I have a workbench to process some raster imagery, each image is stand alone in the processing done, so a clear candidate for parallel processing.

The workbench now looks like

I've set the group by to the image name (all unique), and run in both modes with the following outcomes:

'Process when group changes (Advanced)' - Task Manager shows only only a single FME instance processing, act as blocking, processing all files before it begins writing results.

'Process at End (Blocking)' - Task Manager shows multiple FME instances launched, but only one is doing any work, acts as blocking, processing all files before it begins writing results.

Both paths are giving correct results, I have removed any 'group by' within the custom transformer, I'm trying to understand what is going wrong in running in parallel.

Thanks!