SignalR

SignalR is a library for ASP.NET that enables real-time web functionality by allowing server code to push content to connected clients instantly. It abstracts the underlying transport, using WebSockets when available and gracefully falling back to Server-Sent Events or long polling if needed, to support a wide range of browsers and network configurations.

In ASP.NET Core SignalR, developers define hubs—classes that derive from Hub—and expose methods that clients can invoke. The server can call back into clients through the Clients property, with options to target all connected clients, a specific connection, or groups of connections. SignalR handles connection management, including automatic reconnects and message delivery, and provides features for user-specific messaging and broadcast scenarios.

Clients for SignalR exist across platforms, including JavaScript for web applications, .NET clients, and mobile platforms such as Android and iOS. This enables real-time features in web apps, desktop apps, and mobile apps that share a common server-side hub.

Key features include real-time messaging, broadcasting, group management, user-identity aware messaging, and streaming support. SignalR also integrates with authentication and authorization, allowing access control on hub methods. For scaling to multiple servers, it supports scale-out backplanes such as Redis or the Azure SignalR Service, which can handle large numbers of concurrent connections and distribute messages across instances.

SignalR originated in the ASP.NET ecosystem and evolved into ASP.NET Core SignalR, maintained by Microsoft as part of the ASP.NET Core framework. It is designed to simplify adding real-time capabilities to web applications with a consistent programming model across platforms.