When you run a session, the DTM process reads source data and passes it to the transformations for processing. To help understand DTM processing, consider the following DTM process actions:
Reading source data. The DTM reads the sources in a mapping at different times depending on how you configure the sources, transformations, and targets in the mapping.
Blocking data. The DTM sometimes blocks the flow of data at a transformation in the mapping while it processes a row of data from a different source.
Block processing. The DTM reads and processes a block of rows at a time.