External Procedure transformations operate in conjunction with procedures you create outside of the Designer interface to extend PowerCenter functionality.
Although the standard transformations provide you with a wide range of options, there are occasions when you might want to extend the functionality provided with PowerCenter. For example, the range of standard transformations, such as Expression and Filter transformations, may not provide the functionality you need. If you are an experienced programmer, you may want to develop complex functions within a dynamic link library (DLL) or UNIX shared library, instead of creating the necessary Expression transformations in a mapping.
To get this kind of extensibility, use the Transformation Exchange (TX) dynamic invocation interface built into PowerCenter. Using TX, you can create an Informatica External Procedure transformation and bind it to an external procedure that you have developed. You can bind External Procedure transformations to two kinds of external procedures:
COM external procedures (available on Windows only)
Informatica external procedures (available on Windows, AIX, Linux, and Solaris)
To use TX, you must be an experienced C, C++, or Visual Basic programmer.
Use multi-threaded code in external procedures.