diff --git a/tanks-backend/TanksServer/Endpoints.cs b/tanks-backend/TanksServer/Endpoints.cs
index 8685f63..a002cdd 100644
--- a/tanks-backend/TanksServer/Endpoints.cs
+++ b/tanks-backend/TanksServer/Endpoints.cs
@@ -47,7 +47,7 @@ internal static class Endpoints
                 return Results.BadRequest();
 
             using var ws = await context.WebSockets.AcceptWebSocketAsync();
-            await clientScreenServer.HandleClient(ws, player);
+            await clientScreenServer.HandleClientAsync(ws, player);
             return Results.Empty;
         });
 
@@ -60,7 +60,7 @@ internal static class Endpoints
                 return Results.NotFound();
 
             using var ws = await context.WebSockets.AcceptWebSocketAsync();
-            await controlsServer.HandleClient(ws, player);
+            await controlsServer.HandleClientAsync(ws, player);
             return Results.Empty;
         });
 
diff --git a/tanks-backend/TanksServer/Interactivity/ClientScreenServer.cs b/tanks-backend/TanksServer/Interactivity/ClientScreenServer.cs
index e3861be..e52fcec 100644
--- a/tanks-backend/TanksServer/Interactivity/ClientScreenServer.cs
+++ b/tanks-backend/TanksServer/Interactivity/ClientScreenServer.cs
@@ -1,7 +1,5 @@
-using System.Diagnostics;
 using System.Net.WebSockets;
 using DisplayCommands;
-using Microsoft.Extensions.Hosting;
 using TanksServer.Graphics;
 
 namespace TanksServer.Interactivity;
@@ -10,54 +8,18 @@ internal sealed class ClientScreenServer(
     ILogger<ClientScreenServer> logger,
     ILoggerFactory loggerFactory,
     IOptions<HostConfiguration> hostConfig
-) : IHostedLifecycleService, IFrameConsumer
+) : WebsocketServer<ClientScreenServerConnection>(logger), IFrameConsumer
 {
-    private readonly ConcurrentDictionary<ClientScreenServerConnection, byte> _connections = new();
     private readonly TimeSpan _minFrameTime = TimeSpan.FromMilliseconds(hostConfig.Value.ClientDisplayMinFrameTimeMs);
-    private bool _closing;
 
-    public Task StoppingAsync(CancellationToken cancellationToken)
-    {
-        logger.LogInformation("closing connections");
-        _closing = true;
-        return Task.WhenAll(_connections.Keys.Select(c => c.CloseAsync()));
-    }
-
-    public Task HandleClient(WebSocket socket, Guid? playerGuid)
-    {
-        if (_closing)
-        {
-            logger.LogWarning("ignoring request because connections are closing");
-            return Task.CompletedTask;
-        }
-
-        logger.LogDebug("HandleClient");
-        var connection = new ClientScreenServerConnection(
+    public Task HandleClientAsync(WebSocket socket, Guid? playerGuid)
+        => base.HandleClientAsync(new(
             socket,
             loggerFactory.CreateLogger<ClientScreenServerConnection>(),
-            this,
             _minFrameTime,
-            playerGuid);
-        var added = _connections.TryAdd(connection, 0);
-        Debug.Assert(added);
-        return connection.Done;
-    }
-
-    public void Remove(ClientScreenServerConnection connection) => _connections.TryRemove(connection, out _);
-
-    public IEnumerable<ClientScreenServerConnection> GetConnections() => _connections.Keys;
-
+            playerGuid
+        ));
 
     public Task OnFrameDoneAsync(GamePixelGrid gamePixelGrid, PixelGrid observerPixels)
-    {
-        var tasks = _connections.Keys
-            .Select(c => c.SendAsync(observerPixels, gamePixelGrid));
-        return Task.WhenAll(tasks);
-    }
-
-    public Task StartAsync(CancellationToken cancellationToken) => Task.CompletedTask;
-    public Task StopAsync(CancellationToken cancellationToken) => Task.CompletedTask;
-    public Task StartedAsync(CancellationToken cancellationToken) => Task.CompletedTask;
-    public Task StartingAsync(CancellationToken cancellationToken) => Task.CompletedTask;
-    public Task StoppedAsync(CancellationToken cancellationToken) => Task.CompletedTask;
+        => ParallelForEachConnectionAsync(c => c.SendAsync(observerPixels, gamePixelGrid));
 }
diff --git a/tanks-backend/TanksServer/Interactivity/ClientScreenServerConnection.cs b/tanks-backend/TanksServer/Interactivity/ClientScreenServerConnection.cs
index 75e97d7..1294934 100644
--- a/tanks-backend/TanksServer/Interactivity/ClientScreenServerConnection.cs
+++ b/tanks-backend/TanksServer/Interactivity/ClientScreenServerConnection.cs
@@ -5,11 +5,10 @@ using TanksServer.Graphics;
 
 namespace TanksServer.Interactivity;
 
