Updating segments: REPL, DLET, ISRT, and FLD

You can use REPL, DLET, ISRT, and Field (FLD) calls to update a Data entry database (DEDB) or a Main storage database (MSDB). A single FLD call does what a Get Hold call and a REPL call do in two calls. FLD calls are unique to DEDBs and MSDBs.

You can issue REPL, DLET, and ISRT calls to non-terminal-related MSDBs (without terminal-related keys) or DEDBs. You can issue REPL calls to related or nonrelated MSDBs. Before issuing these calls, you must issue a Get Hold call for the segment that you want to update.

Alternatively, you can use a FLD call to access and change field contents within a segment.

The FLD call has two types. FLD/VERIFY compares a target segment field value to the value that you supply in a field search argument (FSA). FLD/CHANGE changes the target segment field value to your specifications in the FSA. A FLD/CHANGE call succeeds only if the previous FLD/VERIFY call succeeds.

FLD calls can simplify some operations. For example, consider how to address the following scenario, which is based on the ACCOUNT segment that is described in Database hierarchy examples. A bank must perform the following process to determine whether a customer can withdraw a requested amount from their account:

1. Retrieve the customer's account segment.
2. Verify that the customer's account balance is greater than the requested withdrawal amount.
3. Update the balance to reflect the withdrawal if the starting balance is greater than the requested withdrawal amount.

If you do not use FLD calls, you must write a program to issue a GU call to retrieve the segment, verify its contents with program logic, and then issue a REPL call to update the balance to reflect the withdrawal.

If you issue FLD calls with a root segment search argument (SSA), you can retrieve, compare, and change the customer segment. Use a FLD/VERIFY call to compare the BALANCE field with the requested withdrawal amount. If the comparison is satisfactory, use a FLD/CHANGE call to update the BALANCE field.

The FLD call SSA has the same format as SSAs for other calls. If no SSA exists, the first segment in the DEDB or MSDB is retrieved.

The segment retrieved by a FLD call is the same as the segment that can be retrieved by a GHU call. After the FLD call, the position is lost. An unqualified GN call after a FLD call returns the next segment in the current area.