? General<T>(InstanceActionGeneralResult.AgentIsNotResponding);
+	}
+}

Common/Phantom.Common.Data/Replies/LaunchInstanceResult.cs

@@ -2,12 +2,9 @@
 
 public enum LaunchInstanceResult {
 	LaunchInitiated,
-	AgentShuttingDown,
-	InstanceDoesNotExist,
 	InstanceAlreadyLaunching,
 	InstanceAlreadyRunning,
 	InstanceIsStopping,
-	CommunicationError,
 	UnknownError
 }
 
@@ -15,12 +12,9 @@ public static class LaunchInstanceResultExtensions {
 	public static string ToSentence(this LaunchInstanceResult reason) {
 		return reason switch {
 			LaunchInstanceResult.LaunchInitiated          => "Launch initiated.",
-			LaunchInstanceResult.AgentShuttingDown        => "Agent is shutting down.",
-			LaunchInstanceResult.InstanceDoesNotExist     => "Instance does not exist.",
 			LaunchInstanceResult.InstanceAlreadyLaunching => "Instance is already launching.",
 			LaunchInstanceResult.InstanceAlreadyRunning   => "Instance is already running.",
 			LaunchInstanceResult.InstanceIsStopping       => "Instance is stopping.",
-			LaunchInstanceResult.CommunicationError       => "Communication error.",
 			_                                             => "Unknown error."
 		};
 	}

Common/Phantom.Common.Data/Replies/SendCommandToInstanceResult.cs

@@ -2,20 +2,14 @@
 
 public enum SendCommandToInstanceResult {
 	Success,
-	InstanceDoesNotExist,
-	AgentShuttingDown,
-	AgentCommunicationError,
 	UnknownError
 }
 
 public static class SendCommandToInstanceResultExtensions {
 	public static string ToSentence(this SendCommandToInstanceResult reason) {
 		return reason switch {
-			SendCommandToInstanceResult.Success                 => "Command sent.",
+			SendCommandToInstanceResult.Success => "Command sent.",
-			SendCommandToInstanceResult.InstanceDoesNotExist    => "Instance does not exist.",
+			_                                   => "Unknown error."
-			SendCommandToInstanceResult.AgentShuttingDown       => "Agent is shutting down.",
-			SendCommandToInstanceResult.AgentCommunicationError => "Agent did not reply in time.",
-			_                                                   => "Unknown error."
 		};
 	}
 }

Common/Phantom.Common.Data/Replies/StopInstanceResult.cs

@@ -2,11 +2,8 @@
 
 public enum StopInstanceResult {
 	StopInitiated,
-	AgentShuttingDown,
-	InstanceDoesNotExist,
 	InstanceAlreadyStopping,
 	InstanceAlreadyStopped,
-	CommunicationError,
 	UnknownError
 }
 
@@ -14,11 +11,8 @@ public static class StopInstanceResultExtensions {
 	public static string ToSentence(this StopInstanceResult reason) {
 		return reason switch {
 			StopInstanceResult.StopInitiated           => "Stopping initiated.",
-			StopInstanceResult.AgentShuttingDown       => "Agent is shutting down.",
-			StopInstanceResult.InstanceDoesNotExist    => "Instance does not exist.",
 			StopInstanceResult.InstanceAlreadyStopping => "Instance is already stopping.",
 			StopInstanceResult.InstanceAlreadyStopped  => "Instance is already stopped.",
-			StopInstanceResult.CommunicationError      => "Communication error.",
 			_                                          => "Unknown error."
 		};
 	}

Common/Phantom.Common.Messages/BiDirectional/ReplyMessage.cs

@@ -0,0 +1,18 @@
+using MemoryPack;
+using Phantom.Utils.Rpc.Message;
+
+namespace Phantom.Common.Messages.ToServer;
+
+[MemoryPackable]
+public sealed partial record ReplyMessage(
+	[property: MemoryPackOrder(0)] uint SequenceId,
+	[property: MemoryPackOrder(1)] byte[] SerializedReply
+) : IMessageToServer, IMessageToAgent {
+	public Task<NoReply> Accept(IMessageToServerListener listener) {
+		return listener.HandleReply(this);
+	}
+
+	public Task<NoReply> Accept(IMessageToAgentListener listener) {
+		return listener.HandleReply(this);
+	}
+}

Common/Phantom.Common.Messages/IMessageToAgent.cs

@@ -2,4 +2,8 @@
 
 namespace Phantom.Common.Messages;
 
-public interface IMessageToAgent : IMessage<IMessageToAgentListener> {}
+public interface IMessageToAgent<TReply> : IMessage<IMessageToAgentListener, TReply> {}
+
+public interface IMessageToAgent : IMessageToAgent<NoReply> {
+	uint IMessage<IMessageToAgentListener, NoReply>.SequenceId => 0;
+}

Common/Phantom.Common.Messages/IMessageToAgentListener.cs

@@ -1,12 +1,16 @@
-using Phantom.Common.Messages.ToAgent;
+using Phantom.Common.Data.Replies;
+using Phantom.Common.Messages.ToAgent;
+using Phantom.Common.Messages.ToServer;
+using Phantom.Utils.Rpc.Message;
 
 namespace Phantom.Common.Messages;
 
 public interface IMessageToAgentListener {
-	Task HandleRegisterAgentSuccessResult(RegisterAgentSuccessMessage message);
+	Task<NoReply> HandleRegisterAgentSuccess(RegisterAgentSuccessMessage message);
-	Task HandleRegisterAgentFailureResult(RegisterAgentFailureMessage message);
+	Task<NoReply> HandleRegisterAgentFailure(RegisterAgentFailureMessage message);
-	Task HandleConfigureInstance(ConfigureInstanceMessage message);
+	Task<InstanceActionResult<ConfigureInstanceResult>> HandleConfigureInstance(ConfigureInstanceMessage message);
-	Task HandleLaunchInstance(LaunchInstanceMessage message);
+	Task<InstanceActionResult<LaunchInstanceResult>> HandleLaunchInstance(LaunchInstanceMessage message);
-	Task HandleStopInstance(StopInstanceMessage message);
+	Task<InstanceActionResult<StopInstanceResult>> HandleStopInstance(StopInstanceMessage message);
-	Task HandleSendCommandToInstance(SendCommandToInstanceMessage message);
+	Task<InstanceActionResult<SendCommandToInstanceResult>> HandleSendCommandToInstance(SendCommandToInstanceMessage message);
+	Task<NoReply> HandleReply(ReplyMessage message);
 }

Common/Phantom.Common.Messages/IMessageToServer.cs

@@ -2,4 +2,8 @@
 
 namespace Phantom.Common.Messages; 
 
-public interface IMessageToServer : IMessage<IMessageToServerListener> {}
+public interface IMessageToServer<TReply> : IMessage<IMessageToServerListener, TReply> {}
+
+public interface IMessageToServer : IMessageToServer<NoReply> {
+	uint IMessage<IMessageToServerListener, NoReply>.SequenceId => 0;
+}

Common/Phantom.Common.Messages/IMessageToServerListener.cs

@@ -1,14 +1,15 @@
 using Phantom.Common.Messages.ToServer;
+using Phantom.Utils.Rpc.Message;
 
 namespace Phantom.Common.Messages; 
 
 public interface IMessageToServerListener {
 	bool IsDisposed { get; }
-	Task HandleRegisterAgent(RegisterAgentMessage message);
+	Task<NoReply> HandleRegisterAgent(RegisterAgentMessage message);
-	Task HandleUnregisterAgent(UnregisterAgentMessage message);
+	Task<NoReply> HandleUnregisterAgent(UnregisterAgentMessage message);
-	Task HandleAgentIsAlive(AgentIsAliveMessage message);
+	Task<NoReply> HandleAgentIsAlive(AgentIsAliveMessage message);
-	Task HandleAdvertiseJavaRuntimes(AdvertiseJavaRuntimesMessage message);
+	Task<NoReply> HandleAdvertiseJavaRuntimes(AdvertiseJavaRuntimesMessage message);
-	Task HandleReportInstanceStatus(ReportInstanceStatusMessage message);
+	Task<NoReply> HandleReportInstanceStatus(ReportInstanceStatusMessage message);
-	Task HandleInstanceOutput(InstanceOutputMessage message);
+	Task<NoReply> HandleInstanceOutput(InstanceOutputMessage message);
-	Task HandleSimpleReply(SimpleReplyMessage message);
+	Task<NoReply> HandleReply(ReplyMessage message);
 }

