DBRC and data sharing support

Concurrent access to databases by systems in one or more z/OS® operating systems is controlled with a shared Database Recovery Control (DBRC) RECON data set. IMS systems automatically sign on to DBRC, ensuring that DBRC knows which IMS systems and utilities are currently participating in shared access.

Subsequently, a system's eligibility to be authorized to access a database depends on the declared degree of sharing permitted and other status indicators in the RECON data set.

To maintain data integrity, status indicators in the RECON data set control concurrent access and recovery actions for the databases. This common RECON data set is required in a data sharing IMSplex because a given database must have a DMB number that uniquely identifies it to all the sharing IMS systems. The DMB number that DBRC records in its RECON data set is related to the order in which databases are registered to DBRC. Using multiple RECON data sets can result in the same DMB number existing in each RECON data set for different databases. This condition can result in damage to databases.

Registering with DBRC

Databases that are to take part in data sharing must be registered in RECON. Each registered database has a current status that reflects whether it can take part in sharing and the scope of the sharing. The concept of scope combines several ideas:

  • The type of access: read or update
  • Whether more than one access can occur within the database simultaneously
  • Whether an IMS system that needs access is in the same or a different z/OS operating system

You specify the level of sharing on an individual IMS database using the SHARELVL specification on the INIT.DB command.

Databases can be shared at the database level, at the block level, or not at all. The type of sharing used for a database should be based on the data integrity and availability needs of all the application programs using it.

When two or more IMS systems are executing concurrently and sharing data at the database level, they can be in the same or different z/OS operating systems.

Figure 1 shows a database as a shared resource and also illustrates area-level sharing. One or more areas making up the DEDB could be a shared resource. The RECON data set is also shared as a VSAM key-sequenced data set (KSDS), with status being read and updated by a DBRC control portion of the online or batch IMS. For IMS online systems, DBRC executes in a separate region, automatically started by the control region at initialization. For a batch IMS, DBRC control is contained within the batch region, as it is for database-level sharing.

The RECON data set keeps track of the:

  • Sharing level allowed for each database
  • Databases or areas currently authorized for processing
  • IMS systems that are involved
  • Status of all of those systems
  • Database status from a recovery viewpoint
Figure 1. Database-level sharing
Two IMS systems on the same z/OS operating system share data at the database level for full function and at area level for Fast Path. The IMS systems use the same RECON.