Orgnizations & Access methods

There are several types of IMS databases which can be defined by the DBA. Each type has its own access method to perform operation on it. The access method determines the characteristics of the database. There are nine types of databases supported by IMS. Those databases can be grouped by their IMS access method.

1. Hierarchic sequential accessed databases.
2. Hierarchic Direct access databases.

Hierarchic sequential accessed databases -

Hierarchical sequential accessed databases are based on sequential storage and access of database segments. Hierarchic sequential databases are of simpler organization.

The root and dependent segments of a record are stored in physical sequential order. Access to dependent segments is always sequential from the root segment.

Direct deletions of segments are not possible. Deleted dependent segments are not physically removed but marked as deleted. Hierarchic sequential databases can be stored on tape or DASD. Hierarchic sequentially accessed databases include.

• HSAM
• HISAM
• SHSAM
• SHISAM
• GSAM

Hierarchic sequential access methods can be explained in detail here .

Hierarchic direct databases -

The hierarchic direct databases developed to resolve some of the problems of using sequential access databases. HD databases used pointers to relate segments.

Deleted segments will be physically removed from database. VSAM ESDS or OSAM data sets are used for storage. HD datasets can be stored on DASD.Pointers are four-byte address values that provide the offset from the beginning of the data set to the segment being addressed.

The segments can be tied together without the need for segments to be physically adjacent. Segments can be inserted or deleted without the path change of other segments. HD datasets can be used the below two types of access methods.

• HDAM
• HIDAM