WWF - Persisting WorkFlow

Why Persist?
The workflow host, which could be anything from a console application to a full fledged service application like MOSS, is not expected to maintain the state of the workflow instance in memory all the time. This is simply to save server resources and make them available. This considers the fact that workflows could be running for days.

Persisting - The common path
To save/persist/dehydrate/stream/serialize (yes, all denote the same idea conceptually) workflows, you usually use a workflow persistence service object such as the SqlWorkflowPersistenceService. This object could either be consumed directly within your own hosting application codebase or set up via a config file.In either of the case, you could ask for an automatic save when the workflow is 'idle' using the UnloadOnIdle entry.

What needs to be noted here is that all objects used by our workflow should be serializable in order for the host to persist the workflow (and the related objects) OK. An exception is guaranteed otherwise.

When does the save happen?
The workflow runtime appears to persist the workflow on these scenarios ('persist points') :
Against an activity, when it gets completed. (Check out the PersistOnCloseAttribute declared against Activities.)
When the workflow is completed or idle (delays, event waits)
When the workflow is forcefully unloaded.

Writing Custom Persistence Layer
Overriding a few functions by descending from the WorkflowPersistenceService class makes it easy to write a custom persistence class. Further, this new class could be made active against the workflow via the config file. But, most of us should be happy with the out of box SqlWorkflowPersistenceService which does seem to do the job good.

Persistence under MOSS
MOSS as a host has its own persisting service which uses the SPWinOePersistenceService object by default. [haven't tried forcing a different persistence object via the config though]. Waiting for external actions which include delays, waiting for events to fire etc causes the workflow to be persisted/saved to DB. The workflow appears to be serialized to the WorkFlow table (check out the InstanceData column) in the Content DB for the site.