DL/I and CA IDMS/DB › Physical Access Methods

Physical Access Methods

What Physical Access Methods Do

Physical access methods determine the physical organization and available access paths for DL/I databases. Each physical DBD must be assigned an access method, which is specified for the ACCESS parameter in the DBD statement.

Sequential and Direct Access Methods

DL/I provides two general access methods: sequential and direct. The sequential method lays out the segment occurrences as physically contiguous, like records in a tape file. The direct method provides random access via pointers to segment occurrences, like records on a direct access storage device (disk). Each method is further qualified on the basis of whether or not it supports indexing.

DL/I Supports Four Access Methods

The combination of sequential/direct and indexing/no indexing yields the following four access methods for DL/I:

• HSAM──Hierarchical sequential access method
• HISAM──Hierarchical indexed sequential access method
• HDAM──Hierarchical direct access method
• HIDAM──Hierarchical indexed direct access method

Note that all four access methods are hierarchical (H). This reflects the fact that an application always views a database as hierarchical, regardless of the access method used or the physical location of the data.