Common/Phantom.Common.Messages/IMessageWithReply.cs

@@ -1,5 +0,0 @@
-namespace Phantom.Common.Messages;
-
-public interface IMessageWithReply {
-	public uint SequenceId { get; }
-}

Common/Phantom.Common.Messages/MessageRegistries.cs

@@ -1,4 +1,5 @@
-using Phantom.Common.Logging;
+using Phantom.Common.Data.Replies;
+using Phantom.Common.Logging;
 using Phantom.Common.Messages.ToAgent;
 using Phantom.Common.Messages.ToServer;
 using Phantom.Utils.Rpc.Message;
@@ -6,23 +7,24 @@ using Phantom.Utils.Rpc.Message;
 namespace Phantom.Common.Messages; 
 
 public static class MessageRegistries {
-	public static MessageRegistry<IMessageToAgentListener, IMessageToAgent> ToAgent { get; } = new (PhantomLogger.Create("MessageRegistry:ToAgent"));
+	public static MessageRegistry<IMessageToAgentListener> ToAgent { get; } = new (PhantomLogger.Create("MessageRegistry:ToAgent"));
-	public static MessageRegistry<IMessageToServerListener, IMessageToServer> ToServer { get; } = new (PhantomLogger.Create("MessageRegistry:ToServer"));
+	public static MessageRegistry<IMessageToServerListener> ToServer { get; } = new (PhantomLogger.Create("MessageRegistry:ToServer"));
 
 	static MessageRegistries() {
-		ToAgent.Add<RegisterAgentSuccessMessage>(0);
+		ToAgent.Add<RegisterAgentSuccessMessage, NoReply>(0);
-		ToAgent.Add<RegisterAgentFailureMessage>(1);
+		ToAgent.Add<RegisterAgentFailureMessage, NoReply>(1);
-		ToAgent.Add<ConfigureInstanceMessage>(2);
+		ToAgent.Add<ConfigureInstanceMessage, InstanceActionResult<ConfigureInstanceResult>>(2);
-		ToAgent.Add<LaunchInstanceMessage>(3);
+		ToAgent.Add<LaunchInstanceMessage, InstanceActionResult<LaunchInstanceResult>>(3);
-		ToAgent.Add<StopInstanceMessage>(4);
+		ToAgent.Add<StopInstanceMessage, InstanceActionResult<StopInstanceResult>>(4);
-		ToAgent.Add<SendCommandToInstanceMessage>(5);
+		ToAgent.Add<SendCommandToInstanceMessage, InstanceActionResult<SendCommandToInstanceResult>>(5);
+		ToAgent.Add<ReplyMessage, NoReply>(127);
 		
-		ToServer.Add<RegisterAgentMessage>(0);
+		ToServer.Add<RegisterAgentMessage, NoReply>(0);
-		ToServer.Add<UnregisterAgentMessage>(1);
+		ToServer.Add<UnregisterAgentMessage, NoReply>(1);
-		ToServer.Add<AgentIsAliveMessage>(2);
+		ToServer.Add<AgentIsAliveMessage, NoReply>(2);
-		ToServer.Add<AdvertiseJavaRuntimesMessage>(3);
+		ToServer.Add<AdvertiseJavaRuntimesMessage, NoReply>(3);
-		ToServer.Add<ReportInstanceStatusMessage>(4);
+		ToServer.Add<ReportInstanceStatusMessage, NoReply>(4);
-		ToServer.Add<InstanceOutputMessage>(5);
+		ToServer.Add<InstanceOutputMessage, NoReply>(5);
-		ToServer.Add<SimpleReplyMessage>(127);
+		ToServer.Add<ReplyMessage, NoReply>(127);
 	}
 }

Common/Phantom.Common.Messages/ToAgent/ConfigureInstanceMessage.cs

@@ -1,5 +1,6 @@
 using MemoryPack;
 using Phantom.Common.Data.Instance;
+using Phantom.Common.Data.Replies;
 
 namespace Phantom.Common.Messages.ToAgent;
 
@@ -7,8 +8,8 @@ namespace Phantom.Common.Messages.ToAgent;
 public sealed partial record ConfigureInstanceMessage(
 	[property: MemoryPackOrder(0)] uint SequenceId,
 	[property: MemoryPackOrder(1)] InstanceConfiguration Configuration
-) : IMessageToAgent, IMessageWithReply {
+) : IMessageToAgent<InstanceActionResult<ConfigureInstanceResult>> {
-	public Task Accept(IMessageToAgentListener listener) {
+	public Task<InstanceActionResult<ConfigureInstanceResult>> Accept(IMessageToAgentListener listener) {
 		return listener.HandleConfigureInstance(this);
 	}
 }

Common/Phantom.Common.Messages/ToAgent/LaunchInstanceMessage.cs

@@ -1,4 +1,5 @@
 using MemoryPack;
+using Phantom.Common.Data.Replies;
 
 namespace Phantom.Common.Messages.ToAgent; 
 
@@ -6,8 +7,8 @@ namespace Phantom.Common.Messages.ToAgent;
 public sealed partial record LaunchInstanceMessage(
 	[property: MemoryPackOrder(0)] uint SequenceId,
 	[property: MemoryPackOrder(1)] Guid InstanceGuid
-) : IMessageToAgent, IMessageWithReply {
+) : IMessageToAgent<InstanceActionResult<LaunchInstanceResult>> {
-	public Task Accept(IMessageToAgentListener listener) {
+	public Task<InstanceActionResult<LaunchInstanceResult>> Accept(IMessageToAgentListener listener) {
 		return listener.HandleLaunchInstance(this);
 	}
 }

Common/Phantom.Common.Messages/ToAgent/RegisterAgentFailureMessage.cs

@@ -1,5 +1,6 @@
 using MemoryPack;
 using Phantom.Common.Data.Replies;
+using Phantom.Utils.Rpc.Message;
 
 namespace Phantom.Common.Messages.ToAgent;
 
@@ -7,7 +8,7 @@ namespace Phantom.Common.Messages.ToAgent;
 public sealed partial record RegisterAgentFailureMessage(
 	[property: MemoryPackOrder(0)] RegisterAgentFailure FailureKind
 ) : IMessageToAgent {
-	public Task Accept(IMessageToAgentListener listener) {
+	public Task<NoReply> Accept(IMessageToAgentListener listener) {
-		return listener.HandleRegisterAgentFailureResult(this);
+		return listener.HandleRegisterAgentFailure(this);
 	}
 }

Common/Phantom.Common.Messages/ToAgent/RegisterAgentSuccessMessage.cs

@@ -1,6 +1,7 @@
 using System.Collections.Immutable;
 using MemoryPack;
 using Phantom.Common.Data.Instance;
+using Phantom.Utils.Rpc.Message;
 
 namespace Phantom.Common.Messages.ToAgent;
 
@@ -8,7 +9,7 @@ namespace Phantom.Common.Messages.ToAgent;
 public sealed partial record RegisterAgentSuccessMessage(
 	[property: MemoryPackOrder(0)] ImmutableArray<InstanceConfiguration> InitialInstances
 ) : IMessageToAgent {
-	public Task Accept(IMessageToAgentListener listener) {
+	public Task<NoReply> Accept(IMessageToAgentListener listener) {
-		return listener.HandleRegisterAgentSuccessResult(this);
+		return listener.HandleRegisterAgentSuccess(this);
 	}
 }

Common/Phantom.Common.Messages/ToAgent/SendCommandToInstanceMessage.cs

@@ -1,4 +1,5 @@
 using MemoryPack;
+using Phantom.Common.Data.Replies;
 
 namespace Phantom.Common.Messages.ToAgent; 
 
@@ -7,8 +8,8 @@ public sealed partial record SendCommandToInstanceMessage(
 	[property: MemoryPackOrder(0)] uint SequenceId,
 	[property: MemoryPackOrder(1)] Guid InstanceGuid,
 	[property: MemoryPackOrder(2)] string Command
-) : IMessageToAgent, IMessageWithReply {
+) : IMessageToAgent<InstanceActionResult<SendCommandToInstanceResult>> {
-	public Task Accept(IMessageToAgentListener listener) {
+	public Task<InstanceActionResult<SendCommandToInstanceResult>> Accept(IMessageToAgentListener listener) {
 		return listener.HandleSendCommandToInstance(this);
 	}
 }

Common/Phantom.Common.Messages/ToAgent/StopInstanceMessage.cs

@@ -1,5 +1,6 @@
 using MemoryPack;
 using Phantom.Common.Data.Minecraft;
+using Phantom.Common.Data.Replies;
 
 namespace Phantom.Common.Messages.ToAgent; 
 