-internal sealed class ClientScreenServerConnection : IDisposable
+internal sealed class ClientScreenServerConnection : IWebsocketServerConnection, IDisposable
 {
     private readonly ByteChannelWebSocket _channel;
     private readonly ILogger<ClientScreenServerConnection> _logger;
-    private readonly ClientScreenServer _server;
     private readonly SemaphoreSlim _wantedFrames = new(1);
     private readonly Guid? _playerGuid;
     private readonly PlayerScreenData? _playerScreenData;
@@ -20,12 +19,10 @@ internal sealed class ClientScreenServerConnection : IDisposable
     public ClientScreenServerConnection(
         WebSocket webSocket,
         ILogger<ClientScreenServerConnection> logger,
-        ClientScreenServer server,
         TimeSpan minFrameTime,
         Guid? playerGuid = null
     )
     {
-        _server = server;
         _logger = logger;
         _minFrameTime = minFrameTime;
 
@@ -91,9 +88,7 @@ internal sealed class ClientScreenServerConnection : IDisposable
     {
         await foreach (var _ in _channel.ReadAllAsync())
             _wantedFrames.Release();
-
         _logger.LogTrace("done receiving");
-        _server.Remove(this);
     }
 
     public Task CloseAsync()
diff --git a/tanks-backend/TanksServer/Interactivity/ControlsServer.cs b/tanks-backend/TanksServer/Interactivity/ControlsServer.cs
index 1d081ad..aa1fc9b 100644
--- a/tanks-backend/TanksServer/Interactivity/ControlsServer.cs
+++ b/tanks-backend/TanksServer/Interactivity/ControlsServer.cs
@@ -1,115 +1,19 @@
 using System.Net.WebSockets;
-using Microsoft.Extensions.Hosting;
 
 namespace TanksServer.Interactivity;
 
