R – How to persist the workflow when server system is crashed

After being faced with the same problem and spending a lot of time researching and trying out different options, I think I found a really good solution. I'm posting it here for posterity.

1. Create a custom activity that extends SequenceActivity called ReviewActivity
2. The ReviewActivity includes the typical CreateTask -> While -> OnTaskChanged -> CompleteTask scenario
3. In my workflow, I have a Replicator that is creating many instances of the ReviewActivity (and thus many Tasks).
4. The replicator implements an UntilCondition that checks to see if a task was rejected (this is set in the ChildCompleted)
5. After the Replicator, I have an UpdateAllTasks to close the remaining tasks

If you have any experience with this scenario at all, you are getting ready to tell me that UpdateAllTasks also updates the originally rejected task since the "CompleteTask" has not been persisted to the database yet. The magic is in an attribute that you can define for the custom activity (ReviewActivity) called PersistOnClose.

[Designer(typeof(ActivityDesigner), typeof(IDesigner))]
[PersistOnClose]
public partial class ReviewActivity : SequenceActivity

This attribute ensures that once the ReviewActivity is complete, all changes are persisted to the database. Since the last activity in the ReviewActivity is the "CompleteTask", the task is saved to the DB. Therefore, the UpdateAllTasks will not touch it.

I hope this helps someone.

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.