Workflow Foundation and backward compatibility with long running instances

I recently joined a project where Workflow Foundation 4.0 is being used to model business processes.

We have a designer tool so that consultants for clients can customise the workflow definitions. We also persist the workflow instance along with the definition. The workflows can be long running (e.g. months or potentially years).

My question is how do we manage backward compatibility for each release given we don't necessarily know what 开发者_开发问答customisations have been made and what legacy workflows are still in flight? We are loading from XAML but even seemingly minor changes to workflow definitions prevent them from loading. Migration scripts was my initial thought but it seems non-trivial given the complexity of WF workflows.

First off, XOML is 3.0; WF4 uses straight up XAML.

There are two options for doing this. It depends on whether you need to upgrade a long-running workflow in process, or if you want to update the workflow and use it for all new instances, while keeping current instances running on the previous version. Lets call these two options the upgrade and the multiversion strategies.

Re multiversion:
I'm doing this currently. Essentially, you must isolate every different version of the same workflow within an AppDomain. Deserializing from xaml or creating a new instance of a type is the same thing--they both result in an assembly being loaded into the current AppDomain. If v1 of the workflow is defined in assembly A.1, and v2 of the workflow is defined in assembly A.2, you can experience binding issues if you aren't careful. Isolating each version within its own AppDomain helps reduce the chance of this happening.

Re upgrade:
This currently isn't being supported, but there are plans on including this in a (near) future release. Ron Jacobs gave a presentation at PDC10 last October detailing WF4 futures. Three things (I can remember) were mentioned in the presentation--Metadata errors breaking the build, the state machine, and providing an upgrade path for workflows during execution. I can tell you that the state machine was released in the recent Platform Update, and I've been told the metadata-error-breaks-the-build feature is coming soon as well. I'd assume that the upgrade path feature will be coming soon as well.

I've done some research on this.

• WF4 Workflow Versioning Spike
• WF4 Versioning Spike: Planning for Change
• Distributed System Versioning Myth #1