Chapter 9. Data Sharing

An IMS system includes a set of databases that are potentially available to all the declared application programs. Access to an individual database is a characteristic that you define in a program's PSB. Data sharing support makes it possible for application programs in separate IMSs to have concurrent access to the same set of databases. IMSs use lock management to ensure that database changes at the segment level that originate from one program are fully committed before other programs can access that segment's data.

IMS systems can share data in a sysplex environment and in a non-sysplex environment.

• Sysplex data sharing is data sharing between IMS systems on multiple z/OS images. A coupling facility is used by IRLM to control access to databases.

  Related Reading: For more information about IMS and running in a sysplex environment, see Chapter 27, "Introduction to Parallel Sysplex," on page 467.

• Non-sysplex data sharing is data sharing between IMS systems on a single z/OS image. A coupling facility can be used, but is not required.

With data sharing, two levels of control are possible (controlled through DBRC):

• With database-level sharing, an entire database is locked while an application program updates it. Locking prevents concurrent database access and scheduling of application programs that might jeopardize database integrity.

• With block-level sharing, you use a global block-locking scheme to maintain database integrity during concurrent access of a database. The blocks are locked instead of the entire database. Multiple application programs can update a database at the same time if they are updating different blocks.

Differences exist in support for data sharing configurations. Generally, a complete database is regarded as a data resource. When invoked within an IMS online system, or as a batch IMS system, the data resource must be available for an individual application program to process. The resource is not available if, for example, a data resource is either used exclusively by one IMS, is flagged as needing recovery, or backup procedures are in process.

For DEDBs, the data resource is further divided; each individual area is considered a unit of data resource. Throughout this chapter, a "database" is equivalent to a DEDB area unless otherwise noted.

Here are some of the restrictions that apply to data sharing:

• Only IMS online systems can share DEDBs.

• MSDBs and GSAM databases cannot participate in data sharing. Convert MSDBs to VSO DEDBs.

Related Reading:

• For more information about the concepts of IMS data sharing, see IMS Version 9: Administration Guide: System.

• For information about operating an IMS data sharing environment, see IMS Version 9: Operations Guide.

In This Chapter:

• "How Applications Share Data"

• "DBRC and Data Sharing" on page 121