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ABCs of z/OS System Programming Volume 3 - IBM Redbooks

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4.37 VSAM: Typical ESDS processing<br />

CI 1<br />

RBA 0<br />

CI 2<br />

RBA 4096<br />

CI 3<br />

RBA 8192<br />

CI 4<br />

RBA 12288<br />

RECORD<br />

1<br />

RECORD<br />

5<br />

RECORD<br />

9<br />

Figure 4-47 Typical ESDS processing (ESDS)<br />

Typical ESDS processing<br />

For an ESDS, two types <strong>of</strong> processing are supported:<br />

► Sequential access (the most common).<br />

► Direct (or random) access requires the program to give the RBA <strong>of</strong> the record.<br />

Skip sequential is not allowed.<br />

RECORD<br />

2<br />

Existing records can never be deleted. If the application wants to delete a record, it must flag<br />

that record as inactive. As far as VSAM is concerned, the record is not deleted. Records can<br />

be updated, but without length change.<br />

ESDS organization is suited for sequential processing with variable records, but in a few read<br />

accesses you need a direct (random) access by key (here using the AIX cluster).<br />

170 <strong>ABCs</strong> <strong>of</strong> z/<strong>OS</strong> <strong>System</strong> <strong>Programming</strong> <strong>Volume</strong> 3<br />

Application program:<br />

GET NEXT<br />

RECORD<br />

6<br />

RECORD<br />

10<br />

RECORD<br />

3<br />

RECORD<br />

7<br />

UNUSED SPACE<br />

RECORD<br />

4<br />

RECORD<br />

8<br />

UNUSED SPACE<br />

UNUSED SPACE<br />

UNUSED<br />

SPACE<br />

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D<br />

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