Mapping validation fails in the following situations:

The transformation contains stateful variable ports.
The transformation contains unsupported functions in an expression.

If you use a port in an expression in the Aggregator transformation but you do not use the port within an aggregate function, the Spark engine might use any row in the port to process the expression.

The row that the Spark engine uses might not be the last row in the port. Hadoop execution is distributed, and thus the Spark engine might not be able to determine the last row in the port.

You cannot optimize the data cache for the transformation to store data using variable length.