Summary Update.

RelayServer/Services/Chat/ConnectedClientService.cs

@@ -2,12 +2,32 @@ using System.Collections.Concurrent;
 
 namespace RelayServer.Services.Chat;
 
+/// <summary>
+/// Two-way in-memory mapping between WebSocket session IDs and usernames.
+///
+/// Why both directions: when a chat message arrives, we need to look up "which sessions does
+/// this server member have open right now?" (username → sessions) so we can deliver to each
+/// of their devices. When a connection closes, we need to know "which user owned this session?"
+/// (session → username) to clean up correctly.
+///
+/// Multi-device support: one username can have multiple sessions (phone + desktop + web all
+/// connected simultaneously). UsernameToSessions stores a HashSet per username; each lock
+/// is scoped to that specific HashSet so different users never block each other.
+///
+/// Username comparisons are case-insensitive (OrdinalIgnoreCase on the outer dictionary)
+/// because the DB stores usernames lowercase but clients may register with mixed case.
+/// </summary>
 public static class ConnectedClientService
 {
     private static readonly ConcurrentDictionary<string, string> SessionToUsername = new();
     private static readonly ConcurrentDictionary<string, HashSet<string>> UsernameToSessions =
         new(StringComparer.OrdinalIgnoreCase);
 
+    /// <summary>
+    /// Associates a session ID with a username. Called from HandleRegisterKey. If the same
+    /// session re-registers under a different username (rare — basically only if the client
+    /// reauthenticates), the old mapping is cleaned up first to avoid double-bookkeeping.
+    /// </summary>
     public static void Register(string sessionId, string username)
     {
         if (SessionToUsername.TryGetValue(sessionId, out var oldUsername) &&
@@ -26,12 +46,21 @@ public static class ConnectedClientService
             sessions.Add(sessionId);
     }
 
+    /// <summary>
+    /// Removes a session from both mappings. Called from OnClose. Idempotent — calling for
+    /// a session that's already gone is a no-op.
+    /// </summary>
     public static void Unregister(string sessionId)
     {
         if (SessionToUsername.TryRemove(sessionId, out var username))
             RemoveSessionFromUsername(sessionId, username);
     }
 
+    /// <summary>
+    /// Returns every active session ID for a given username (case-insensitive lookup).
+    /// Empty collection if the user is offline. Snapshot-safe: the returned list is a copy,
+    /// not a live view of the underlying HashSet.
+    /// </summary>
     public static IReadOnlyCollection<string> GetSessionsForUser(string username)
     {
         if (UsernameToSessions.TryGetValue(username, out var sessions))
@@ -43,11 +72,19 @@ public static class ConnectedClientService
         return Array.Empty<string>();
     }
 
+    /// <summary>
+    /// Reverse lookup: which user owns this session? Returns the mixed-case username the
+    /// client registered with (preserves casing for display). Null if the session is unknown.
+    /// </summary>
     public static string? GetUsernameForSession(string sessionId)
     {
         return SessionToUsername.TryGetValue(sessionId, out var u) ? u : null;
     }
 
+    /// <summary>
+    /// Internal cleanup: pulls a session out of the username→sessions HashSet, and removes
+    /// the username entry entirely if no sessions remain (keeps the dictionary lean).
+    /// </summary>
     private static void RemoveSessionFromUsername(string sessionId, string username)
     {
         if (!UsernameToSessions.TryGetValue(username, out var sessions))