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A TOP-DOWN
SOFTWARE REUSE
SUPPORT ENVIRONMENT

Sukesh Patel, William Chu, Rich Baxter, Brian Sayrs, and Steve Sherman

Lockheed Software Technology Center
Lockheed Palo Alto Research Laboratories
3251 Hanover Street, B/254E, O/96-10
Palo Alto, CA. 94304-1191
[e-mail]

ABSTRACT

The true potential of software reuse lies in providing facilities for reusing
large scale software systems. Given appropriate tool support software engineers
can leverage existing software investments by recovering the design of the
software and then systematically partitioning and extracting reusable
components from existing systems. It is also suggested that reusable components
must be subjected to a certification process prior to inclusion in a reuse
library. Moreover, simply retrieving large scale reusable components from a
library is not enough and appropriate software understanding tools must be
provided to permit interactive component understanding and selection.

Keywords: Reuse, Certification, Reverse Engineering, Design Recovery,
Restructuring.

        INTRODUCTION

The potential benefits of Software Reuse has been extensively reported, both in
commercial and defense publications and in conferences world wide. [2,5,14].
Libraries of mathematical functions (e.g., the IMSL library collection of
mathematical routines [6]) and Booch components [4] are instances of successful
(albeit limited) reuse programs. This is what we term bottom-up reuse: the
development of compact general purpose software components that can be combined
to synthesize a class of applications.

We believe that the true potential of software reuse is in exploiting the reuse
of very large scale software systems[2]. Although one would like reuse to
inform the complete software life-cycle, many times such investments are cost
prohibitive and software engineers must turn to existing systems/components for
reuse opportunities. Disparagingly, such reuse has been termed code scavenging.
But given appropriate tools, our belief is that incremental component
identification and improvement supports evolutionary development and is a cost
effective means of leveraging existing software investments. Large scale reuse
requires top-down reuse: the development of tools and techniques to understand,
certify, extract, and restructure components from existing software systems.
Our research suggests that solutions to the following problems must be pursued
to facilitate top-down reuse(1):

    Reusable Component Extraction: Tools and techniques to understand and
extract components from existing software systems.

    Reusability Certification Procedures/Metrics: A set of documentation,
coding, quality and reusability checks that instill confidence in reusable
software.

    Component Location and Composition Support: Processes and tools for
designing new applications by glueing together reusable and newly developed
software components.

We believe that (1) and (2) are critical technologies for leveraging existing
software investments. Issue (3) provides a framework to put pieces of the
puzzle together, and although important, must be established in the context of
particular applications and will not be addressed here in any depth.  This
paper is organized as follows. We cover our key technology thrusts in Sections
2 (software re-engineering and restructuring) and 3 (reuse certification).
Section 4 illustrates our overall top-down reuse support environment and
provides a context for our technology thrusts. Section 5 summarizes our
progress.

        SOFTWARE RE-ENGINEERING & RESTRUCTURING

Our first key technology thrust is software re-engineering and restructuring
[1]. Software re-engineering and software restructuring are techniques that
facilitate the process of design recovery [3]. Design recovery attempts to
create design concepts and relationships based on analyzing source code and
available software artifacts. Our approach recognizes that full scale
automation for design recovery is difficult and instead supports tool
development that emphasizes automation of ``routine'' chores like system
partitioning and fact finding tasks. The principle steps that have been
implemented are:

    Partitioning systems based on minimizing the coupling relationships.

    Extracting ADTs that act as communication links between independent
components.

    Exploiting the visibility relation to yield sets of independent
components (If A and B both, call C and C does not contain state information,
then copies of C can be made to yield two components {{A,C},{B,C}}. If C
contains recognizable traits for state information, then {{A,B,C}} must be
treated as a primitive component).

    Exploiting cohesion properties to identify misplaced components (e.g.,
a integer add operation in a STACK package can be identified and moved to some
INTEGER-OPS utilities package that can be with'ed)

Ultimately, isolation of independent components facilitates complexity
management and permits reuse/maintenance personnel to acquire system knowledge
on a piecemeal basis. This in turn provides knowledge of past systems that can
be used to supplement domain analysis to define application architectures.

