dotnet-realtime-communication

What this skill does

# dotnet-realtime-communication

Real-time communication patterns for .NET applications. Compares SignalR (full-duplex over WebSockets with automatic fallback), Server-Sent Events (SSE, built-in to ASP.NET Core in .NET 10), JSON-RPC 2.0 (structured request-response over any transport), and gRPC streaming (high-performance binary streaming). Provides decision guidance for choosing the right protocol based on requirements.

**Out of scope:** HTTP client factory patterns and resilience pipelines -- see [skill:dotnet-http-client] and [skill:dotnet-resilience]. Native AOT architecture and trimming strategies -- see [skill:dotnet-native-aot] for AOT compilation, [skill:dotnet-aot-architecture] for AOT-first design patterns, and [skill:dotnet-trimming] for trim-safe development. Blazor-specific SignalR usage (component integration, Blazor Server circuit management, render mode interaction) -- see [skill:dotnet-blazor-patterns] for Blazor hosting models and circuit patterns.

Cross-references: [skill:dotnet-grpc] for gRPC streaming implementation details and all four streaming patterns. See [skill:dotnet-integration-testing] for testing real-time communication endpoints. See [skill:dotnet-blazor-patterns] for Blazor-specific SignalR circuit management and render mode interaction.

---

## Protocol Comparison

| Protocol | Direction | Transport | Format | Browser Support | Best For |
|----------|-----------|-----------|--------|-----------------|----------|
| **SignalR** | Full-duplex | WebSocket, SSE, Long Polling (auto-negotiation) | JSON or MessagePack | Yes (JS/TS client) | Interactive apps, chat, dashboards, collaborative editing |
| **SSE (.NET 10)** | Server-to-client only | HTTP/1.1+ | Text (typically JSON lines) | Yes (native EventSource API) | Notifications, live feeds, status updates |
| **JSON-RPC 2.0** | Request-response | Any (HTTP, WebSocket, stdio) | JSON | Depends on transport | Tooling protocols (LSP), structured RPC over simple transports |
| **gRPC streaming** | All four patterns | HTTP/2 | Protobuf (binary) | Limited (gRPC-Web) | Service-to-service, high-throughput, low-latency streaming |

### When to Choose What

- **SignalR**: You need bidirectional real-time communication with browser clients. SignalR handles transport negotiation automatically (WebSocket preferred, falls back to SSE, then Long Polling). Use when clients need to both send and receive in real time.
- **SSE (.NET 10 built-in)**: You only need server-to-client push. Simpler than SignalR when bidirectional communication is not required. Built into ASP.NET Core in .NET 10 -- no additional packages needed. Works with the browser's native `EventSource` API.
- **JSON-RPC 2.0**: You need structured request-response semantics over a simple transport. Used by Language Server Protocol (LSP) and some .NET tooling. Not a streaming protocol -- use when you need named methods with typed parameters over WebSocket or stdio.
- **gRPC streaming**: Service-to-service streaming with maximum performance. Supports all four streaming patterns (unary, server streaming, client streaming, bidirectional). Best when both endpoints are .NET services or gRPC-compatible. See [skill:dotnet-grpc] for implementation details.

---

## SignalR

SignalR provides real-time web functionality with automatic connection management and transport negotiation.

### Server Setup

```csharp
var builder = WebApplication.CreateBuilder(args);

builder.Services.AddSignalR(options =>
{
    options.EnableDetailedErrors = builder.Environment.IsDevelopment();
    options.MaximumReceiveMessageSize = 64 * 1024; // 64 KB
    options.KeepAliveInterval = TimeSpan.FromSeconds(15);
});

var app = builder.Build();

app.MapHub<NotificationHub>("/hubs/notifications");
```