@@ -8,8 +9,8 @@ public sealed partial record StopInstanceMessage(
 	[property: MemoryPackOrder(0)] uint SequenceId,
 	[property: MemoryPackOrder(1)] Guid InstanceGuid,
 	[property: MemoryPackOrder(2)] MinecraftStopStrategy StopStrategy
-) : IMessageToAgent, IMessageWithReply {
+) : IMessageToAgent<InstanceActionResult<StopInstanceResult>> {
-	public Task Accept(IMessageToAgentListener listener) {
+	public Task<InstanceActionResult<StopInstanceResult>> Accept(IMessageToAgentListener listener) {
 		return listener.HandleStopInstance(this);
 	}
 }

Common/Phantom.Common.Messages/ToServer/AdvertiseJavaRuntimesMessage.cs

@@ -1,6 +1,7 @@
 using System.Collections.Immutable;
 using MemoryPack;
 using Phantom.Common.Data.Java;
+using Phantom.Utils.Rpc.Message;
 
 namespace Phantom.Common.Messages.ToServer; 
 
@@ -8,7 +9,7 @@ namespace Phantom.Common.Messages.ToServer;
 public sealed partial record AdvertiseJavaRuntimesMessage(
 	[property: MemoryPackOrder(0)] ImmutableArray<TaggedJavaRuntime> Runtimes
 ) : IMessageToServer {
-	public Task Accept(IMessageToServerListener listener) {
+	public Task<NoReply> Accept(IMessageToServerListener listener) {
 		return listener.HandleAdvertiseJavaRuntimes(this);
 	}
 }

Common/Phantom.Common.Messages/ToServer/AgentIsAliveMessage.cs

@@ -1,10 +1,11 @@
 using MemoryPack;
+using Phantom.Utils.Rpc.Message;
 
 namespace Phantom.Common.Messages.ToServer; 
 
 [MemoryPackable]
 public sealed partial record AgentIsAliveMessage : IMessageToServer {
-	public Task Accept(IMessageToServerListener listener) {
+	public Task<NoReply> Accept(IMessageToServerListener listener) {
 		return listener.HandleAgentIsAlive(this);
 	}
 }

Common/Phantom.Common.Messages/ToServer/InstanceOutputMessage.cs

@@ -1,5 +1,6 @@
 using System.Collections.Immutable;
 using MemoryPack;
+using Phantom.Utils.Rpc.Message;
 
 namespace Phantom.Common.Messages.ToServer;
 
@@ -8,7 +9,7 @@ public sealed partial record InstanceOutputMessage(
 	[property: MemoryPackOrder(0)] Guid InstanceGuid,
 	[property: MemoryPackOrder(1)] ImmutableArray<string> Lines
 ) : IMessageToServer {
-	public Task Accept(IMessageToServerListener listener) {
+	public Task<NoReply> Accept(IMessageToServerListener listener) {
 		return listener.HandleInstanceOutput(this);
 	}
 }

Common/Phantom.Common.Messages/ToServer/RegisterAgentMessage.cs

@@ -1,5 +1,6 @@
 using MemoryPack;
 using Phantom.Common.Data.Agent;
+using Phantom.Utils.Rpc.Message;
 
 namespace Phantom.Common.Messages.ToServer;
 
@@ -8,7 +9,7 @@ public sealed partial record RegisterAgentMessage(
 	[property: MemoryPackOrder(0)] AgentAuthToken AuthToken,
 	[property: MemoryPackOrder(1)] AgentInfo AgentInfo
 ) : IMessageToServer {
-	public Task Accept(IMessageToServerListener listener) {
+	public Task<NoReply> Accept(IMessageToServerListener listener) {
 		return listener.HandleRegisterAgent(this);
 	}
 }

Common/Phantom.Common.Messages/ToServer/ReportInstanceStatusMessage.cs

@@ -1,5 +1,6 @@
 using MemoryPack;
 using Phantom.Common.Data.Instance;
+using Phantom.Utils.Rpc.Message;
 
 namespace Phantom.Common.Messages.ToServer;
 
@@ -8,7 +9,7 @@ public sealed partial record ReportInstanceStatusMessage(
 	[property: MemoryPackOrder(0)] Guid InstanceGuid,
 	[property: MemoryPackOrder(1)] IInstanceStatus InstanceStatus
 ) : IMessageToServer {
-	public Task Accept(IMessageToServerListener listener) {
+	public Task<NoReply> Accept(IMessageToServerListener listener) {
 		return listener.HandleReportInstanceStatus(this);
 	}
 }

Common/Phantom.Common.Messages/ToServer/SimpleReplyMessage.cs

@@ -1,22 +0,0 @@
-using System.Runtime.CompilerServices;
-using MemoryPack;
-
-namespace Phantom.Common.Messages.ToServer;
-
-[MemoryPackable]
-public sealed partial record SimpleReplyMessage(
-	[property: MemoryPackOrder(0)] uint SequenceId,
-	[property: MemoryPackOrder(1)] int EnumValue
-) : IMessageToServer {
-	public static SimpleReplyMessage FromEnum<TEnum>(uint sequenceId, TEnum enumValue) where TEnum : Enum {
-		if (Unsafe.SizeOf<TEnum>() != Unsafe.SizeOf<int>()) {
-			throw new ArgumentException("Enum type " + typeof(TEnum).Name + " is not compatible with int.", nameof(TEnum));
-		}
-
-		return new SimpleReplyMessage(sequenceId, Unsafe.As<TEnum, int>(ref enumValue));
-	}
-
-	public Task Accept(IMessageToServerListener listener) {
-		return listener.HandleSimpleReply(this);
-	}
-}

Common/Phantom.Common.Messages/ToServer/UnregisterAgentMessage.cs

@@ -1,4 +1,5 @@
 using MemoryPack;
+using Phantom.Utils.Rpc.Message;
 
 namespace Phantom.Common.Messages.ToServer;
 
@@ -6,7 +7,7 @@ namespace Phantom.Common.Messages.ToServer;
 public sealed partial record UnregisterAgentMessage(
 	[property: MemoryPackOrder(0)] Guid AgentGuid
 ) : IMessageToServer {
-	public Task Accept(IMessageToServerListener listener) {
+	public Task<NoReply> Accept(IMessageToServerListener listener) {
 		return listener.HandleUnregisterAgent(this);
 	}
 }

Packages.props

@@ -1,11 +1,11 @@
 <Project>
   
   <ItemGroup>
-    <PackageReference Update="Microsoft.AspNetCore.Components.Authorization"     Version="7.0.0-rc.1.22427.2" />
+    <PackageReference Update="Microsoft.AspNetCore.Components.Authorization"     Version="7.0.1" />
-    <PackageReference Update="Microsoft.AspNetCore.Components.Web"               Version="7.0.0-rc.1.22427.2" />
+    <PackageReference Update="Microsoft.AspNetCore.Components.Web"               Version="7.0.1" />
-    <PackageReference Update="Microsoft.AspNetCore.Identity.EntityFrameworkCore" Version="7.0.0-rc.1.22427.2" />
+    <PackageReference Update="Microsoft.AspNetCore.Identity.EntityFrameworkCore" Version="7.0.1" />
-    <PackageReference Update="Microsoft.EntityFrameworkCore.Tools"               Version="7.0.0-rc.1.22426.7" />
+    <PackageReference Update="Microsoft.EntityFrameworkCore.Tools"               Version="7.0.1" />
-    <PackageReference Update="Npgsql.EntityFrameworkCore.PostgreSQL"             Version="7.0.0-rc.1" />
+    <PackageReference Update="Npgsql.EntityFrameworkCore.PostgreSQL"             Version="7.0.1" />
   </ItemGroup>
   
   <ItemGroup>
@@ -13,22 +13,22 @@
   </ItemGroup>
   
   <ItemGroup>
-    <PackageReference Update="MemoryPack" Version="1.4.1" />
+    <PackageReference Update="MemoryPack" Version="1.9.7" />
     <PackageReference Update="NetMQ"      Version="4.0.1.10" />
   </ItemGroup>
   
   <ItemGroup>
     <PackageReference Update="Serilog"               Version="2.12.0" />
-    <PackageReference Update="Serilog.AspNetCore"    Version="6.0.1" />
+    <PackageReference Update="Serilog.AspNetCore"    Version="6.1.0" />
     <PackageReference Update="Serilog.Sinks.Console" Version="4.1.0" />
   </ItemGroup>
   
