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Prior to DB2 V4, when an application required DB2 access, the teleprocessing monitor (TSO, CICS, or IMS/TM) had to reside on the same MVS system as DB2. This situation created a critical mass, which is the set of subsystems tied by a single common attribute; they must access DB2 resources. For example, if a data-processing shop uses both CICS and IMS/TM to develop DB2 applications, the shop's critical mass would consist of the following:
All of them had to operate on the same CPU. Additionally, when an error occurred, they could not be moved independently without losing DB2 access. A large shop could quickly use up the resources of its machine if all DB2 applications were developed on a single DB2 subsystem. However, data sharing enables multiple DB2 subsystems to access the same data, which frees up resources, enables flexible configuration and management, expands capacity, and improves availability. Prior to data sharing, organizations had to slice applications into disparate, independently operating units in one of the following ways:
However, today the preferred method of avoiding the critical mass is to implement data sharing, which is covered in the next chapter. |
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