-internal sealed class ControlsServer(ILogger<ControlsServer> logger, ILoggerFactory loggerFactory)
-    : IHostedLifecycleService
+internal sealed class ControlsServer(
+    ILogger<ControlsServer> logger,
+    ILoggerFactory loggerFactory
+) : WebsocketServer<ControlsServerConnection>(logger)
 {
-    private readonly List<ControlsServerConnection> _connections = [];
-
-    public Task StoppingAsync(CancellationToken cancellationToken)
-    {
-        return Task.WhenAll(_connections.Select(c => c.CloseAsync()));
-    }
-
-    public Task HandleClient(WebSocket ws, Player player)
+    public async Task HandleClientAsync(WebSocket ws, Player player)
     {
         logger.LogDebug("control client connected {}", player.Id);
         var clientLogger = loggerFactory.CreateLogger<ControlsServerConnection>();
-        var sock = new ControlsServerConnection(ws, clientLogger, this, player);
-        _connections.Add(sock);
-        return sock.Done;
-    }
-
-    private void Remove(ControlsServerConnection connection) => _connections.Remove(connection);
-
-    public Task StartAsync(CancellationToken cancellationToken) => Task.CompletedTask;
-    public Task StopAsync(CancellationToken cancellationToken) => Task.CompletedTask;
-    public Task StartedAsync(CancellationToken cancellationToken) => Task.CompletedTask;
-    public Task StartingAsync(CancellationToken cancellationToken) => Task.CompletedTask;
-    public Task StoppedAsync(CancellationToken cancellationToken) => Task.CompletedTask;
-
-    private sealed class ControlsServerConnection
-    {
-        private readonly ByteChannelWebSocket _binaryWebSocket;
-        private readonly ILogger<ControlsServerConnection> _logger;
-        private readonly Player _player;
-        private readonly ControlsServer _server;
-
-        public ControlsServerConnection(WebSocket socket, ILogger<ControlsServerConnection> logger,
-            ControlsServer server, Player player)
-        {
-            _logger = logger;
-            _server = server;
-            _player = player;
-            _binaryWebSocket = new ByteChannelWebSocket(socket, logger, 2);
-            Done = ReceiveAsync();
-        }
-
-        public Task Done { get; }
-
-        private async Task ReceiveAsync()
-        {
-            await foreach (var buffer in _binaryWebSocket.ReadAllAsync())
-            {
-                var type = (MessageType)buffer.Span[0];
-                var control = (InputType)buffer.Span[1];
-
-                _logger.LogTrace("player input {} {} {}", _player.Id, type, control);
-
-                var isEnable = type switch
-                {
-                    MessageType.Enable => true,
-                    MessageType.Disable => false,
-                    _ => throw new ArgumentException("invalid message type")
-                };
-
-                _player.LastInput = DateTime.Now;
-
-                switch (control)
-                {
-                    case InputType.Forward:
-                        _player.Controls.Forward = isEnable;
-                        break;
-                    case InputType.Backward:
-                        _player.Controls.Backward = isEnable;
-                        break;
-                    case InputType.Left:
-                        _player.Controls.TurnLeft = isEnable;
-                        break;
-                    case InputType.Right:
-                        _player.Controls.TurnRight = isEnable;
-                        break;
-                    case InputType.Shoot:
-                        _player.Controls.Shoot = isEnable;
-                        break;
-                    default:
-                        throw new ArgumentException("invalid control type");
-                }
-            }
-
-            _server.Remove(this);
-        }
-
-        public Task CloseAsync()
-        {
-            return _binaryWebSocket.CloseAsync();
-        }
-
-        private enum MessageType : byte
-        {
-            Enable = 0x01,
-            Disable = 0x02
-        }
-
-        private enum InputType : byte
-        {
-            Forward = 0x01,
-            Backward = 0x02,
-            Left = 0x03,
-            Right = 0x04,
-            Shoot = 0x05
-        }
+        var sock = new ControlsServerConnection(ws, clientLogger, player);
+        await AddConnection(sock);
+        await sock.Done;
+        await RemoveConnection(sock);
     }
 }
diff --git a/tanks-backend/TanksServer/Interactivity/ControlsServerConnection.cs b/tanks-backend/TanksServer/Interactivity/ControlsServerConnection.cs
new file mode 100644
index 0000000..e30095a
--- /dev/null
+++ b/tanks-backend/TanksServer/Interactivity/ControlsServerConnection.cs
@@ -0,0 +1,78 @@
+using System.Net.WebSockets;
+
+namespace TanksServer.Interactivity;
+
+internal sealed class ControlsServerConnection : IWebsocketServerConnection
+{
+    private readonly ByteChannelWebSocket _binaryWebSocket;
+    private readonly ILogger<ControlsServerConnection> _logger;
+    private readonly Player _player;
+
+    public ControlsServerConnection(WebSocket socket, ILogger<ControlsServerConnection> logger, Player player)
+    {
+        _logger = logger;
+        _player = player;
+        _binaryWebSocket = new ByteChannelWebSocket(socket, logger, 2);
+        Done = ReceiveAsync();
+    }
+
+    public Task Done { get; }
+
+    private async Task ReceiveAsync()
+    {
+        await foreach (var buffer in _binaryWebSocket.ReadAllAsync())
+        {
+            var type = (MessageType)buffer.Span[0];
+            var control = (InputType)buffer.Span[1];
+
+            _logger.LogTrace("player input {} {} {}", _player.Id, type, control);
+
+            var isEnable = type switch
+            {
+                MessageType.Enable => true,
+                MessageType.Disable => false,
+                _ => throw new ArgumentException("invalid message type")
+            };
+
+            _player.LastInput = DateTime.Now;
+
+            switch (control)
+            {
+                case InputType.Forward:
+                    _player.Controls.Forward = isEnable;
+                    break;
+                case InputType.Backward:
+                    _player.Controls.Backward = isEnable;
+                    break;
+                case InputType.Left:
+                    _player.Controls.TurnLeft = isEnable;
+                    break;
+                case InputType.Right:
+                    _player.Controls.TurnRight = isEnable;
+                    break;
+                case InputType.Shoot:
+                    _player.Controls.Shoot = isEnable;
+                    break;
+                default:
+                    throw new ArgumentException("invalid control type");
+            }
+        }
+    }
+
+    public Task CloseAsync() => _binaryWebSocket.CloseAsync();
+
+    private enum MessageType : byte
+    {
+        Enable = 0x01,
+        Disable = 0x02
+    }
+
+    private enum InputType : byte
+    {
+        Forward = 0x01,
+        Backward = 0x02,
+        Left = 0x03,
+        Right = 0x04,
+        Shoot = 0x05
+    }
+}
diff --git a/tanks-backend/TanksServer/Interactivity/IWebsocketServerConnection.cs b/tanks-backend/TanksServer/Interactivity/IWebsocketServerConnection.cs
new file mode 100644
index 0000000..10210cb
--- /dev/null
+++ b/tanks-backend/TanksServer/Interactivity/IWebsocketServerConnection.cs
@@ -0,0 +1,8 @@
+namespace TanksServer.Interactivity;
+
+internal interface IWebsocketServerConnection
+{
+    Task CloseAsync();
+
+    Task Done { get; }
+}
diff --git a/tanks-backend/TanksServer/Interactivity/PlayerServer.cs b/tanks-backend/TanksServer/Interactivity/PlayerServer.cs
index a78d4a9..e1d7c80 100644
--- a/tanks-backend/TanksServer/Interactivity/PlayerServer.cs
+++ b/tanks-backend/TanksServer/Interactivity/PlayerServer.cs
@@ -3,7 +3,10 @@ using TanksServer.GameLogic;
 
 namespace TanksServer.Interactivity;
 
