The importance of keeping track of system elements and their modifications becomes readily apparent when we examine the z/OS® maintenance process.

Often, a PTF contains multiple element replacements. In the example shown in Figure 1, PTF1 contains replacements for two modules, MOD1 and MOD2. Although load module LMOD2 contains four modules, only two of those modules are being replaced.

Figure 1. PTF replacement

But what happens if a second PTF replaces some of the code in a module that was replaced by PTF1, as shown in Figure 2?

Figure 2. PTF prerequisite

In this example, PTF2 contains replacements for MOD2 and MOD3. For MOD1, MOD2, and MOD3 to interface successfully, PTF1 must be installed before PTF2. That's because MOD3 supplied in PTF2 may depend on the PTF1 version of MOD1 to be present. It is this dependency that constitutes a prerequisite. SYSMOD prerequisites are identified in the modification control statements (MCS) part of the SYSMOD package.

In addition to tracking prerequisites, there is another important reason to track system elements. The same module is often part of many different load modules, as shown in Figure 3.

Figure 3. Load module constructions

In Figure 3, the same MOD2 module is present in LMOD1, LMOD2, and LMOD3. When a PTF is introduced that replaces the element MOD2, that module must be replaced in all the load modules in which it exists. Therefore, it is imperative that we keep track of all load modules and the modules they contain.