   <ItemGroup>
-    <PackageReference Update="coverlet.collector"     Version="3.1.2" />
+    <PackageReference Update="coverlet.collector"     Version="3.2.0" />
-    <PackageReference Update="Microsoft.NET.Test.Sdk" Version="17.3.2" />
+    <PackageReference Update="Microsoft.NET.Test.Sdk" Version="17.4.1" />
     <PackageReference Update="NUnit"                  Version="3.13.3" />
-    <PackageReference Update="NUnit.Analyzers"        Version="3.3.0" />
+    <PackageReference Update="NUnit.Analyzers"        Version="3.5.0" />
-    <PackageReference Update="NUnit3TestAdapter"      Version="4.2.1" />
+    <PackageReference Update="NUnit3TestAdapter"      Version="4.3.1" />
   </ItemGroup>
 
 </Project>

Server/Phantom.Server.Rpc/RpcClientConnection.cs

@@ -1,6 +1,8 @@
 using NetMQ;
 using NetMQ.Sockets;
 using Phantom.Common.Messages;
+using Phantom.Common.Messages.ToServer;
+using Phantom.Utils.Rpc.Message;
 
 namespace Phantom.Server.Rpc;
 
@@ -8,12 +10,15 @@ public sealed class RpcClientConnection {
 	private readonly ServerSocket socket;
 	private readonly uint routingId;
 
+	private readonly MessageReplyTracker messageReplyTracker;
+
 	internal event EventHandler<RpcClientConnectionClosedEventArgs>? Closed;
 	private bool isClosed;
 
-	internal RpcClientConnection(ServerSocket socket, uint routingId) {
+	internal RpcClientConnection(ServerSocket socket, uint routingId, MessageReplyTracker messageReplyTracker) {
 		this.socket = socket;
 		this.routingId = routingId;
+		this.messageReplyTracker = messageReplyTracker;
 	}
 
 	public bool IsSame(RpcClientConnection other) {
@@ -29,14 +34,32 @@ public sealed class RpcClientConnection {
 		}
 	}
 
-	public async Task Send<TMessage>(TMessage message) where TMessage : IMessageToAgent {
+	private byte[] WriteBytes<TMessage, TReply>(TMessage message) where TMessage : IMessageToAgent<TReply> {
-		if (isClosed) {
+		return isClosed ? Array.Empty<byte>() : MessageRegistries.ToAgent.Write<TMessage, TReply>(message).ToArray();
-			return; // TODO
+	}
-		}
 
-		byte[] bytes = MessageRegistries.ToAgent.Write(message).ToArray();
+	public async Task Send<TMessage>(TMessage message) where TMessage : IMessageToAgent {
+		var bytes = WriteBytes<TMessage, NoReply>(message);
 		if (bytes.Length > 0) {
 			await socket.SendAsync(routingId, bytes);
 		}
 	}
+
+	public async Task<TReply?> Send<TMessage, TReply>(Func<uint, TMessage> messageFactory, TimeSpan waitForReplyTime, CancellationToken cancellationToken) where TMessage : IMessageToAgent<TReply> where TReply : class {
+		var sequenceId = messageReplyTracker.RegisterReply();
+		var message = messageFactory(sequenceId);
+		
+		var bytes = WriteBytes<TMessage, TReply>(message);
+		if (bytes.Length == 0) {
+			messageReplyTracker.ForgetReply(sequenceId);
+			return null;
+		}
+
+		await socket.SendAsync(routingId, bytes);
+		return await messageReplyTracker.WaitForReply<TReply>(message.SequenceId, waitForReplyTime, cancellationToken);
+	}
+
+	public void Receive(ReplyMessage message) {
+		messageReplyTracker.ReceiveReply(message.SequenceId, message.SerializedReply);
+	}
 }

Server/Phantom.Server.Rpc/RpcLauncher.cs

@@ -2,6 +2,7 @@
 using Phantom.Common.Messages;
 using Phantom.Common.Messages.ToServer;
 using Phantom.Utils.Rpc;
+using Phantom.Utils.Rpc.Message;
 using Phantom.Utils.Runtime;
 using Serilog.Events;
 using ILogger = Serilog.ILogger;
@@ -38,7 +39,7 @@ public sealed class RpcLauncher : RpcRuntime<ServerSocket> {
 		logger.Information("ZeroMQ server initialized, listening for agent connections on port {Port}.", config.Port);
 	}
 
-	protected override void Run(ServerSocket socket, TaskManager taskManager) {
+	protected override void Run(ServerSocket socket, MessageReplyTracker replyTracker, TaskManager taskManager) {
 		var logger = config.Logger;
 		var clients = new Dictionary<ulong, Client>();
 
@@ -60,19 +61,17 @@ public sealed class RpcLauncher : RpcRuntime<ServerSocket> {
 					continue;
 				}
 
-				var connection = new RpcClientConnection(socket, routingId);
+				var connection = new RpcClientConnection(socket, routingId, replyTracker);
 				connection.Closed += OnConnectionClosed;
 
-				client = new Client(connection, listenerFactory);
+				client = new Client(connection, listenerFactory, logger, taskManager, cancellationToken);
 				clients[routingId] = client;
 			}
 
 			LogMessageType(logger, routingId, data);
-			MessageRegistries.ToServer.Handle(data, client.Listener, taskManager, cancellationToken);
+			MessageRegistries.ToServer.Handle(data, client);
-
+			
-			if (client.Listener.IsDisposed) {
+			client.CloseIfDisposed();
-				client.Connection.Close();
-			}
 		}
 
 		foreach (var client in clients.Values) {
@@ -102,13 +101,21 @@ public sealed class RpcLauncher : RpcRuntime<ServerSocket> {
 		return false;
 	}
 
-	private readonly struct Client {
+	private sealed class Client : MessageHandler<IMessageToServerListener> {
 		public RpcClientConnection Connection { get; }
-		public IMessageToServerListener Listener { get; }
 
-		public Client(RpcClientConnection connection, Func<RpcClientConnection, IMessageToServerListener> listenerFactory) {
+		public Client(RpcClientConnection connection, Func<RpcClientConnection, IMessageToServerListener> listenerFactory, ILogger logger, TaskManager taskManager, CancellationToken cancellationToken) : base(listenerFactory(connection), logger, taskManager, cancellationToken) {
 			Connection = connection;
-			Listener = listenerFactory(connection);
+		}
+
+		protected override Task SendReply(uint sequenceId, byte[] serializedReply) {
+			return Connection.Send(new ReplyMessage(sequenceId, serializedReply));
+		}
+
+		public void CloseIfDisposed() {
+			if (Listener.IsDisposed) {
+				Connection.Close();
+			}
 		}
 	}
 }

Server/Phantom.Server.Services/Agents/AgentConnection.cs

@@ -1,7 +1,7 @@
 using Phantom.Common.Messages;
 using Phantom.Server.Rpc;
 
-namespace Phantom.Server.Services.Agents; 
+namespace Phantom.Server.Services.Agents;
 
 sealed class AgentConnection {
 	private readonly RpcClientConnection connection;
@@ -18,7 +18,11 @@ sealed class AgentConnection {
 		connection.Close();
 	}
 
-	public async Task SendMessage<TMessage>(TMessage message) where TMessage : IMessageToAgent {
+	public Task Send<TMessage>(TMessage message) where TMessage : IMessageToAgent {
-		await connection.Send(message);
+		return connection.Send(message);
+	}
+
+	public Task<TReply?> Send<TMessage, TReply>(Func<uint, TMessage> messageFactory, TimeSpan waitForReplyTime, CancellationToken cancellationToken) where TMessage : IMessageToAgent<TReply> where TReply : class {
+		return connection.Send<TMessage, TReply>(messageFactory, waitForReplyTime, cancellationToken);
 	}
 }

Server/Phantom.Server.Services/Agents/AgentManager.cs

@@ -7,7 +7,6 @@ using Phantom.Common.Messages.ToAgent;
 using Phantom.Server.Database;
 using Phantom.Server.Rpc;
 using Phantom.Server.Services.Instances;
-using Phantom.Server.Services.Rpc;
 using Phantom.Utils.Collections;
 using Phantom.Utils.Events;
 using Phantom.Utils.Runtime;
@@ -114,28 +113,14 @@ public sealed class AgentManager {
 		}
 	}
 
