R – Modifying a sharepoint workflow while it’s running

There are two parts to a workflow in SharePoint - .net code and xml configuration.

For changes in the code, you can make changes and redeploy your solution package - as long as the strong name of the assembly doesn't change the new code will get called the next time an event occurs on an existing workflow. You just need to be careful when making changes that your code doesn't assume that the workflow was started using the current code.

Any changes in the xml configuration (effectively the interface definition for the workflow) will require the more complete new deployment.

I'm not sure what you mean by "handled soley through persistence services"? Persistence refers only to the storing of an idle workflow.

You could have a Delay and a Code activity in a Sequence in a While loop. When in the Delay the workflow will go idle and may be persisted if necessary. However depending on how much state is needed when persisting the workflow and/or how many such workflows you would have running at any one time may mean that a leaner approach is necessary.

A leaner approach would be to externalise the DB watching and have some "DB watching" workflow service raise an event when the desired change has occured. This service would be added to Workflow runtime.

To that end you need a service contract which is defined by an Inteface with the [ExternalDataExchange] attribute. This interface in turn defines an event that the service will raise when the desired DB change is detected. It also defines a method that a Workflow can call to specify what what change this service should be looking for. The method should accept an instance GUID so that the requesting instance can be found when the DB change is detected.

In the workflow you use a CallExternalMethodActivity to call this services method. You then flow to a HandleExternalEventActivity which listen for the event. At this point the workflow will go idle and can be persisted. It will remain there until the service raises the event.