Creating FOCUS Descriptions

In IMS, a PSB regulates an application's authority to access databases. The PSB consists of PCBs, each of which describes one view of one database that the program can access. You must describe your PSBs and PCBs to FOCUS before FOCUS can use them to access an IMS database:

• A FOCPSB describes a PSB (see The FOCPSB).
• A Master File describes a PCB (see The Master File).

Each Master File describes the segments and fields that are accessible through its corresponding PCB. The database made accessible by the PCB can be a physical or logical database.

• Optionally, in the DBCTL environment, an Access File can select the appropriate PSB for a request (see The Access File).

In most environments, the user selects a specific PSB at allocation time. However, if you access IMS through the DBCTL environment, you select the PSB during your session, either by issuing a SET command or by creating an Access File to select the PSB.

As discussed in IMS Overview and Mapping Concepts, IMS database descriptions (DBDs) often do not describe every field in the database. However, your site may maintain a COBOL FD description of the database. If it does, you can use the FOCUS COBOL FD Translator to help create Master Files.

A Master File can describe an entire PCB or any part of the PCB that represents a subtree of the hierarchy starting from the root. (See IMS Overview and Mapping Concepts, for a discussion of hierarchies and subtrees.)

When you issue a request such as TABLE FILE SALES, FOCUS processes the request with the following steps:

1. It locates the Master File named SALES.

For MVS, Master Files are stored in an MVS partitioned dataset (PDS) allocated to ddname MASTER; the SALES Master File is member SALES in this dataset.

For CMS, Master Files are stored as files with filetype MASTER; the SALES Master File is the file named SALES MASTER.

2. It examines the SUFFIX attribute in the Master File. Each Master File that describes an IMS database includes the attribute SUFFIX=IMS. When FOCUS detects this SUFFIX, it passes control to the Interface.

3. The Interface examines the user-selected FOCPSB file to identify the PCB that corresponds to the SALES Master File. It uses the information contained in both descriptions to generate the DL/I calls required by the report request. It passes these calls to the IMS DBMS.

4. The Interface retrieves the data generated by the IMS DBMS and returns control to FOCUS. For some requests, FOCUS may perform additional processing on the returned data.