I don't understand why the method "configure" has a DataTableSpec return value.
The format of the columns is set in the method "execute". Could the method "configure" make a cast after the execution of the "execute"?
When eventually this return value is useful?
Some consequent nodes might need the DataTableSpec. If you don't provide the DTS in the configure the next node can only be configured if the previous one is executed, this is not really practical in big workflows or when some nodes take more time. Whenever you want to change something the previous nodes would all have to be fully executed before.
Typical you would use one method for execute and configure to generate the DTS.