-	private async Task<bool> SendMessage<TMessage>(Guid guid, TMessage message) where TMessage : IMessageToAgent {
+	internal async Task<TReply?> SendMessage<TMessage, TReply>(Guid guid, Func<uint, TMessage> messageFactory, TimeSpan waitForReplyTime) where TMessage : IMessageToAgent<TReply> where TReply : class {
 		var connection = agents.ByGuid.TryGetValue(guid, out var agent) ? agent.Connection : null;
-		if (connection != null) {
+		if (connection == null) {
-			await connection.SendMessage(message);
-			return true;
-		}
-		else {
 			// TODO handle missing agent?
-			return false;
-		}
-	}
-
-	internal async Task<int?> SendMessageWithReply<TMessage>(Guid guid, Func<uint, TMessage> messageFactory, TimeSpan waitForReplyTime) where TMessage : IMessageToAgent, IMessageWithReply {
-		var sequenceId = MessageReplyTracker.Instance.RegisterReply();
-		var message = messageFactory(sequenceId);
-
-		if (!await SendMessage(guid, message)) {
-			MessageReplyTracker.Instance.ForgetReply(sequenceId);
 			return null;
 		}
 
-		return await MessageReplyTracker.Instance.WaitForReply(sequenceId, waitForReplyTime, cancellationToken);
+		return await connection.Send<TMessage, TReply>(messageFactory, waitForReplyTime, cancellationToken);
 	}
 
 	private sealed class ObservableAgents : ObservableState<ImmutableArray<Agent>> {

Server/Phantom.Server.Services/Instances/AddInstanceResult.cs

@@ -6,10 +6,8 @@ public enum AddInstanceResult {
 	InstanceNameMustNotBeEmpty,
 	InstanceMemoryMustNotBeZero,
 	AgentNotFound,
-	AgentShuttingDown,
 	AgentInstanceLimitExceeded,
 	AgentMemoryLimitExceeded,
-	AgentCommunicationError,
 	UnknownError
 }
 
@@ -20,10 +18,8 @@ public static class AddInstanceResultExtensions {
 			AddInstanceResult.InstanceNameMustNotBeEmpty  => "Instance name must not be empty.",
 			AddInstanceResult.InstanceMemoryMustNotBeZero => "Memory must not be 0 MB.",
 			AddInstanceResult.AgentNotFound               => "Agent not found.",
-			AddInstanceResult.AgentShuttingDown           => "Agent is shutting down.",
 			AddInstanceResult.AgentInstanceLimitExceeded  => "Agent instance limit exceeded.",
 			AddInstanceResult.AgentMemoryLimitExceeded    => "Agent memory limit exceeded.",
-			AddInstanceResult.AgentCommunicationError     => "Agent did not reply in time.",
 			_                                             => "Unknown error."
 		};
 	}

Server/Phantom.Server.Services/Instances/InstanceManager.cs

@@ -54,21 +54,21 @@ public sealed class InstanceManager {
 		}
 	}
 
-	public async Task<AddInstanceResult> AddInstance(InstanceConfiguration configuration) {
+	public async Task<InstanceActionResult<AddInstanceResult>> AddInstance(InstanceConfiguration configuration) {
 		var agent = agentManager.GetAgent(configuration.AgentGuid);
 		if (agent == null) {
-			return AddInstanceResult.AgentNotFound;
+			return InstanceActionResult.Concrete(AddInstanceResult.AgentNotFound);
 		}
 
 		var instance = new Instance(configuration);
 		if (!instances.ByGuid.TryAdd(instance.Configuration.InstanceGuid, instance)) {
-			return AddInstanceResult.InstanceAlreadyExists;
+			return InstanceActionResult.Concrete(AddInstanceResult.InstanceAlreadyExists);
 		}
 
 		var agentName = agent.Name;
 
-		var reply = (ConfigureInstanceResult?) await agentManager.SendMessageWithReply(configuration.AgentGuid, sequenceId => new ConfigureInstanceMessage(sequenceId, configuration), TimeSpan.FromSeconds(10));
+		var reply = (await agentManager.SendMessage<ConfigureInstanceMessage, InstanceActionResult<ConfigureInstanceResult>>(configuration.AgentGuid, sequenceId => new ConfigureInstanceMessage(sequenceId, configuration), TimeSpan.FromSeconds(10))).DidNotReplyIfNull();
-		if (reply == ConfigureInstanceResult.Success) {
+		if (reply.Is(ConfigureInstanceResult.Success)) {
 			using (var scope = databaseProvider.CreateScope()) {
 				InstanceEntity entity = scope.Ctx.InstanceUpsert.Fetch(configuration.InstanceGuid);
 
@@ -87,20 +87,18 @@ public sealed class InstanceManager {
 			}
 
 			Logger.Information("Added instance \"{InstanceName}\" (GUID {InstanceGuid}) to agent \"{AgentName}\".", configuration.InstanceName, configuration.InstanceGuid, agentName);
-			return AddInstanceResult.Success;
+			return InstanceActionResult.Concrete(AddInstanceResult.Success);
 		}
 		else {
 			instances.ByGuid.Remove(configuration.InstanceGuid);
 
-			var result = reply switch {
+			var result = reply.Map(static result => result switch {
-				null                                          => AddInstanceResult.AgentCommunicationError,
-				ConfigureInstanceResult.AgentShuttingDown     => AddInstanceResult.AgentShuttingDown,
 				ConfigureInstanceResult.InstanceLimitExceeded => AddInstanceResult.AgentInstanceLimitExceeded,
 				ConfigureInstanceResult.MemoryLimitExceeded   => AddInstanceResult.AgentMemoryLimitExceeded,
 				_                                             => AddInstanceResult.UnknownError
-			};
+			});
-
+			
-			Logger.Information("Failed adding instance \"{InstanceName}\" (GUID {InstanceGuid}) to agent \"{AgentName}\". {ErrorMessage}", configuration.InstanceName, configuration.InstanceGuid, agentName, result.ToSentence());
+			Logger.Information("Failed adding instance \"{InstanceName}\" (GUID {InstanceGuid}) to agent \"{AgentName}\". {ErrorMessage}", configuration.InstanceName, configuration.InstanceGuid, agentName, result.ToSentence(AddInstanceResultExtensions.ToSentence));
 			return result;
 		}
 	}
@@ -121,28 +119,28 @@ public sealed class InstanceManager {
 		instances.ByGuid.ReplaceAllIf(instance => instance with { Status = instanceStatus }, instance => instance.Configuration.AgentGuid == agentGuid);
 	}
 
-	public async Task<LaunchInstanceResult> LaunchInstance(Guid instanceGuid) {
+	public async Task<InstanceActionResult<LaunchInstanceResult>> LaunchInstance(Guid instanceGuid) {
 		var instance = GetInstance(instanceGuid);
 		if (instance == null) {
-			return LaunchInstanceResult.InstanceDoesNotExist;
+			return InstanceActionResult.General<LaunchInstanceResult>(InstanceActionGeneralResult.InstanceDoesNotExist);
 		}
 
 		await SetInstanceShouldLaunchAutomatically(instanceGuid, true);
 
-		var reply = (LaunchInstanceResult?) await agentManager.SendMessageWithReply(instance.Configuration.AgentGuid, sequenceId => new LaunchInstanceMessage(sequenceId, instanceGuid), TimeSpan.FromSeconds(10));
+		var reply = await agentManager.SendMessage<LaunchInstanceMessage, InstanceActionResult<LaunchInstanceResult>>(instance.Configuration.AgentGuid, sequenceId => new LaunchInstanceMessage(sequenceId, instanceGuid), TimeSpan.FromSeconds(10));
-		return reply ?? LaunchInstanceResult.CommunicationError;
+		return reply.DidNotReplyIfNull();
 	}
 
-	public async Task<StopInstanceResult> StopInstance(Guid instanceGuid, MinecraftStopStrategy stopStrategy) {
+	public async Task<InstanceActionResult<StopInstanceResult>> StopInstance(Guid instanceGuid, MinecraftStopStrategy stopStrategy) {
 		var instance = GetInstance(instanceGuid);
 		if (instance == null) {
-			return StopInstanceResult.InstanceDoesNotExist;
+			return InstanceActionResult.General<StopInstanceResult>(InstanceActionGeneralResult.InstanceDoesNotExist);
 		}
 
 		await SetInstanceShouldLaunchAutomatically(instanceGuid, false);
 
