SignalR Patterns — Notifications, Live Updates and Presence

Real-world SignalR applications require more than basic hub methods — they need reliable reconnection, presence tracking, strongly-typed hub interfaces, and thoughtful architecture decisions. The patterns in this lesson represent production-grade SignalR usage: typed hubs for compile-time safety, presence tracking for “who is online” features, background services for periodic push updates, and clear guidelines on when SignalR is the right tool versus simpler alternatives.

Typed Hub — Strongly-Typed Client Interface #

// ── Define the client interface (methods the server calls on the client) ───
public interface IPostHubClient
{
    Task PostPublished(PostSummaryDto post);
    Task PostUpdated(PostUpdateDto update);
    Task NewComment(CommentDto comment);
    Task ViewCountUpdated(int postId, int viewCount);
    Task UserJoined(string userId, string displayName);
    Task UserLeft(string userId);
}

// ── Typed Hub — compile-time checked client method names ─────────────────
public class PostHub : Hub<IPostHubClient>
{
    // Now Clients.Caller, Clients.All, etc. are typed to IPostHubClient
    public async Task SubscribeToPost(int postId)
    {
        await Groups.AddToGroupAsync(Context.ConnectionId, $"post-{postId}");
        // Type-safe — PostSummaryDto shape is verified at compile time
        await Clients.Caller.PostPublished(new PostSummaryDto { Id = postId });
    }
}

// ── IHubContext with typed interface ──────────────────────────────────────
public class PostService(IHubContext<PostHub, IPostHubClient> hubContext) : IPostService
{
    public async Task PublishAsync(int id, CancellationToken ct)
    {
        var post = await _repo.PublishAsync(id, ct);
        // Compile-time checked — PostSummaryDto must match IPostHubClient.PostPublished
        await hubContext.Clients.All.PostPublished(post.ToSummaryDto());
    }
}

Presence Tracking #

// ── Track who is currently viewing each post ─────────────────────────────
public class PostHub(
    IConnectionMultiplexer redis,
    IHubContext<PostHub, IPostHubClient> hubContext) : Hub<IPostHubClient>
{
    private const string PresenceKeyPrefix = "presence:post:";

    public async Task StartViewingPost(int postId)
    {
        var userId      = Context.User?.FindFirstValue(ClaimTypes.NameIdentifier);
        var displayName = Context.User?.FindFirstValue("displayName") ?? "Anonymous";
        var groupName   = $"post-{postId}";

        await Groups.AddToGroupAsync(Context.ConnectionId, groupName);

        // Track in Redis (TTL auto-expires on connection drop)
        if (userId is not null)
        {
            var db = redis.GetDatabase();
            await db.HashSetAsync($"{PresenceKeyPrefix}{postId}", userId, displayName);
            await db.KeyExpireAsync($"{PresenceKeyPrefix}{postId}", TimeSpan.FromMinutes(5));
        }

        // Notify others in the group that this user joined
        await Clients.OthersInGroup(groupName).UserJoined(userId ?? "anon", displayName);
    }

    public override async Task OnDisconnectedAsync(Exception? exception)
    {
        var userId = Context.User?.FindFirstValue(ClaimTypes.NameIdentifier);
        if (userId is not null)
        {
            // Remove from all post presence sets
            // In production: track which groups this connection is in
        }
        await base.OnDisconnectedAsync(exception);
    }
}

When to Use SignalR vs Polling #

Common Mistakes #

Mistake 1 — Using untyped SendAsync for all client calls (silent runtime failures on typos) #

❌ Wrong — Clients.All.SendAsync("PostPublihsed", dto) — typo compiles, silently fails at runtime.

✅ Correct — use typed Hub (Hub<IPostHubClient>) for compile-time method name and type checking.

Mistake 2 — Using SignalR for scenarios where polling is simpler and sufficient #

❌ Wrong — SignalR for a dashboard that refreshes every minute; adds complexity for no perceptible benefit.

✅ Correct — evaluate latency requirement; use SignalR only when sub-5-second push latency is genuinely needed.