Dev Blog

Summary ​

Conclusions ​

ASP.NET Core SignalR simplifies building real-time applications: the server calls client methods as easily as clients call server methods. The library negotiates the transport (WebSockets, Server-Sent Events, or long polling) and the protocol (JSON or MessagePack) on its own. On the server, logic is described by a short-lived hub: Clients determines the recipients, Groups manages groups, OnConnectedAsync and OnDisconnectedAsync track connections, and state is stored in separate services. A strongly typed hub Hub<T> checks client method calls at compile time, and HubException passes understandable errors to the client. A .NET client creates a HubConnection with a builder, registers On handlers, and calls hub methods with InvokeAsync; in Windows Forms the handlers update the form through InvokeAsync. Automatic reconnection restores the connection, but not the groups. IHubContext sends messages from background services and REST endpoints, and streaming methods with IAsyncEnumerable<T> transfer data in parts.

Self-check questions ​

  1. How do real-time applications differ from the "request – response" exchange in REST?
  2. How do polling and long polling work? What are their drawbacks?
  3. How do Server-Sent Events differ from WebSocket?
  4. Which transports does SignalR support, and how are they negotiated?
  5. What is a hub? How do you register a hub on the server?
  6. Why must you not store state in hub fields? Where should it be stored?
  7. Who receives messages sent to Clients.All, Clients.Caller, Clients.Others, and Clients.OthersInGroup?
  8. How does InvokeAsync differ from SendAsync in the .NET client?
  9. Why do you override OnConnectedAsync and OnDisconnectedAsync?
  10. How do groups work? What happens to group membership after a disconnect?
  11. What advantages does a strongly typed hub Hub<T> give?
  12. What is HubException used for?
  13. Why can't On handlers in Windows Forms change controls directly?
  14. How does WithAutomaticReconnect work? What events does the connection raise?
  15. How do you send a message to clients from a background service or a REST endpoint?
  16. How do you create a streaming hub method and read the stream in the client? How do you cancel a stream?
  17. When does it make sense to use MessagePack? What are its limitations?