Database Program Translation - Information Systems Reengineering, Integration and Normalization - page 181

Path-group EMP-LR is the logical-record name. It enables users to retrieve Em-
ployee, Department, and Office records of the same employee. Database navigation
is done in the subschema, retrieving a sequence Employee, Department, and Office
records. The “Next” statement validates the return code of DML. “0000” indicates
a successful operation. LRF locates an appropriate path by matching the selection
criteria specified in the program request to the selectors specified in the path.

5.3 Emulation

This approach includes auxiliary support software or firmware in the target system
to map source program commands to functionally equivalent commands in the tar-
get system. Each nonrelational DML is substituted by relational DML statements to
access the converted relational database (Housel 1977 ).
The DML emulation strategy preserves the behavior of the application program
by intercepting the individual DML calls at execution time and invoking equiva-
lent DML calls to the restructured database. For example, Computer Associate's
ESCAPE/DL1 (CA 1992c ) translates input-output statements in Information Man-
agement System (IMS—a hierarchical database) to IDMS/R (a relational-like data-
base) DML. An IMS application program can access and update an IDMS/R data-
base through a run-time interpreter.
In the run-time environment of Computer Associate's ESCAPE/DL1 package,
two components are used to translate DL/1 to IDMS/R requests. One component,
the ESCAPE DL/1 Run-time Interface, receives DL/1 requests from the applica-
tion program; it then accesses the IDMS/R database and presents the appropriate
IMS segments to the application program. The other component, the interface pro-
gram specification block (IPSB) compiler, describes the correspondence between
the IDMS/R database structure and the simulated IMS database structure that the
application program will view. The IPSB source contains user-supplied control in-
formation that is compiled by the IPSB compiler; the resulting IPSB load module is
loaded by the ESCAPE/DL1 run-time interface as shown in Fig. 1.4.

5.4 Decompilation

This approach first transforms a program written in a low-level language into an
equivalent but more abstract version and then, based on this abstract representation,
implements new programs to fit the new environment, database files, and DBMS
requirements. Decompilation algorithms have been developed to transform pro-
grams written with the procedural operators of CODASYL DML into programs that
interact with a relational system via a nonprocedural query specification. This is
done through the analysis of the database access path.