IMS DB Characteristics
An IMS database consists of many number of database records.
The size of the database is limited by the constraint of VSAM and DASD.
A maximum of 15 levels are allowed.
A level can be defined as all segments that have the same number of segments above them. The root segment is always level one.
Each immediate child of the root is level two and the child of level two is level three.
IMS allows up to 255 different segment types in single database.
The Database positioning will be set like below.
The sequential processing of an IMS Hierarchy is always top-to-bottom, left-to-right.
IMS keeps track of its place in the database after every DL/I call.
Before the first call, the position is prior to the first root segment.
The current position places just before the segment that DL/I would retrieve next during unrestricted sequential processing.
After any read/ write, the position is after the segment just processed.
The database position needs to be remembered when processing a database sequentially.
IMS moves only forward, no backward.
If the program tries to find the segment already processed, IMS will not find the segment.
IMSDB Database Path IMSDB DL/I Control Blocks