Choosing replace, insert, and delete rules for logical relationships

You must establish insert, delete, and replace rules when a segment is involved in a logical relationship, because such segments can be updated from two paths: a physical path and a logical path.

The following figure and Figure 2 show example insert, delete, and replace rules. Consider the following questions:

  1. Should the CUSTOMER segment in the following figure be able to be inserted by both its physical and logical paths?
  2. Should the BORROW segment be replaceable using only the physical path, or using both the physical and logical paths?
  3. If the LOANS segment is deleted using its physical path, should it be erased from the database? Or should it be marked as physically deleted but remain accessible using its logical path?
  4. If the logical child segment BORROW or the concatenated segment BORROW/LOANS is deleted from the physical path, should the logical path CUST/CUSTOMER also be automatically deleted? Or should the logical path remain?
Figure 1. Example of the replace, insert, and delete rules
Root: CUSTOMER (PP). Child is BORROW. Logical relationship between BORROW (LC) and LOANS (LP). LOANS has VLC named CUST. BORROW has child PAYMENTS.
Abbreviation
Explanation
PP
Physical parent segment type
LC
Logical child segment type
LP
Logical parent segment type
VLC
Virtual logical child segment type
Figure 2. Example of the replace, insert, and delete rules: before and after
Two paths: Physical path to CUSTOMER and BORROW/LOANS and logical path to LOANS. Physical path to LOANS and logical path to CUSTOMER and BORROW.

The answer to these questions depends on the application. The enforcement of the answer depends on your choosing the correct insert, delete, and replace rules for the logical child, logical parent, and physical parent segments. You must first determine your application processing requirements and then the rules that support those requirements.

For example, the answer to question 1 depends on whether the application requires that a CUSTOMER segment be inserted into the database before accepting the loan. An insert rule of physical (P) on the CUSTOMER segment prohibits insertion of the CUSTOMER segment except by the physical path. An insert rule of virtual (V) allows insertion of the CUSTOMER segment by either the physical or logical path. It probably makes sense for a customer to be checked (past credit, time on current job, and so on.) and the CUSTOMER segment inserted before approving the loan and inserting the BORROW segment. Thus, the insert rule for the CUSTOMER segment should be P to prevent the segment from being inserted logically. (Using the insert rule in this example provides better control of the application.)

Or consider question 3. If it is possible for this loan institution to cancel a type of loan (cancel 10% car loans, for instance, and create 12% car loans) before everyone with a 10% loan has fully paid it, then it is possible for the LOANS segment to be physically deleted and still be accessible from the logical path. This can be done by specifying the delete rule for LOANS as either logical (L) or V, but not as P.

The P delete rule prohibits physically deleting a logical parent segment before all its logical children have been physically deleted. This means the logical path to the logical parent is deleted first.

You need to examine all your application requirements and decide who can insert, delete, and replace segments involved in logical relationships and how those updates should be made (physical path only, or physical and logical path). The insert, delete, and replace rules in the physical DBD and the PROCOPT= parameter in the PCB are the means of control.