You can include multiple input group transformations in a mapping. The DTM passes data to the input groups concurrently. However, sometimes the transformation logic of a multiple input group transformation requires that the DTM block data on one input group while it waits for a row from a different input group.
Blocking is the suspension of the data flow into an input group of a multiple input group transformation. When the DTM blocks data, it reads data from the source connected to the input group until it fills the reader and transformation buffers. After the DTM fills the buffers, it does not read more source rows until the transformation logic allows the DTM to stop blocking the source. When the DTM stops blocking a source, it processes the data in the buffers and continues to read from the source.
The DTM blocks data at one input group when it needs a specific row from a different input group to perform the transformation logic. After the DTM reads and processes the row it needs, it stops blocking the source.