In summary software re-engineering and restructuring technology provide the
following services for top-down reuse:

    Software Understanding: Large scale software is selectively displayed
and understood.

    Software Selection: Independent components can be recognized and
extracted and reused in different contexts.

    Software Translation: Existing software must be super-imposed with
different structural views (e.g., a system in C must be partitioned into CSCIs,
CSCs, CSUs, ADTs etc.) before the system can be syntactically converted to
Ada(2).

    Software Generalization: Software components comprised of independent
sub-components can be recognized and generalized by domain experts.

        REUSE CERTIFICATION

Reuse certification seeks to extract domain specific characteristics of
software components at all levels of abstractions that help to facilitate
reuse. In general, reusable software is quality software. Our strategy for
certification has been to survey characteristics of software components that
encourage reuse and then attempt to provide automated techniques for ensuring
that reusable software complies with or displays the required characteristics.
In cases where automated detection is ineffective or cost-prohibitive,
check-lists must be designed and manual procedures must be outlined as part of
a reusable software inspection process. For example, semantic checks can be
developed to recognize incomplete or inconsistent components.

Aspects of the certification process that can be automated are:

    Documentation Standards. Documentation prologues and certain
consistency checks on documentation (e.g., checks can be automated to ensure
that a module's call list is complete (all calls are mentioned) and consistent
(mentioned names are called somewhere in the program body)). Enforcement of
Variable and Type naming standards etc..

    Identification of system dependent features like primitive type
representations and the use of primitive I/O packages.

    Path analysis to determine if functions and procedures return valid
results on all logical paths.

    Measurable system quality indicators (cohesion, coupling, and
complexity)(3).

Notice that the above checks not only facilitate software reuse but are also
required to ensure quality software. Our research indicates that there are two
facets of reusable software: (a) a quality facet that all software should
satisfy (simple interface, minimal complexity, minimal testing complexity,
etc.); and (b) a domain appropriateness facet that determines to a large extent
whether a piece of software can be used in several applications (e.g., a
generalized stack is such a component). The later facet is much more difficult
to recognize and evaluate. Issues here include:

    Component Granularity: Is this component a valid primitive in the
domain, or does the component require further decomposition into primitive
operations and primitive data elements?

    Component Naming Conventions: Given a potentially reusable component,
are the objects and operations named with semantically meaningfully taxonomic
terms?

    Component Flexibility: Are the objects and operations sufficient to
synthesize required components in many situations?

    Component Generality: Is the component applicable to various problem
situations?

In view of the difficulty of recognizing the domain specific attributes of
reusable components we have initiated a research project for circumscribing the
domain appropriateness facet. More specifically, we have initiated a pilot
project whereby software engineers and researchers will interact with experts
in particular domains and establish a set of enforceable guidelines that can
help the process of identifying reusable software as (i) either one of the
output of domain analysis, or (ii) extracting such components from existing
systems. Certain progress has been made for (ii) at the Software Productivity
Consortium [12] and we shall extend both the scope and applicability of this
research.

        OVERALL APPROACH

Fig. 1 depicts the support necessary for depositing components into a reuse
repository. As shown in Fig. 1 reusable components are generated from two
primary sources: (1) the extraction and restructuring of components from
existing systems written in conventional imperative languages (C, Fortran, and
Ada), and (2) the development of domain specific components given broad
knowledge of intended applications.

Given a collection of reusable components, we must provide for some means to
organize these components in a user friendly manner. Software components can be
organized as appropriate to the intended applications: (a) a large set of
independent domain specific components for libraries, and (b) organization
along application architectures for narrow domains. Library organization is
common and is not discussed further.

Fig. 2 displays our vision of structures to support the reuse of existing
components for narrow domains.  This vision follows the design memory metaphor
outlined in [7,8] and seeks to provide designers a platform for visualizing and
inferring the consequences of design decisions. A user typically enters the
reuse environment and is given a top level view of a typical application
architecture. The architectures displays the principle top-level architectural
components, their associated information flows and relationships, and the
top-level storage structures. Components that be further decomposed into
sub-components are shaded. Such components have sub-components that may be
organized and controlled to perform the task required of the higher level
component. Components that are hatched indicate that there is no available
executable image associated with the module(4). This can occur when a component
is functionally coupled [13] to environmental objects that are likely to
change; for example a set of routines that read radar sensors and organize the
data in specific formats.

