<add key="owin:appStartup" value="UmbracoDefaultOwinStartup" />
Before 7.2.8 it didn't use this. Most people would normally have a Startup.cs class in their Visual Studio project to register the use of SignalR. Your startup class will not get called in the new 7.3.0 scenario. To fix this, you can create a class that inherits from UmbracoDefaultOwinStartup and add the SignalR usage. Change the web.config entry to the friendly name you give your OWIN class.
<add key="owin:appStartup" value="TestStartup" />
With Startup.cs looking like this (please note the OwinStartupAttribute, its first parameter is the friendly name you refer to in the web.config):
using System;
using System.Threading.Tasks;
using Microsoft.Owin;
using Owin;
using Umbraco.Web;
[assembly: OwinStartup("TestStartup", typeof(WebApplication1.Startup))]
namespace WebApplication1
{
public class Startup : UmbracoDefaultOwinStartup
{
public override void Configuration(IAppBuilder app)
{
base.Configuration(app);
app.MapSignalR();
}
}
}
By also calling base.Configuration(app); you make sure Umbraco gets to do all its specific things and afterwards you load SignalR. And there you go, SignalR should work again in Umbraco 7.3.0 and above!