@zhenlan We use workflow foundation's WorkflowApplication, Host Extensions (for email, data access layer, security, kernel, licensing) and TrackingProfile. This is connected to a database where the XAML of the Workflows is stored, they are dynamically loaded via DynamicActivity (using AppDomain's AssemblyResolve with Assembly.Load to load any dependency assemblies).
Our server application uses the workflows to push data around, start the optimization process, retrieve the results, send emails about status and so on. We provided a re-hosted workflow designer to our clients with new activities in dynamically loaded DLLs so that they can extend and modify their workflows. Ideally, we would want this to be functional on .Net Core. This would remove our dependency on Windows servers for the application server (calculation servers run on Win or Lin - Calculations are done in Fortran anyway).
A web hosted workflow designer would be nice, but it's not a primary focus for me at this point. That said, congrats @gary_b, you did some excellent work on your designer already.