-		var reply = (StopInstanceResult?) await agentManager.SendMessageWithReply(instance.Configuration.AgentGuid, sequenceId => new StopInstanceMessage(sequenceId, instanceGuid, stopStrategy), TimeSpan.FromSeconds(10));
+		var reply = await agentManager.SendMessage<StopInstanceMessage, InstanceActionResult<StopInstanceResult>>(instance.Configuration.AgentGuid, sequenceId => new StopInstanceMessage(sequenceId, instanceGuid, stopStrategy), TimeSpan.FromSeconds(10));
-		return reply ?? StopInstanceResult.CommunicationError;
+		return reply.DidNotReplyIfNull();
 	}
 
 	private async Task SetInstanceShouldLaunchAutomatically(Guid instanceGuid, bool shouldLaunchAutomatically) {
@@ -158,14 +156,14 @@ public sealed class InstanceManager {
 		}
 	}
 
-	public async Task<SendCommandToInstanceResult> SendCommand(Guid instanceGuid, string command) {
+	public async Task<InstanceActionResult<SendCommandToInstanceResult>> SendCommand(Guid instanceGuid, string command) {
 		var instance = GetInstance(instanceGuid);
-		if (instance != null) {
+		if (instance == null) {
-			var reply = (SendCommandToInstanceResult?) await agentManager.SendMessageWithReply(instance.Configuration.AgentGuid, sequenceId => new SendCommandToInstanceMessage(sequenceId, instanceGuid, command), TimeSpan.FromSeconds(10));
+			return InstanceActionResult.General<SendCommandToInstanceResult>(InstanceActionGeneralResult.InstanceDoesNotExist);
-			return reply ?? SendCommandToInstanceResult.AgentCommunicationError;
 		}
 
-		return SendCommandToInstanceResult.InstanceDoesNotExist;
+		var reply = await agentManager.SendMessage<SendCommandToInstanceMessage, InstanceActionResult<SendCommandToInstanceResult>>(instance.Configuration.AgentGuid, sequenceId => new SendCommandToInstanceMessage(sequenceId, instanceGuid, command), TimeSpan.FromSeconds(10));
+		return reply.DidNotReplyIfNull();
 	}
 
 	internal ImmutableArray<InstanceConfiguration> GetInstanceConfigurationsForAgent(Guid agentGuid) {

Server/Phantom.Server.Services/Rpc/MessageReplyTracker.cs

@@ -1,58 +0,0 @@
-using System.Collections.Concurrent;
-using Phantom.Common.Logging;
-using Phantom.Common.Messages.ToServer;
-using Serilog;
-
-namespace Phantom.Server.Services.Rpc;
-
-sealed class MessageReplyTracker {
-	private static readonly ILogger Logger = PhantomLogger.Create<MessageReplyTracker>();
-
-	public static MessageReplyTracker Instance { get; } = new ();
-	
-	private uint lastSequenceId;
-	private readonly ConcurrentDictionary<uint, TaskCompletionSource<int?>> simpleReplyTasks = new (4, 16);
-	
-	private MessageReplyTracker() {}
-
-	public uint RegisterReply() {
-		var sequenceId = Interlocked.Increment(ref lastSequenceId);
-		simpleReplyTasks[sequenceId] = new TaskCompletionSource<int?>(TaskCreationOptions.None);
-		return sequenceId;
-	}
-
-	public async Task<int?> WaitForReply(uint sequenceId, TimeSpan waitForReplyTime, CancellationToken cancellationToken) {
-		if (!simpleReplyTasks.TryGetValue(sequenceId, out var completionSource)) {
-			Logger.Warning("No reply callback for id {SequenceId}.", sequenceId);
-			return null;
-		}
-		
-		try {
-			return await completionSource.Task.WaitAsync(waitForReplyTime, cancellationToken);
-		} catch (TimeoutException) {
-			return null;
-		} catch (OperationCanceledException) {
-			return null;
-		} catch (Exception e) {
-			Logger.Warning(e, "Error processing reply with id {SequenceId}.", sequenceId);
-			return null;
-		} finally {
-			ForgetReply(sequenceId);
-		}
-	}
-
-	public void ForgetReply(uint sequenceId) {
-		if (simpleReplyTasks.TryRemove(sequenceId, out var task)) {
-			task.SetCanceled();
-		}
-	}
-
-	public void ReceiveReply(SimpleReplyMessage message) {
-		if (simpleReplyTasks.TryRemove(message.SequenceId, out var task)) {
-			task.SetResult(message.EnumValue);
-		}
-		else {
-			Logger.Warning("Received a reply with id {SequenceId} but no registered callback.", message.SequenceId);
-		}
-	}
-}

Server/Phantom.Server.Services/Rpc/MessageToServerListener.cs

@@ -6,6 +6,7 @@ using Phantom.Common.Messages.ToServer;
 using Phantom.Server.Rpc;
 using Phantom.Server.Services.Agents;
 using Phantom.Server.Services.Instances;
+using Phantom.Utils.Rpc.Message;
 
 namespace Phantom.Server.Services.Rpc;
 
@@ -31,7 +32,7 @@ public sealed class MessageToServerListener : IMessageToServerListener {
 		this.instanceLogManager = instanceLogManager;
 	}
 
-	public async Task HandleRegisterAgent(RegisterAgentMessage message) {
+	public async Task<NoReply> HandleRegisterAgent(RegisterAgentMessage message) {
 		if (agentGuid != null && agentGuid != message.AgentInfo.Guid) {
 			await connection.Send(new RegisterAgentFailureMessage(RegisterAgentFailure.ConnectionAlreadyHasAnAgent));
 		}
@@ -40,42 +41,46 @@ public sealed class MessageToServerListener : IMessageToServerListener {
 			agentGuid = guid;
 			agentGuidWaiter.SetResult(guid);
 		}
+		
+		return NoReply.Instance;
 	}
 
 	private async Task<Guid> WaitForAgentGuid() {
 		return await agentGuidWaiter.Task.WaitAsync(cancellationToken);
 	}
 	
-	public Task HandleUnregisterAgent(UnregisterAgentMessage message) {
+	public Task<NoReply> HandleUnregisterAgent(UnregisterAgentMessage message) {
 		IsDisposed = true;
 		
 		if (agentManager.UnregisterAgent(message.AgentGuid, connection)) {
 			instanceManager.SetInstanceStatesForAgent(message.AgentGuid, InstanceStatus.Offline);
 		}
 
-		return Task.CompletedTask;
+		return Task.FromResult(NoReply.Instance);
 	}
 
-	public async Task HandleAgentIsAlive(AgentIsAliveMessage message) {
+	public async Task<NoReply> HandleAgentIsAlive(AgentIsAliveMessage message) {
 		agentManager.NotifyAgentIsAlive(await WaitForAgentGuid());
+		return NoReply.Instance;
 	}
 
-	public async Task HandleAdvertiseJavaRuntimes(AdvertiseJavaRuntimesMessage message) {
+	public async Task<NoReply> HandleAdvertiseJavaRuntimes(AdvertiseJavaRuntimesMessage message) {
 		agentJavaRuntimesManager.Update(await WaitForAgentGuid(), message.Runtimes);
+		return NoReply.Instance;
 	}
 
-	public Task HandleReportInstanceStatus(ReportInstanceStatusMessage message) {
+	public Task<NoReply> HandleReportInstanceStatus(ReportInstanceStatusMessage message) {
 		instanceManager.SetInstanceState(message.InstanceGuid, message.InstanceStatus);
-		return Task.CompletedTask;
+		return Task.FromResult(NoReply.Instance);
 	}
 
-	public Task HandleInstanceOutput(InstanceOutputMessage message) {
+	public Task<NoReply> HandleInstanceOutput(InstanceOutputMessage message) {
 		instanceLogManager.AddLines(message.InstanceGuid, message.Lines);
-		return Task.CompletedTask;
+		return Task.FromResult(NoReply.Instance);
 	}
 
-	public Task HandleSimpleReply(SimpleReplyMessage message) {
+	public Task<NoReply> HandleReply(ReplyMessage message) {
-		MessageReplyTracker.Instance.ReceiveReply(message);
+		connection.Receive(message);
-		return Task.CompletedTask;
+		return Task.FromResult(NoReply.Instance);
 	}
 }

Server/Phantom.Server.Web/Pages/InstanceCreate.razor

@@ -294,12 +294,12 @@
 
     var instance = new InstanceConfiguration(selectedAgent.Agent.Guid, instanceGuid, form.InstanceName, serverPort, rconPort, form.MinecraftVersion, form.MinecraftServerKind, memoryAllocation, javaRuntimeGuid, jvmArguments, LaunchAutomatically: false);
     var result = await InstanceManager.AddInstance(instance);
-    if (result == AddInstanceResult.Success) {
+    if (result.Is(AddInstanceResult.Success)) {
       await AuditLog.AddInstanceCreatedEvent(instance.InstanceGuid);
       Nav.NavigateTo("instances/" + instance.InstanceGuid);
     }
     else {
-      form.SubmitModel.StopSubmitting(result.ToSentence());
+      form.SubmitModel.StopSubmitting(result.ToSentence(AddInstanceResultExtensions.ToSentence));
     }
   }
 

