SignalR Groups.Add times out and fails

问题:

I'm trying to add a member to a Group using SignalR 2.2. Every single time, I hit a 30 second timeout and get a "System.Threading.Tasks.TaskCanceledException: A task was canceled." error.

From a GroupSubscriptionController that I've written, I'm calling:

 var hubContext = GlobalHost.ConnectionManager.GetHubContext<ProjectHub>();
 await hubContext.Groups.Add(connectionId, groupName);

I've found this issue where people are periodically encountering this, but it happens to me every single time. I'm running the backend (ASP.NET 4.5) on one VS2015 launched localhost port, and the frontend (AngularJS SPA) on another VS 2015 launched localhost port.

I had gotten SignalR working to the point where messages were being broadcast to every connected client. It seemed so easy. Now, adding in the Groups part (so that people only get select messages from the server) has me pulling my hair out...

回答1:

That task cancellation error could be being thrown because the connectionId can't be found in the SignalR registry of connected clients.

How are you getting this connectionId? You have multiple servers/ports going - is it possible that you're getting your wires crossed?

回答2:

I know there is an accepted answer to this, but I came across this once for a different reason.

First off, do you know what Groups.Add does?

I had expected Groups.Add's task to complete almost immediately every time, but not so. Groups.Add returns a task that only completes, when the client (i.e. Javascript) acknowledges that it has been added to a group - this is useful for reconnecting so it can resubscribe to all its old groups. Note this acknowledgement is not visible to the developer code and nicely covered up for you.

The problem is that the client may not respond because they have disconnected (i.e. they've navigated to another page). This will mean that the await call will have to wait until the connection has disconnected (default timeout 30 seconds) before giving up by throwing a TaskCanceledException.

See http://www.asp.net/signalr/overview/guide-to-the-api/working-with-groups for more detail on groups