What causes the "DataAlreadyUsed" error when PSP data flow is enabled?

Question

I get a "DataAlreadyUsed" error when a PSP data flow is enabled for a data flow for a data flow that is drawn from the outside to the inside of a loop operation.

Answer

This error is caused by the PSP (parallel steam processing) specification limitaion.

Multi-Thread Processing
https://www.hulft.com/help/en-us/HULFTSquare/Content/Data_Integration/Detailed/multi_stream_processing.htm

• In the Loop, Conditional Loop, or Loop by Number of Data operation, the DataAlreadyUsedException error occurs during execution when the data flows directly from the Read CSV File operation that is set with Enable multi-thread processing setting which is outside the loop to the components that perform Multi-Thread Processing within the loop.

This error may occur due to the loop operations start processing before the data processing is completed in former operations, since PSP is a mechanism that process data in parallel.

If an error occurs during processing with PSP enabled, even if the error occurs in reading or conversion components, it will be logged as a writing component error, and this makes it difficult to identify where the error occurred in the script.

Please refer to the above manual page for other limitations of PSP.