Custom transformations operate in conjunction with procedures you create outside of the Designer interface to extend PowerCenter functionality. You can create a Custom transformation and bind it to a procedure that you develop using the Custom transformation functions. The Custom transformation can be an active or passive transformation.
Use the Custom transformation to create transformation applications, such as sorting and aggregation, which require all input rows to be processed before outputting any output rows. To support this process, the input and output functions occur separately in Custom transformations compared to External Procedure transformations.
The Integration Service passes the input data to the procedure using an input function. The output function is a separate function that you must enter in the procedure code to pass output data to the Integration Service. In contrast, in the External Procedure transformation, an external procedure function does both input and output, and its parameters consist of all the ports of the transformation.
You can also use the Custom transformation to create a transformation that requires multiple input groups, multiple output groups, or both. A group is the representation of a row of data entering or leaving a transformation. For example, you might create a Custom transformation with one input group and multiple output groups that parses XML data. Or, you can create a Custom transformation with two input groups and one output group that merges two streams of input data into one stream of output data.