Reusing a process in Workflow

I was recently a question which took me back to my very first Workflow project where we needed to re-use a sub-process repeatedly:

When I try to save the workflow, I am getting the error below:

What’s the problem?

The error is caused because Workflow will not allow the developer to re-use a sub-process within the same hierarchy, i.e. Start -> SUB1 -> SUB2 -> SUB1 -> End.  When I did my first Workflow project, we had a very complex process flow which managed sending messages via AQ from one system to a transformation hub, which then passed them onto a back-office system.  What was a simple message from the front-end became a series of simple messages in the back-end, and Workflow was used to manage the co-ordination between the systems.

What we found, however, was that in the front-end, we could build a message which allowed the user to enter a mailing address for the customer, for the product that the customer was buying (so different products could be posted to different addresses), for a proxy for a product and so forth – essentially we had four or five different levels where we wanted to do the same thing.  Unfortunately, because the “create an address” process could be called a number of times within the same hierarchy, we hit the limitation in Workflow that you cannot reuse the process.  As a workaround, we ended up creating copies of the sub-process and naming them differently (we added a different letter to the name), which became pretty annoying when it came to maintenance later on – if we had to make a change to one sub-process, we had to make sure that we were making the same change to each copy as well….

Why can’t you do it?

Here’s the background about WHY you can’t do it – feel free to skip ahead to my thoughts on bypassing the problem now.

Because of the way that the Workflow definition is stored in the database, if the same process is re-used, there is no way that the Workflow Engine can determine which time through the activities are being run.  For a single activity, when it is revisited, the previous visit is archived in the Workflow history tables.  For a series of activities (i.e. a sub-process), the same holds true when it is revisited – the whole process is recorded in the history tables.  However, there is a big difference between revisiting a process and reusing it.  If you are reusing the process, then there is nothing to archive – the previous use should remain in the current run.  When the activity then completes, the WF_ITEM_ACTIVITY_STATUSES table would have two records for the same activity – the first run and then the re-used process.  Sadly, the code which manages the update cannot take the two into account, so wouldn’t know which record to update.

If Workflow was being written today, then if the activity statuses used TIMESTAMP rather than a DATE to store the execution time (or even included a sequence number), then the Workflow Engine could more easily determine which record to update, to help bypass this limitation.  However, if you had a parallel branch, which routed to two identical re-used processes (I can’t see WHY you would do that, but it could be possible), then you hit the same limitation again – how do you identify within the Workflow tables which activity in the process is running???  Overall, it makes sense to include the limitation, IMO.

Ways Round The Problem

If there are only going to be a limited number of times that the process is going to be re-used, then I would be inclined to make a copy of the process and rename it slightly and use that.  That process is significantly easier if you know that the process is finalized, so that you won’t be needing to make the same change to multiple copies.  If you are going this route, then you need to ensure that the different process copies are sufficiently clearly labelled / documented / commented to allow you (or someone else) to maintain the definition going forwards.

If there are a large number of processes that need to be copied (I remember having copies A through to K of one sub-process!), then I would define the process as a separate, runnable process which is invoked as a child process with it’s own item type and item key.  There are no limitations on the number of times that the process can be re-used, and if the process is still in the same item type, then when it is created, the item attribute definitions will be the same – you just need to include something in your code which copies them from one instance to another.

One downside of implementing a parent-child relationship, however, is that when you look at the entire history of the process, it isn’t going to be all in one place – there will be some parts recorded in one process and others in other bits.  Also, I would be wary of using the standard WAITFORFLOW / CONTINUEFLOW activities to manage the flow, if there is a way that there could be more than one waiting process.  Instead, I would define some custom business events – have the main flow wait to receive the event, and the child flows raise the event.  This way, when you raise the event, you can pass attribute values easily back into the parent without having to directly call the WF_ENGINE APIs.


If I were building that first project from scratch, there are many things that I would change.  This would definitely be one of those.

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