For existing software systems without an associated knowledge base, the system
hierarchy is generated by reverse engineering tools that act as selective
display mechanisms. Here the system provides little inferencing ability but
allows a user to traverse the system and select components of various
granularity.

        CONCLUSIONS

In this paper we have illustrated a promising architecture for solving problems
associated with large scale reuse. Currently, we are tackling both angles of
our overall approach (Figs. 1 and 2). We are in the process of developing a
prototype system that can accomplish the objectives of Fig. 2 for the target
tracker domain. Here our intent is to provide knowledge based support for
enhancing reuse opportunity. We have made substantial strides in re-engineering
and restructuring technology. We have applied our techniques to large software
systems in C and Ada with encouraging results. Currently, we are actively
involved in the following: (1) refining our re-engineering and restructuring
capabilities and expanding the scope of our techniques by experimenting with
other imperative languages like COBOL and FORTRAN, and (2) establishing
cooperative efforts to flesh out reuse certification procedures.

        REFERENCES

 R.S. Arnold (Ed.), Tutorial: On Software Restructuring, IEEE Press, Washington
D.C., 1986.

 T. J. Biggerstaff, & A. J. Perlis (Eds.), Software Reusability Volume 1:
Concepts and Models, and Volume 2: Applications and Experience, ACM Press,
Addison-Wesley Publishing Company, New York, NY, 1989.

 Biggerstaff, T. J., ``Design Recovery for Maintenance and Reuse'', IEEE
Computer, July 1989, pp. 36-49.

 G. Booch, Software Components with Ada: Structures, Tools, and Subsystems, The
Benjamin/Cummings Publishing Company, Inc., Menlo Park, CA., 1987.

 P. Freeman (Ed.), Tutorial: Software Reusability, IEEE Press, Washington D.C.,
1987.

 IMSL Inc., Library Reference Manual, Huston TX, 1987.

 Mark, W., ``Software Design Memory'', Proc. Workshop Notes AAAI-91: Automating
Software Design: Interactive Design, AAAI, Anaheim, CA., July 15, 1991, pp.
115-120.

 Mark, W., & Schlossberg, J., ``Interactive Acquisition of Design Decisions'',
Proc. 5th Knowledge Knowledge Acquisition for Knowledge-Based Systems, Banff,
Canada, Nov. 1990.

 Neighbors, J.M., ``The Draco Approach to Constructing Software from Reusable
Components'', In P. Freeman (Ed.), Tutorial: Software Reusability, IEEE Press,
Washington D.C., 1987, pp. 181-191.

 Patel, S. J., Chu, W., and Baxter, R. W., ``A Measure for Composite Module
Cohesion'', Submitted, Proc. 14-th Intl. Conf. Soft. Engr., Melbourn,
Austrilia, 1992.

 Prieto-Diaz, R., ``Domain Analysis for Reusability'', InW. Tracz (Ed.),
Tutorial: Software Reuse: Emerging Technology, IEEE Press, Washington D.C.,
1988, pp. 347-353.

 Software Productivity Consortium (Author: R.S. Arnold), ``Heuristics for
Salvaging Reusable Parts from Ada Source Code'', SPC Tech. Report: Ada Reuse
Heuristics-90011-N, Version 01.00.00, March 1990.

 Stevens, W. P., Myers, G. J., & Constantine, L.L., ``Structured Design'', IBM
Systems Journal, Vol. 13, No. 2, 1974, pp. 115-139.

 W. Tracz (Ed.), Tutorial: Software Reuse: Emerging Technology, IEEE Press,
Washington D.C., 1988.

 Tracz, W., ``Software Reuse Myths'' in W. Tracz (Ed.),  Tutorial: Software
Reuse: Emerging Technology, IEEE Press, Washington D.C., 1988, pp. 18-22.

