DBD
Database description: the IMS definition of a database's structure, its segments, fields and the method used to store it.
Also written database description, DBDGEN, HDAM, HIDAM, PHDAM, PHIDAM
A DBD, or database description, defines an IMS database. It lists the segments and how they fit in the hierarchy, the fields in each segment, which field is the key, and how the database is stored on disk.
The storage method is called the access method, and it is chosen for how the data is used. HDAM finds root segments directly with a calculation on the key, which is fast for looking up one customer. HIDAM uses an index on the root key, which also allows reading roots in key order. PHDAM and PHIDAM are partitioned versions of the same, for very large databases. The data itself is held in VSAM or OSAM datasets.
DBDs are written as source statements and generated with DBDGEN, and then built into control blocks that IMS uses at run time.
Programs never refer to the DBD directly. They see the database through the PCBs in their PSB, which are based on the DBD. Changing a DBD, such as adding a segment, is a database administrator's job and usually means unloading and reloading the database.
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- IMSAn older IBM transaction manager and hierarchical database, still running some of the highest-volume systems in the world.
- IMS segmentThe basic unit of data in an IMS database, arranged in a hierarchy of parents and children.
- PSBProgram specification block: the IMS definition of which databases and segments a program may use, and how.
- DL/IData Language/I: the set of calls a program uses to read and update an IMS database.
- VSAMA family of dataset types built for fast record access by key, used where applications need to look records up rather than read them in order.