The Critical Mass

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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:

  • IMS/TM subsystem

  • All CICS subsystems requiring DB2 access

  • DB2 subsystem

  • TSO subsystem if DB2I access is required

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:

  • You could develop IMS/TM applications on one DB2 subsystem, develop CICS applications on another, and develop TSO applications on yet another. This approach reduces the critical mass so that IMS/TM and CICS are not married together.

  • Another method is to provide the separate DB2 subsystems with distributed access to DB2 data that must be shared.

  • Yet another method is to choose a single teleprocessing environment for all DB2 applications.

  • Last, by avoiding DB2I and QMF access, you can eliminate TSO from the critical mass. Instead, you submit SQL and DSN commands as batch invocations of TSO. Because this hampers ease of use and detracts from the overall user -friendliness of DB2, however, doing so is not recommended.

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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DB2 Developers Guide
DB2 Developers Guide (5th Edition)
ISBN: 0672326132
EAN: 2147483647
Year: 2004
Pages: 388

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