Types of IMS databases

IMS™ allows you to define many different database types. You define the database type that best suits your application's processing requirements.

You need to know that each IMS database has its own access method, because IMS runs under control of the z/OS® operating system. The operating system does not know what a segment is because it processes logical records, not segments. IMS access methods therefore manipulate segments in a database record. When a logical record needs to be read, operating system access methods (or IMS) are used.

The following table lists the IMS database types you can define, the IMS access methods they use and the operating system access methods you can use with them. Although each type of database varies slightly in its access method, they all use database records.

Table 1. Types of IMS databases and their z/OS access methods
Type of IMS database	Full name of database type	IMS or operating system access methods that can be used
DEDB ¹	Data Entry Database	Media Manager
GSAM	Generalized Sequential Access Method	QSAM/BSAM or VSAM
HDAM	Hierarchical Direct Access Method	VSAM or OSAM
HIDAM	Hierarchical Indexed Direct Access Method	VSAM or OSAM
HISAM	Hierarchical Indexed Sequential Access Method	VSAM
HSAM	Hierarchical Sequential Access Method	BSAM or QSAM
MSDB ²	Main Storage Database	N/A
PHDAM	Partitioned Hierarchical Direct Access Method	VSAM or OSAM
PHIDAM	Partitioned Hierarchical Indexed Direct Access Method	VSAM or OSAM
PSINDEX	Partitioned Secondary Index	VSAM
SHSAM	Simple Hierarchical Sequential Access Method	BSAM or QSAM
SHISAM	Simple Hierarchical Indexed Sequential Access Method	VSAM

Table notes:

For DBCTL, available only to BMPs
Not applicable to DBCTL

The databases listed in the above table are divided into two categories: Full-function database types and Fast Path database types. DEDB and MSDB are the only two Fast Path database types. All other databases in the above table are considered full-function database types.