### Hub Implementation

```csharp
public sealed class NotificationHub(
    ILogger<NotificationHub> logger) : Hub
{
    public override async Task OnConnectedAsync()
    {
        var userId = Context.UserIdentifier;
        if (userId is not null)
        {
            await Groups.AddToGroupAsync(Context.ConnectionId, $"user:{userId}");
        }

        await base.OnConnectedAsync();
    }

    // Client-to-server method
    public async Task SendMessage(string channel, string message)
    {
        // Broadcast to all clients in the channel group
        await Clients.Group(channel).SendAsync("ReceiveMessage",
            Context.UserIdentifier, message);
    }

    // Server-to-client streaming
    public async IAsyncEnumerable<StockPrice> StreamPrices(
        string symbol,
        [EnumeratorCancellation] CancellationToken cancellationToken)
    {
        while (!cancellationToken.IsCancellationRequested)
        {
            yield return await GetLatestPrice(symbol, cancellationToken);
            await Task.Delay(1000, cancellationToken);
        }
    }
}
```

### Strongly-Typed Hubs

Use interfaces to get compile-time safety for client method calls:

```csharp
public interface INotificationClient
{
    Task ReceiveMessage(string user, string message);
    Task OrderStatusChanged(int orderId, string status);
}

public sealed class NotificationHub(
    ILogger<NotificationHub> logger) : Hub<INotificationClient>
{
    public async Task SendMessage(string channel, string message)
    {
        // Compile-time checked -- no magic strings
        await Clients.Group(channel).ReceiveMessage(
            Context.UserIdentifier!, message);
    }
}
```

### Sending from Outside Hubs

Inject `IHubContext` to send messages from background services or controllers:

```csharp
public sealed class OrderService(
    IHubContext<NotificationHub, INotificationClient> hubContext)
{
    public async Task UpdateOrderStatus(int orderId, string userId, string status)
    {
        // Send to specific user group
        await hubContext.Clients.Group($"user:{userId}")
            .OrderStatusChanged(orderId, status);
    }
}
```

### Transport Negotiation

SignalR automatically negotiates the best transport:

1. **WebSocket** (preferred) -- full-duplex, lowest latency
2. **Server-Sent Events** -- server-to-client only, falls back when WebSockets unavailable
3. **Long Polling** -- universal fallback, highest latency

Force a specific transport when needed:

```csharp
// Server: disable specific transports
app.MapHub<NotificationHub>("/hubs/notifications", options =>
{
    options.Transports = HttpTransportType.WebSockets |
                         HttpTransportType.ServerSentEvents;
    // Disables Long Polling
});
```

### MessagePack Protocol

Use MessagePack for smaller payloads and faster serialization:

```csharp
// Server
builder.Services.AddSignalR()
    .AddMessagePackProtocol();

// Client (JavaScript)
// new signalR.HubConnectionBuilder()
//     .withUrl("/hubs/notifications")
//     .withHubProtocol(new signalR.protocols.msgpack.MessagePackHubProtocol())
//     .build();
```

### Connection Lifecycle

Override `OnConnectedAsync` and `OnDisconnectedAsync` to manage connection state:

```csharp
public sealed class NotificationHub(
    ILogger<NotificationHub> logger,
    IConnectionTracker tracker) : Hub<INotificationClient>
{
    public override async Task OnConnectedAsync()
    {
        var userId = Context.UserIdentifier;
        var connectionId = Context.ConnectionId;

        logger.LogInformation("Client {ConnectionId} connected (user: {UserId})",
            connectionId, userId);

        // Track connection for presence features
        if (userId is not null)
        {
            await tracker.AddConnectionAsync(userId, connectionId);
            await Groups.AddToGroupAsync(connectionId, $"user:{userId}");
        }

        await base.OnConnectedAsync();
    }

    public override async Task OnDisconnectedAsync(Exception? exception)
    {
        var userId = Context.UserIdentifier;
        var connectionId = Context.ConnectionId;

        if (exception is not null)
        {
            logger.LogWarning(exception,
                "Client {Connectio