SignalR lets the server send messages to clients the moment something happens, without the browser constantly polling for updates. It picks the best transport on its own: WebSockets when available, otherwise compatible alternatives.

A hub

public class NotificationsHub : Hub
{
    public async Task JoinGroup(string department)
    {
        await Groups.AddToGroupAsync(Context.ConnectionId, department);
    }
}

builder.Services.AddSignalR();
app.MapHub<NotificationsHub>("/hubs/notifications");

Sending from the server, from anywhere

With IHubContext an application service can notify clients even outside the hub, for example when an order changes status:

public class OrderService(IHubContext<NotificationsHub> hub)
{
    public async Task ShippedAsync(Order o)
    {
        // ...update the database...
        await hub.Clients.Group("logistics").SendAsync("orderShipped", o.Id);
    }
}

const conn = new signalR.HubConnectionBuilder()
    .withUrl("/hubs/notifications")
    .withAutomaticReconnect()
    .build();

conn.on("orderShipped", id => showAlert(`Order ${id} shipped`));
await conn.start();
await conn.invoke("JoinGroup", "logistics");

Good practices

• Authenticate connections and check permissions in every hub method: a client can call any public method.
• Send small events (identifiers, statuses) and let the client load details with a normal call.
• With several server instances you need a backplane, such as Redis or Azure SignalR Service, so that messages reach everyone.
• Plan for reconnection: after an interruption the client must recover what it missed.