We are implementing scale-out for our SignalR app and trying to avoid a single point of failure in our cluster. Thus, more than one Redis message bus server is required.
The problem with implementing Redis Sentinel is that upon fail-over, a the client needs to connect to a new endpoint [address], which would require the SignalR application to be restarted (Redis endpoint defined in Application_Start()).
Not an option.
I'm trying to understand if/how Booksleeve will help, and would like some explain this.
The issue is that we can only have one single endpoint defined for message bus. A hardware solution is not currently an option.
Would the SignalR application connect to a Booksleeve wrapper, which maintains the list of master/slaves?
Another option using Azure Service Bus. However, the instructions for Wiring Up the Windows Azure Service Bus Provider indicate there are still problems with this:
Note, this web site is an ASP.NET site that runs in an Azure web role. As of 1.0alpha2 there are some bugs in AzureWebSites due to which ServiceBus Scale out scenarios do not work well. We are working on resolving this for the future
I don't know the specifics of how SignalR does the connect, but: BookSleeve already offers some concessions towards failover nodes. In particular, the ConnectionUtils.Connect method takes a string (ideal for web.config configuration values etc), which can include multiple redis nodes, and BookSleeve will then try to locate the most appropriate node to connect to. If the nodes mentioned in the string are regular redis nodes, it will attempt to connect to a master, otherwise falling back to a slave (optionally promoting the slave in the process). If the nodes mentioned are sentinel nodes, it will ask sentinel to nominate a serer to connect to.
What BookSleeve doesn't offer at the moment is a redundant connection wrapper that will automatically reconnect. That is on the road-map, but isn't difficult to do in the calling code. I plan to add more support for this at the same time as implementing redis-cluster support.
But: all that is from a BookSleeve perspective - I can't comment on SignalR specifically.