-internal sealed class PlayerServer(ILogger<PlayerServer> logger, TankSpawnQueue tankSpawnQueue)
+internal sealed class PlayerServer(
+    ILogger<PlayerServer> logger,
+    TankSpawnQueue tankSpawnQueue
+)
 {
     private readonly ConcurrentDictionary<string, Player> _players = new();
 
diff --git a/tanks-backend/TanksServer/Interactivity/WebsocketServer.cs b/tanks-backend/TanksServer/Interactivity/WebsocketServer.cs
new file mode 100644
index 0000000..a4bd0eb
--- /dev/null
+++ b/tanks-backend/TanksServer/Interactivity/WebsocketServer.cs
@@ -0,0 +1,87 @@
+using Microsoft.Extensions.Hosting;
+
+namespace TanksServer.Interactivity;
+
+internal class WebsocketServer<T>(
+    ILogger logger
+) : IHostedLifecycleService, IDisposable
+    where T : IWebsocketServerConnection
+{
+    private readonly SemaphoreSlim _mutex = new(1, 1);
+    private bool _closing;
+    private readonly HashSet<T> _connections = [];
+
+    public async Task StoppingAsync(CancellationToken cancellationToken)
+    {
+        logger.LogInformation("closing connections");
+        await Locked(async () =>
+        {
+            _closing = true;
+            await Task.WhenAll(_connections.Select(c => c.CloseAsync()));
+        }, cancellationToken);
+        logger.LogInformation("closed connections");
+    }
+
+    protected Task ParallelForEachConnectionAsync(Func<T, Task> body)
+    {
+        _mutex.Wait();
+        try
+        {
+            return Task.WhenAll(_connections.Select(body));
+        }
+        finally
+        {
+            _mutex.Release();
+        }
+    }
+
+    protected Task AddConnection(T connection) => Locked(() =>
+    {
+        if (_closing)
+        {
+            logger.LogWarning("refusing connection because server is shutting down");
+            return connection.CloseAsync();
+        }
+
+        _connections.Add(connection);
+        return Task.CompletedTask;
+    }, CancellationToken.None);
+
+    protected Task RemoveConnection(T connection) => Locked(() =>
+    {
+        _connections.Remove(connection);
+        return Task.CompletedTask;
+    }, CancellationToken.None);
+
+    protected async Task HandleClientAsync(T connection)
+    {
+        await AddConnection(connection);
+        await connection.Done;
+        await RemoveConnection(connection);
+    }
+
+    private async Task Locked(Func<Task> action, CancellationToken cancellationToken)
+    {
+        await _mutex.WaitAsync(cancellationToken);
+        try
+        {
+            await action();
+        }
+        finally
+        {
+            _mutex.Release();
+        }
+    }
+
+    public void Dispose() => _mutex.Dispose();
+
+    public Task StartAsync(CancellationToken cancellationToken) => Task.CompletedTask;
+
+    public Task StopAsync(CancellationToken cancellationToken) => Task.CompletedTask;
+
+    public Task StartedAsync(CancellationToken cancellationToken) => Task.CompletedTask;
+
+    public Task StartingAsync(CancellationToken cancellationToken) => Task.CompletedTask;
+
+    public Task StoppedAsync(CancellationToken cancellationToken) => Task.CompletedTask;
+}
diff --git a/tanks-backend/TanksServer/Models/Scores.cs b/tanks-backend/TanksServer/Models/Scores.cs
index 9c651f3..afa730d 100644
--- a/tanks-backend/TanksServer/Models/Scores.cs
+++ b/tanks-backend/TanksServer/Models/Scores.cs
@@ -1,10 +1,10 @@
 namespace TanksServer.Models;
 
-internal sealed record class Scores(int Kills = 0, int Deaths = 0)
+internal sealed record class Scores
 {
-    public int Kills { get; set; } = Kills;
+    public int Kills { get; set; }
 
-    public int Deaths { get; set; } = Deaths;
+    public int Deaths { get; set; }
 
     public double Ratio
     {
@@ -14,7 +14,7 @@ internal sealed record class Scores(int Kills = 0, int Deaths = 0)
                 return 0;
             if (Deaths == 0)
                 return Kills;
-            return Kills / (double)Deaths;
+            return Math.Round(Kills / (double)Deaths, 3);
         }
     }