Return Records in Control Interval Format

Learn to return records in control interval format with Datacom VSAM Transparency.
Datacom VSAM Transparency
's sequential read-only support for control interval processing is designed to accommodate batch applications such as those that sort a file to create an extract or produce a report. Such support enables you to execute VSAM applications issuing control interval reads to files converted to
Datacom/DB
. Unlike access using native VSAM, access by using the Transparency is strictly sequential in ascending key-value order of the records from one control interval to the next. Control interval support extends to spanned records (records which are too long to fit within a single control interval).
We recommend that you request blocked GETIT processing for files using Control Interval access.

Prerequisites

With the parameter
CISIZE=
provided with the VSAM Interface Table macros, you specify the size of the control interval. Control interval access is initialized for a file that is defined with CISIZE > 0 under either of the following conditions:
  • If the first VSAM action request after OPEN of the ACB is a GET request with the following options: SEQ, CNV, FWD
  • When a POINT request is issued with option CNV and an RBA-value of zero specified in the RPL
For more information about
CSIZE=
, see Code the DBVUFIL Macro.

Processing Description

When a read request is issued,
Datacom VSAM Transparency
builds the control interval image with all the
Datacom/DB
records that fit into the defined area, creates the RDFs (Record Definition Fields) and CIDF (Control Interval Definition Field) accordingly, and then returns the control interval to the application program. The Control Interval Definition Field (CIDF) describes the location and length of the unused space in the control interval. The Record Definition Fields (RDFs) describe the records.
Any request resulting in an RPL-feedback code other than zero or EOF terminates CI-access. To reestablish control interval access after it has been so terminated, issue a POINT request with option CNV and an RBA of
zero
. Control interval processing then restarts with the first control interval of the file.
You cannot reestablish access with an RBA greater than zero, that is, you cannot access or position to Control Intervals directly using an RBA.