A lot is demanded from Web applications today. Web apps are expected not only to function properly, but they need to do so with a great user experience. Users expect applications to be fast. They also expect applications to deliver information in real-time, without the need to refresh the browser.
All major programming platforms have frameworks that make it easier to build real-time applications. The most well-known real-time framework is probably Socket.IO on Node.js. ASP.NET has a popular real-time framework called SignalR.
In this article, you’ll learn about the origins of SignalR and how it has been rewritten to run on ASP.NET Core and address the needs of today’s real-time applications. You’ll also learn how to get started building ASP.NET Core SignalR applications.
The History of SignalR
SignalR was created in 2011 by David Fowler and Damian Edwards, who now play key roles in the direction and development of ASP.NET. It was brought into the ASP.NET project and released as part of ASP.NET in 2013. At the time, the WebSocket protocol had just been standardized and most browsers didn’t support it.
To achieve real-time messaging for the Web, developers used inefficient techniques such as AJAX polling and long-polling, and technologies like server-sent events that weren’t broadly implemented by browsers. SignalR was set up to solve this problem and provide easy support for real-time capabilities on the ASP.NET stack by creating server- and client-side libraries that abstract away the complications of these technologies.
Behind the scenes, SignalR negotiates the best protocol to use for a specified connection based on what’s supported by both the server and the client. It then provides a consistent API for sending and receiving messages in real-time. Because it’s so easy to use, ASP.NET developers quickly adopted SignalR, making it the de facto stack for real-time ASP.NET development.
Other features in SignalR were intended to make it easier to use, such as automatic reconnection and turnkey scale-out, but they ended up adding complexity and inefficiencies to the framework.
So when ASP.NET was reimagined from the ground up to create a faster, cross-platform ASP.NET Core, the team also took the time to rewrite SignalR from scratch, taking into account all that was learned from the first two versions of SignalR, as well as making it extensible enough to future-proof against new protocols and transport technologies that may emerge.
Introducing ASP.NET Core SignalR
SignalR on .NET Core runs on ASP.NET Core 2.1, which can be downloaded at http://aka.ms/DotNetCore21.
Overall, ASP.NET Core SignalR maintains a lot of the same core concepts and capabilities as SignalR. Hubs continue to be the main connection point between the server and its clients. Clients can invoke methods on the hub, and the hub can invoke methods on the clients. The hub has control over the connections on which to invoke a certain method. For example, it can send a message to a single connection, to all connections belonging to a single user, or to connections that have been placed in arbitrary groups.
There were several noteworthy changes between ASP.NET SignalR and ASP.NET Core SignalR; let’s go over some of them right now.
Built-in and Custom Protocols
ASP.NET Core SignalR ships with a new JSON message protocol that’s incompatible with earlier versions of SignalR. In addition, it has a second built-in protocol based on MessagePack, which is a binary protocol that has smaller payloads than the text-based JSON.
If you want to implement a custom message protocol, ASP.NET Core SignalR has extensibility points that allow new protocols to be plugged in.
ASP.NET didn’t have dependency injection built in, so SignalR provided a GlobalHost class that included its own dependency resolver. Now that ASP.NET Core ships with an inversion of control (IoC) container, ASP.NET Core SignalR simply leverages the built-in framework for dependency injection.
Hubs in ASP.NET Core SignalR now support constructor dependency injection without extra configuration, just like ASP.NET Core controllers or razor pages do. It’s also easy to gain access to a hub’s context from outside the hub itself by retrieving an IHubContext<T> from the IoC container and using its methods to send messages to the hub’s clients.
SignalR shipped with built-in support for scale-out using Redis, Service Bus, or SQL Server as a backplane. A backplane allows different instances of the same ASP.NET SignalR application to communicate with one another to broadcast messages to the correct clients, regardless of which instance the clients are connected to. This proved to be difficult to implement correctly and added a lot of overhead; it also didn’t consider that different applications have different scale-out needs. The result was a scale-out functionality that was complex, inefficient, and didn’t work well for many scenarios.
ASP.NET Core SignalR was redesigned with a simpler and more extensible scale-out model. It no longer allows a single client to connect to different server-side instances between requests. This means that sticky sessions are required to ensure server affinity for clients using protocols other than WebSockets. ASP.NET Core SignalR currently provides a scale-out plug-in for Redis.
Later in this article, you’ll also learn about a new, fully managed Azure SignalR Service that allows you to massively scale out your ASP.NET Core SignalR applications with only minor code changes. Azure SignalR Service also enables non-.NET and serverless applications to provide real-time messaging to SignalR compatible clients.
Another design decision that seemed like a good idea when SignalR first came out was automatic reconnections. SignalR included reconnection logic on both the clients and the server. Clients attempted to reconnect if a connection was lost, and the server buffered unsent messages and replayed them when a client reconnected. This also proved to be buggy and inefficient, and the implementation didn’t make sense for all applications.
ASP.NET Core SignalR doesn’t support automatic reconnection or automatic buffering of messages. Instead, it’s up to the client application to decide when it needs to reconnect; and it’s up to the server to implement message buffering if required.
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