Server/Phantom.Server.Web/Pages/InstanceDetail.razor

@@ -71,11 +71,11 @@ else {
       }
 
       var result = await InstanceManager.LaunchInstance(InstanceGuid);
-      if (result == LaunchInstanceResult.LaunchInitiated) {
+      if (result.Is(LaunchInstanceResult.LaunchInitiated)) {
         await AuditLog.AddInstanceLaunchedEvent(InstanceGuid);
       }
       else {
-        lastError = result.ToSentence();
+        lastError = result.ToSentence(LaunchInstanceResultExtensions.ToSentence);
       }
     } finally {
       isLaunchingInstance = false;

Server/Phantom.Server.Web/Shared/InstanceCommandInput.razor

@@ -1,6 +1,6 @@
-@using Phantom.Common.Data.Replies
+@using Phantom.Server.Services.Instances
 @using Phantom.Server.Services.Audit
-@using Phantom.Server.Services.Instances
+@using Phantom.Common.Data.Replies
 @inherits PhantomComponent
 @inject InstanceManager InstanceManager
 @inject AuditLog AuditLog
@@ -40,13 +40,13 @@
     }
 
     var result = await InstanceManager.SendCommand(InstanceGuid, form.Command);
-    if (result == SendCommandToInstanceResult.Success) {
+    if (result.Is(SendCommandToInstanceResult.Success)) {
       await AuditLog.AddInstanceCommandExecutedEvent(InstanceGuid, form.Command);
       form.Command = string.Empty;
       form.SubmitModel.StopSubmitting();
     }
     else {
-      form.SubmitModel.StopSubmitting(result.ToSentence());
+      form.SubmitModel.StopSubmitting(result.ToSentence(SendCommandToInstanceResultExtensions.ToSentence));
     }
 
     await commandInputElement.FocusAsync(preventScroll: true);

Server/Phantom.Server.Web/Shared/InstanceStopDialog.razor

@@ -1,8 +1,8 @@
-@using Phantom.Common.Data.Replies
+@using Phantom.Server.Services.Instances
 @using Phantom.Server.Services.Audit
-@using Phantom.Server.Services.Instances
 @using System.ComponentModel.DataAnnotations
 @using Phantom.Common.Data.Minecraft
+@using Phantom.Common.Data.Replies
 @inherits PhantomComponent
 @inject IJSRuntime Js;
 @inject InstanceManager InstanceManager;
@@ -57,13 +57,13 @@
     }
     
     var result = await InstanceManager.StopInstance(InstanceGuid, new MinecraftStopStrategy(form.StopInSeconds));
-    if (result == StopInstanceResult.StopInitiated) {
+    if (result.Is(StopInstanceResult.StopInitiated)) {
       await AuditLog.AddInstanceStoppedEvent(InstanceGuid, form.StopInSeconds);
       await Js.InvokeVoidAsync("closeModal", ModalId);
       form.SubmitModel.StopSubmitting();
     }
     else {
-      form.SubmitModel.StopSubmitting(result.ToSentence());
+      form.SubmitModel.StopSubmitting(result.ToSentence(StopInstanceResultExtensions.ToSentence));
     }
   }
 

Utils/Phantom.Utils.Rpc/Message/IMessage.cs

@@ -1,5 +1,6 @@
 namespace Phantom.Utils.Rpc.Message; 
 
-public interface IMessage<TListener> {
+public interface IMessage<TListener, TReply> {
-	Task Accept(TListener listener);
+	public uint SequenceId { get; }
+	Task<TReply> Accept(TListener listener);
 }

Utils/Phantom.Utils.Rpc/Message/MessageHandler.cs

@@ -0,0 +1,40 @@
+using Phantom.Utils.Runtime;
+using Serilog;
+
+namespace Phantom.Utils.Rpc.Message; 
+
+public abstract class MessageHandler<TListener> {
+	protected TListener Listener { get; }
+	
+	private readonly ILogger logger;
+	private readonly TaskManager taskManager;
+	private readonly CancellationToken cancellationToken;
+
+	protected MessageHandler(TListener listener, ILogger logger, TaskManager taskManager, CancellationToken cancellationToken) {
+		this.Listener = listener;
+		this.logger = logger;
+		this.taskManager = taskManager;
+		this.cancellationToken = cancellationToken;
+	}
+	
+	internal void Enqueue<TMessage, TReply>(TMessage message) where TMessage : IMessage<TListener, TReply> {
+		cancellationToken.ThrowIfCancellationRequested();
+		taskManager.Run(async () => {
+			try {
+				await Handle<TMessage, TReply>(message);
+			} catch (Exception e) {
+				logger.Error(e, "Failed to handle message {Type}.", message.GetType().Name);
+			}
+		});
+	}
+
+	private async Task Handle<TMessage, TReply>(TMessage message) where TMessage : IMessage<TListener, TReply> {
+		TReply reply = await message.Accept(Listener);
+		
+		if (reply is not NoReply) {
+			await SendReply(message.SequenceId, MessageSerializer.Serialize(reply));
+		}
+	}
+	
+	protected abstract Task SendReply(uint sequenceId, byte[] serializedReply);
+}

Utils/Phantom.Utils.Rpc/Message/MessageRegistry.cs

@@ -1,27 +1,26 @@
 using System.Buffers;
 using System.Diagnostics.CodeAnalysis;
-using Phantom.Utils.Runtime;
 using Serilog;
 using Serilog.Events;
 
 namespace Phantom.Utils.Rpc.Message;
 
-public sealed class MessageRegistry<TListener, TMessageBase> where TMessageBase : class, IMessage<TListener> {
+public sealed class MessageRegistry<TListener> {
 	private const int DefaultBufferSize = 512;
-	
+
 	private readonly ILogger logger;
 	private readonly Dictionary<Type, ushort> typeToCodeMapping = new ();
 	private readonly Dictionary<ushort, Type> codeToTypeMapping = new ();
-	private readonly Dictionary<ushort, Func<ReadOnlyMemory<byte>, TMessageBase>> codeToDeserializerMapping = new ();
+	private readonly Dictionary<ushort, Action<ReadOnlyMemory<byte>, ushort, MessageHandler<TListener>>> codeToHandlerMapping = new ();
 
 	public MessageRegistry(ILogger logger) {
 		this.logger = logger;
 	}
 
-	public void Add<TMessage>(ushort code) where TMessage : TMessageBase {
+	public void Add<TMessage, TReply>(ushort code) where TMessage : IMessage<TListener, TReply> {
 		typeToCodeMapping.Add(typeof(TMessage), code);
 		codeToTypeMapping.Add(code, typeof(TMessage));
-		codeToDeserializerMapping.Add(code, MessageSerializer.Deserialize<TMessage, TMessageBase, TListener>());
+		codeToHandlerMapping.Add(code, HandleInternal<TMessage, TReply>);
 	}
 
 	public bool TryGetType(ReadOnlyMemory<byte> data, [NotNullWhen(true)] out Type? type) {
@@ -34,7 +33,11 @@ public sealed class MessageRegistry<TListener, TMessageBase> where TMessageBase
 		}
 	}
 
-	public ReadOnlySpan<byte> Write<TMessage>(TMessage message) where TMessage : TMessageBase {
+	public ReadOnlySpan<byte> Write<TMessage>(TMessage message) where TMessage : IMessage<TListener, NoReply> {
+		return Write<TMessage, NoReply>(message);
+	}
+
+	public ReadOnlySpan<byte> Write<TMessage, TReply>(TMessage message) where TMessage : IMessage<TListener, TReply> {
 		if (!typeToCodeMapping.TryGetValue(typeof(TMessage), out ushort code)) {
 			logger.Error("Unknown message type {Type}.", typeof(TMessage));
 			return default;
@@ -44,12 +47,12 @@ public sealed class MessageRegistry<TListener, TMessageBase> where TMessageBase
 
 		try {
 			MessageSerializer.WriteCode(buffer, code);
-			MessageSerializer.Serialize<TMessage, TListener>(buffer, message);
+			MessageSerializer.Serialize(buffer, message);
 
 			if (buffer.WrittenCount > DefaultBufferSize && logger.IsEnabled(LogEventLevel.Verbose)) {
 				logger.Verbose("Serializing {Type} exceeded default buffer size: {WrittenSize} B > {DefaultBufferSize} B", typeof(TMessage).Name, buffer.WrittenCount, DefaultBufferSize);
 			}
-			
+
 			return buffer.WrittenSpan;
 		} catch (Exception e) {
 			logger.Error(e, "Failed to serialize message {Type}.", typeof(TMessage).Name);
@@ -57,7 +60,7 @@ public sealed class MessageRegistry<TListener, TMessageBase> where TMessageBase
 		}
 	}
 
