Improve gateway reliability and dashboard docs

src/MxGateway.Server/Sessions/SessionManager.cs

@@ -23,6 +23,7 @@ public sealed class SessionManager : ISessionManager
     private readonly TimeProvider _timeProvider;
     private readonly ILogger<SessionManager> _logger;
     private readonly GatewayOptions _options;
+    private readonly SemaphoreSlim _sessionSlots;
 
     public SessionManager(
         ISessionRegistry registry,
@@ -39,6 +40,7 @@ public sealed class SessionManager : ISessionManager
         _timeProvider = timeProvider ?? TimeProvider.System;
         _logger = logger ?? NullLogger<SessionManager>.Instance;
         _options = options.Value;
+        _sessionSlots = new SemaphoreSlim(_options.Sessions.MaxSessions, _options.Sessions.MaxSessions);
     }
 
     public async Task<GatewaySession> OpenSessionAsync(
@@ -49,16 +51,17 @@ public sealed class SessionManager : ISessionManager
         ArgumentNullException.ThrowIfNull(request);
         EnsureSessionCapacity();
 
-        GatewaySession session = CreateSession(request, clientIdentity);
-        if (!_registry.TryAdd(session))
-        {
-            throw new SessionManagerException(
-                SessionManagerErrorCode.OpenFailed,
-                $"Session id collision while opening session {session.SessionId}.");
-        }
-
+        GatewaySession? session = null;
         try
         {
+            session = CreateSession(request, clientIdentity);
+            if (!_registry.TryAdd(session))
+            {
+                throw new SessionManagerException(
+                    SessionManagerErrorCode.OpenFailed,
+                    $"Session id collision while opening session {session.SessionId}.");
+            }
+
             session.TransitionTo(SessionState.StartingWorker);
             IWorkerClient workerClient = await _workerClientFactory
                 .CreateAsync(session, cancellationToken)
@@ -72,18 +75,23 @@ public sealed class SessionManager : ISessionManager
         }
         catch (Exception exception)
         {
-            session.MarkFaulted(exception.Message);
-            _registry.TryRemove(session.SessionId, out _);
-            await session.DisposeAsync().ConfigureAwait(false);
+            session?.MarkFaulted(exception.Message);
+            if (session is not null)
+            {
+                _registry.TryRemove(session.SessionId, out _);
+                await session.DisposeAsync().ConfigureAwait(false);
+            }
+
+            ReleaseSessionSlot();
             _metrics.Fault(SessionManagerErrorCode.OpenFailed.ToString());
             _logger.LogWarning(
                 exception,
                 "Failed to open gateway session {SessionId}.",
-                session.SessionId);
+                session?.SessionId ?? "<not-created>");
 
             throw new SessionManagerException(
                 SessionManagerErrorCode.OpenFailed,
-                $"Failed to open session {session.SessionId}.",
+                session is null ? "Failed to create session." : $"Failed to open session {session.SessionId}.",
                 exception);
         }
     }
@@ -177,6 +185,7 @@ public sealed class SessionManager : ISessionManager
                     "Graceful shutdown failed for session {SessionId}; killing worker.",
                     session.SessionId);
                 session.KillWorker(GatewayShutdownReason);
+                await RemoveSessionAsync(session).ConfigureAwait(false);
             }
         }
     }
@@ -195,6 +204,7 @@ public sealed class SessionManager : ISessionManager
                 _metrics.SessionClosed();
             }
 
+            await RemoveSessionAsync(session).ConfigureAwait(false);
             return result;
         }
         catch (Exception exception)
@@ -222,7 +232,7 @@ public sealed class SessionManager : ISessionManager
 
     private void EnsureSessionCapacity()
     {
-        if (_registry.ActiveCount >= _options.Sessions.MaxSessions)
+        if (!_sessionSlots.Wait(0))
         {
             throw new SessionManagerException(
                 SessionManagerErrorCode.SessionLimitExceeded,
@@ -230,6 +240,29 @@ public sealed class SessionManager : ISessionManager
         }
     }
 
+    private async Task RemoveSessionAsync(GatewaySession session)
+    {
+        if (!_registry.TryRemove(session.SessionId, out GatewaySession? removedSession))
+        {
+            return;
+        }
+
+        _metrics.RemoveSessionEvents(session.SessionId);
+        ReleaseSessionSlot();
+        await removedSession.DisposeAsync().ConfigureAwait(false);
+    }
+
+    private void ReleaseSessionSlot()
+    {
+        try
+        {
+            _sessionSlots.Release();
+        }
+        catch (SemaphoreFullException)
+        {
+        }
+    }
+
     private GatewaySession CreateSession(
         SessionOpenRequest request,
         string? clientIdentity)
@@ -244,6 +277,7 @@ public sealed class SessionManager : ISessionManager
         string pipeName = $"mxaccess-gateway-{Environment.ProcessId}-{sessionId}";
         string nonce = CreateNonce();
         DateTimeOffset openedAt = _timeProvider.GetUtcNow();
+        string clientCorrelationId = CreateClientCorrelationId(request.ClientSessionName, sessionId);
 
         return new GatewaySession(
             sessionId,
@@ -252,13 +286,24 @@ public sealed class SessionManager : ISessionManager
             nonce,
             clientIdentity,
             request.ClientSessionName,
-            request.ClientCorrelationId,
+            clientCorrelationId,
             commandTimeout,
             startupTimeout,
             shutdownTimeout,
             openedAt);
     }
 
+    private static string CreateClientCorrelationId(
+        string? clientSessionName,
+        string sessionId)
+    {
+        string clientName = string.IsNullOrWhiteSpace(clientSessionName)
+            ? "client"
+            : clientSessionName!;
+
+        return $"{clientName}-{sessionId}";
+    }
+
     private TimeSpan ResolveCommandTimeout(Duration? requestedTimeout)
     {
         if (requestedTimeout is null)