Choosing between a USERMOD and a function SYSMOD

Software products available from IBM® provide you with many functions. However, these functions may not always exactly meet your processing requirements. Many of these products provide interfaces, such as user exit routines or dummy modules, that you can use to customize the functions to your needs. Sometimes, however, you may need to change a function substantially. You can do this by either:

Constructing a user modification as USERMODs to an existing function
Constructing an additional function SYSMOD

Although you might be able to make these changes without SMP/E, there are advantages to creating them either as USERMODs or as function SYSMODs so that SMP/E can install them.

When SMP/E installs the changes, it does the following:

Keeps a record of the changes
Reports any intersections with SYSMODs provided by IBM
Ensures that the changes are not regressed
Ensures that the changes are installed properly in the correct libraries
Lets you remove the changes if there are problems

Before creating your changes, you must decide whether to build a USERMOD type SYSMOD or a function SYSMOD. Table 1 lists considerations that will help you make this decision.

Table 1. Comparison of USERMODs and function SYSMODs
USERMOD	Function SYSMOD
Provides changes for elements owned by an existing function. May provide new elements for an existing function.	Provides new elements or new element versions for a new function.
SMP/E reports on changes attempted by PTFs, APARs, or USERMODs.	SMP/E does not report on changes attempted by PTFs, APARs, or USERMODs.
Other SYSMODs for the same function can update the elements.	Because the SYSMOD owns the element, SYSMODs for other functions cannot update the element without also changing the owner of the element.
Better for small changes that affect only a few elements.	Better for major additions to the system.