-	public void Handle(ReadOnlyMemory<byte> data, TListener listener, TaskManager taskManager, CancellationToken cancellationToken) {
+	public void Handle(ReadOnlyMemory<byte> data, MessageHandler<TListener> handler) {
 		ushort code;
 		try {
 			code = MessageSerializer.ReadCode(ref data);
@@ -66,28 +69,23 @@ public sealed class MessageRegistry<TListener, TMessageBase> where TMessageBase
 			return;
 		}
 
-		if (!codeToDeserializerMapping.TryGetValue(code, out var deserialize)) {
+		if (!codeToHandlerMapping.TryGetValue(code, out var handle)) {
 			logger.Error("Unknown message code {Code}.", code);
 			return;
 		}
 
-		TMessageBase message;
+		handle(data, code, handler);
+	}
+
+	private void HandleInternal<TMessage, TReply>(ReadOnlyMemory<byte> data, ushort code, MessageHandler<TListener> handler) where TMessage : IMessage<TListener, TReply> {
+		TMessage message;
 		try {
-			message = deserialize(data);
+			message = MessageSerializer.Deserialize<TMessage>(data);
 		} catch (Exception e) {
 			logger.Error(e, "Failed to deserialize message with code {Code}.", code);
 			return;
 		}
 
-		async Task HandleMessage() {
+		handler.Enqueue<TMessage, TReply>(message);
-			try {
-				await message.Accept(listener);
-			} catch (Exception e) {
-				logger.Error(e, "Failed to handle message {Type}.", message.GetType().Name);
-			}
-		}
-
-		cancellationToken.ThrowIfCancellationRequested();
-		taskManager.Run(HandleMessage);
 	}
 }

Utils/Phantom.Utils.Rpc/Message/MessageReplyTracker.cs

@@ -0,0 +1,57 @@
+using System.Collections.Concurrent;
+using Serilog;
+
+namespace Phantom.Utils.Rpc.Message;
+
+public sealed class MessageReplyTracker {
+	private readonly ILogger logger;
+	private readonly ConcurrentDictionary<uint, TaskCompletionSource<byte[]>> replyTasks = new (4, 16);
+	
+	private uint lastSequenceId;
+
+	internal MessageReplyTracker(ILogger logger) {
+		this.logger = logger;
+	}
+
+	public uint RegisterReply() {
+		var sequenceId = Interlocked.Increment(ref lastSequenceId);
+		replyTasks[sequenceId] = new TaskCompletionSource<byte[]>(TaskCreationOptions.None);
+		return sequenceId;
+	}
+
+	public async Task<TReply?> WaitForReply<TReply>(uint sequenceId, TimeSpan waitForReplyTime, CancellationToken cancellationToken) where TReply : class {
+		if (!replyTasks.TryGetValue(sequenceId, out var completionSource)) {
+			logger.Warning("No reply callback for id {SequenceId}.", sequenceId);
+			return null;
+		}
+		
+		try {
+			byte[] replyBytes = await completionSource.Task.WaitAsync(waitForReplyTime, cancellationToken);
+			return MessageSerializer.Deserialize<TReply>(replyBytes);
+		} catch (TimeoutException) {
+			return null;
+		} catch (OperationCanceledException) {
+			return null;
+		} catch (Exception e) {
+			logger.Warning(e, "Error processing reply with id {SequenceId}.", sequenceId);
+			return null;
+		} finally {
+			ForgetReply(sequenceId);
+		}
+	}
+
+	public void ForgetReply(uint sequenceId) {
+		if (replyTasks.TryRemove(sequenceId, out var task)) {
+			task.SetCanceled();
+		}
+	}
+
+	public void ReceiveReply(uint sequenceId, byte[] serializedReply) {
+		if (replyTasks.TryRemove(sequenceId, out var task)) {
+			task.SetResult(serializedReply);
+		}
+		else {
+			logger.Warning("Received a reply with id {SequenceId} but no registered callback.", sequenceId);
+		}
+	}
+}

Utils/Phantom.Utils.Rpc/Message/MessageSerializer.cs

@@ -5,14 +5,18 @@ using MemoryPack;
 namespace Phantom.Utils.Rpc.Message;
 
 static class MessageSerializer {
-	private static readonly MemoryPackSerializeOptions SerializerOptions = MemoryPackSerializeOptions.Utf8;
+	private static readonly MemoryPackSerializerOptions SerializerOptions = MemoryPackSerializerOptions.Utf8;
 
-	public static void Serialize<TMessage, TListener>(IBufferWriter<byte> destination, TMessage message) where TMessage : IMessage<TListener> {
+	public static byte[] Serialize<T>(T message) {
-		MemoryPackSerializer.Serialize(typeof(TMessage), destination, message, SerializerOptions);
+		return MemoryPackSerializer.Serialize(typeof(T), message, SerializerOptions);
 	}
 
-	public static Func<ReadOnlyMemory<byte>, TMessageBase> Deserialize<TMessage, TMessageBase, TListener>() where TMessageBase : IMessage<TListener> where TMessage : TMessageBase {
+	public static void Serialize<T>(IBufferWriter<byte> destination, T message) {
-		return static memory => MemoryPackSerializer.Deserialize<TMessage>(memory.Span) ?? throw new NullReferenceException();
+		MemoryPackSerializer.Serialize(typeof(T), destination, message, SerializerOptions);
+	}
+
+	public static T Deserialize<T>(ReadOnlyMemory<byte> memory) {
+		return MemoryPackSerializer.Deserialize<T>(memory.Span) ?? throw new NullReferenceException();
 	}
 
 	public static void WriteCode(IBufferWriter<byte> destination, ushort value) {

Utils/Phantom.Utils.Rpc/Message/NoReply.cs

@@ -0,0 +1,5 @@
+namespace Phantom.Utils.Rpc.Message; 
+
+public readonly struct NoReply {
+	public static NoReply Instance { get; } = new ();
+}

Utils/Phantom.Utils.Rpc/RpcRuntime.cs

@@ -1,4 +1,5 @@
 using NetMQ;
+using Phantom.Utils.Rpc.Message;
 using Phantom.Utils.Runtime;
 using Serilog;
 
@@ -25,6 +26,7 @@ static class RpcRuntime {
 
 public abstract class RpcRuntime<TSocket> where TSocket : ThreadSafeSocket, new() {
 	private readonly TSocket socket;
+	private readonly MessageReplyTracker replyTracker;
 	private readonly TaskManager taskManager;
 	private readonly ILogger logger;
 
@@ -33,6 +35,7 @@ public abstract class RpcRuntime<TSocket> where TSocket : ThreadSafeSocket, new(
 		RpcRuntime.SetDefaultSocketOptions(socket.Options);
 		this.socket = socket;
 		this.logger = logger;
+		this.replyTracker = new MessageReplyTracker(logger);
 		this.taskManager = new TaskManager();
 	}
 
@@ -40,7 +43,7 @@ public abstract class RpcRuntime<TSocket> where TSocket : ThreadSafeSocket, new(
 		Connect(socket);
 
 		void RunTask() {
-			Run(socket, taskManager);
+			Run(socket, replyTracker, taskManager);
 		}
 		
 		try {
@@ -50,7 +53,7 @@ public abstract class RpcRuntime<TSocket> where TSocket : ThreadSafeSocket, new(
 		} finally {
 			logger.Information("Stopping task manager...");
 			await taskManager.Stop();
-			await Disconnect(socket);
+			await Disconnect();
 			
 			socket.Dispose();
 			NetMQConfig.Cleanup();
@@ -59,9 +62,9 @@ public abstract class RpcRuntime<TSocket> where TSocket : ThreadSafeSocket, new(
 	}
 	
 	protected abstract void Connect(TSocket socket);
-	protected abstract void Run(TSocket socket, TaskManager taskManager);
+	protected abstract void Run(TSocket socket, MessageReplyTracker replyTracker, TaskManager taskManager);
 	
-	protected virtual Task Disconnect(TSocket socket) {
+	protected virtual Task Disconnect() {
 		return Task.CompletedTask;
 	}
 }