I am planning on using Windows Workflow Foundation (version 4) to implement a user management system. For persistence, I would use the Instance Store with SQL Server.
I would have a workflow instance per user, and there would be 10000+ users. This means there would be, at all times, more than 10k running instances, most of them being waiting.
The question is: will WF be able to handle so many workflow instances? In other terms: will it scale?
It will scale - the issue is how much infrastructure it will take. You can distribute WF and have them resume (via bookmarks) on a cluster of machines using (content/context) correlation. You can also cluster SQL DBs depending on how data intensive your WFs are. Without more information on what your WF will contain - this is a pretty relative question.
Microsoft provides some throughput comparison with WF3 vs. WF4. WF4 claims to have 150K workflows/sec.
