Chapter 8. Implementing the IMS Hierarchical Database Model

Chapter 7, "Overview of the IMS Hierarchical Database Model," on page 67 described the logical model for IMS databases. This chapter describes how the hierarchical database model is physically implemented using IMS DB and z/OS services.

For both IMS DB and IMS TM, application programs interface with IMS through functions provided by the IMS DL/I application programming interface (API). IMS access methods and the operating system access methods are used to physically store the data. See Figure 8-1 on page 84. This chapter addresses only the functions that are relevant to IMS DB.

Figure 8-1. Elements of the Physical Storage of Data

The individual elements that make up the database, segments, and database records are organized using different IMS access methods. The choice of access method can influence the functionality available to your application, the order in which data is returned to the application, and the performance the application receives from IMS DB.

Underlying the IMS access methods, IMS uses VSAM or OSAM to store the data on DASD and move the data between the DASD and the buffers in the IMS address space, where the data is manipulated.

The structure of the IMS databases, and a program's access to them, is defined by a set of IMS control blocks:

• The database description block (DDB)

• The program specification block (PSB)

• The application control block (ACB)

These control blocks are coded as sets of source statements that are then generated into control blocks for use by IMS DB and the application.

In This Chapter:

• "Segments, Records, and Pointers"

• "IMS Hierarchical Access Methods" on page 87

• "Operating System Access Methods" on page 109

• "IMS Checkpoints" on page 111

• "Locking Data" on page 114