Add Columns changes are safe to deploy before all active processes are completed?
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In general all processes will run with whatever configurations were there when you started.
If you add columns to a CDT, and add the columns to the database, it shouldn't cause any problems so long as you allow those columns to be null.
You'll see that the PM has a caret in the name of CDT, such as myCDT ^ 2. This means that it's running an outdated version of the CDT. After you update the PM to run the most recent version of the CDT, that will only affect new process instances that start afterward.
This can cause all sorts of horrific nightmare problems depending on what you update in the CDT. But adding columns that can be null should be safe. I've encountered a few errors in the past caused by developers never updating the CDT in the PM, but I've never encountered an error due to adding columns and updating the PM with the new CDT.
Thank you