PSBs, PCBs, and DBDs in logical relationships
When a logical relationship is used, you must define the physical databases involved in the relationship to IMS by using a physical DBD. In addition, many times you must define the logical structure of the databases to IMS because this is the structure that the application program perceives. This is done using a logical DBD.
A logical DBD is needed because the application program's PCB references a DBD, and the physical DBD does not reflect the logical data structure the application program needs to access. Finally, the application program needs a PSB, consisting of one or more PCBs. The PCB that is used when processing with a logical relationship points to the logical DBD when one has been defined. This PCB indicates which segments in the logical database the application program can process. It also indicates what type of processing the application program can perform on each segment.
Internally, the PSB and PCBs, the logical DBD, and the physical DBD are represented to IMS as control blocks. The following figure shows the relationship between these three control blocks. It assumes that the logical relationship is established between two physical databases.