NOTES

(1) As noted in Tracz [15], management and economic reasons can also hinder
software reuse. However, in the following we assume the existence of
appropriate management structures that encourage and reward software reuse.

(2) Ada is a registered trademark of the U.S. Government (Ada Joint Program
Office).

(3) We have successfully developed a metric for module cohesion [10]. We have
also established some criteria that  lead to undesiriable coupling and are in
the process of proposing a measure for module coupling.

(4) Note that it is not necessary to always have a executable object file of
the component. Rather the component may itself be a application generator or
some expert system that can synthesize the required executable image.

Dr. Sukesh Patel is the  Principal Investigator for the Software Reuse and
Maintenance project at Lockheed's Software Technology Center (STC). Sukesh
received his B.S. in Electrical Engineering from Sadar Patel University (India)
and his M.S. and Ph.D. degrees in Computer Engineering from the Center for
Advanced Computer Studies at the University of Southwestern Louisiana. His
primary interests are design automation, design recovery, and the development
of evaluation techniques for large scale hardware and software systems. Sukesh
is a member of the IEEE and AAAI societies.

Dr. William Chu received his B.S. degree from the Tamkung University (Taipei,
Taiwan) in 1982 and his M.S. and Ph.D. degrees from Northwestern University,
Evaston, Illinois in 1987 and 1989, respectively, all in computer science. He
is currently a research scientist at the Software Technology Center of Lockheed
Missiles and Space Company, Palo Alto, California. His current interests are
software maintenance and reuse, with special focus on software re-engineering
and restructuring, software engineering, and software quality.

Mr. Rich Baxter received his M.S. degree in Computer Science in 1985 from the
New Mexico Institute of Mining and Technology (NMIMT). Mr Baxter joined
Lockheed Missiles and Space, Co. (LMSC) Research and Development Division's
Software Technology Center's (STC) Ada Technology group as a Scientific
Programmer/Analyst in 1988. Mr. Baxter is participating in several Ada related
internal research and development (IRAD) projects in the STC including the
Software Reuse in Large Ada Systems project. As a member of the Ada technology
group, Mr. Baxter is actively participating in the group's infusion of Ada
technology into LMSC and other Lockheed companies. Mr. Baxter has co-authored
several papers dealing with Ada Software topics while at Lockheed. Mr. Baxter
has been a member of the ACM since 1981 and of the IEEE Computer society since
1989.

Mr. Brian Sayrs is the principal investigator for Software Re-engineering at
the Software Technology Center (STC). Brian has been involved with several
software re-engineering projects and is currently developing methods and
techniques for integrating forward and reverse engineering.

Dr. Steve Sherman is manager of the Software Technology Center (STC), Research
and Development Division, Lockheed Missiles and Space Company (LMSC). Dr.
Sherman received his B.S. in Mathematics from North Carolina State University.
After working for Pratt & Whitney in West Palm Beach, FL, Dr. Sherman joined
the University of Wisconsin where he received his M.S. in Computer Science in
1968. Dr. Sherman worked for Sandia Corporation in the systems and nuclear
simulation groups in Albuquerque and subsequently received his Ph.D. in
Computer Science from the University of Texas at Austin in 1972. His
dissertation addressed the simulation of operating system heuristics for
performance prediction.

Dr. Sherman taught at the University of Houston for 2 years in the Computer
Science Department and left on leave in 1974 to work for the Institute for
Computer Applications in Engineering at NASA Langley in Hampton, VA where he
conducted research in data base management. In 1976, Dr. Sherman joined the
University of Nevada at Las Vegas as a faculty member in the Computer Science
department and was promoted to full professor in 1984.In 1985, Dr. Sherman
joined LMSC working for the STC in Austin, Texas where he led the technology
transfer group and the Ada environment project. In 1987, STC teamed with the
Space Systems Division (SSD) of LMSC to merge the Ada environment with SSDs
language independent software engineering environment. In 1988 Dr. Sherman led
his group to support a Lockheed Space Operations Company proposal and training
effort. Dr. Sherman led the Software Automation research group in Palo Alto and
his current group, also in Palo Alto, is conducting research in software
engineering.