CASE STUDY
`
`EXPERT SYSTEMS FOR CONFIGURATION
`
`AT DIGITAL: XCON AND BEYOND
`
`Members of Digital Equipment Corporation’s team of expert system experts
`reflect and recount a decade’s worth of lessons learned in designing and
`building a core of configuration systems
`
`VIRGINIA E. BARKER and DENNIS E. O’CONNOR
`
`The XCON configuration system at Digital Equipment
`Corporation was the first expert system in daily pro—
`duction use in industry [2, 10, 11]. It is the cornerstone
`of Digital’s knowledge network vision: a number of ex-
`pert systems embedded in both the company’s order
`process cycle and its new product introduction cycle
`[4]. Today, Digital is continuing to extend the knowl-
`edge network, as well as using expert systems technol-
`ogy in many additional aspects of the company’s busi-
`ness. In fact, XCON is only one of several expert
`systems dealing with hardware and software config-
`uration which are currently in use or under develop-
`ment at Digital. The configuration systems “family”
`includes four expert systems in production use. Several
`additional configuration expert systems are in the
`research, advanced development, or prototype stage of
`development.
`The development of these expert systems has
`spanned nearly 10 years; Digital’s Configuration Sys-
`tems Development Group [CSDG] has thereby gained
`considerable understanding of all phases of the life-
`cycle of production quality eXpert systems: design, de-
`velopment, production, and ongoing support. In this ar—
`ticle we highlight some of the key lessons we have
`learned:
`
`0 Building a successful expert system involves much
`more than simply putting rules into a knowledge
`base. Rather, to successfully develop and provide
`ongoing support for expert systems and to integrate
`them into the fabric of one’s business, as has been
`done at Digital, one must attend to the needs of the
`business and to human resource and organizational
`issues as well as to technical issues.
`
`0 Although many software engineering practices
`carry over to the engineering of expert systems, we
`
`© 1939 ACM 0001~0782/89/0300-0298 $1.50
`
`have nonetheless needed to be creative in develop-
`ing practices specifically tuned to this relatively
`new, and still evolving, technology.
`
`While the accompanying article by Bachant and Solo-
`way focuses specifically on the technical issues under-
`lying XCON and the other configuration systems, we
`take a more holistic, integrative approach in this article
`and attempt to put the aforementioned concerns into a
`coherent perspective.
`
`CONFIGURATION SYSTEMS AT DIGITAL TODAY
`
`The Digital Configuration Systems Timeline (p. 302]
`shows, for each of the configuration systems, key stage-
`of—life milestones, including initial production use
`where appropriate.
`XCON is used to validate the technical correctness
`
`(configurability) of customer orders and to guide the
`actual assembly of these orders. It provides the follow-
`ing functionality:
`
`0 Configures CPUS, memory, boxes, backplanes, cab—
`inets, and power supplies, disks, tapes, HSC /CI,
`printers, etc.
`0 Diagrams complete system configuration (Figure 1
`overlays selected pages of a sample XCON diagram]
`0 Checks marketing restrictions, system building
`block menus, and prerequisites
`0 Assigns addresses/vectors and determines box
`power status
`0 Partitions multiple-cpu orders and cluster systems
`0 Determines and lists cabling information
`0 Lists components ordered with configuration-
`related comments
`
`0 Generates warning messages on issues affecting
`technical validity.
`
`XSEL [7, 9] is used interactively to assist in the selec-
`tion of saleable parts which make up a customer order.
`
`Communications of the ACM
`
`March 1989 Volume 32 Number 3
`
`CONFIGIT 1044
`
`CONFIGIT 1044
`
`1
`
`

`

`Case Study
`
`It provides the following functionality:
`
`I Allows interactive selection by generic component
`name, partial or full model number
`0 Performs completeness checking, adding and sug-
`gesting required parts
`0 Checks software compatibility, prerequisites, 1i-
`cense and media completeness
`0 Checks standard and system building block menus,
`marketing and engineering restrictions
`0 Provides computer room environmental data and
`requirements
`0 Links to Automated Quotation System.
`
`XFL is used to diagram a computer room floor layout
`for the configuration(s) under consideration. It provides
`the following functionality:
`
`0 Provides “minimum footprint” floor layout of com-
`ponents
`0 Allows custom rooms (user-specified dimensions
`and placement)
`0 Can include several configurations or a cluster in
`one site layout.
`
`XCLUSTER is used to assist in configuring clusters. It
`provides the following functionality:
`
`0 Clusters multiple—node configurations for validation
`0 Specifies device quantity for dual porting
`0 Upgrades appropriate clusterable tapes.
`
`In addition to these four systems currently in produc-
`tion use, two other configuration systems are under
`development:
`XNET is an expert system which will be used to
`design local area networks, to select appropriate compo-
`nents for such networks, and to validate the technical
`correctness of the resultant network configurations.
`SIZER is a research effort addressing the need for
`tools to assist in the sizing of computing resources re-
`quired for any of a wide variety of uses in various types
`of organizations.
`To support the ongoing development of the configura-
`tion systems in production use and to enable us to more
`effectively build new configuration systems, we have
`developed a software engineering methodology, called
`RIME [1, 12, 13], expressly for expert systems. Because
`RIME provides substantial insight into the engineering
`issues that require particular attention in building ex-
`pert systems, it is discussed at some length in “The
`Engineering of XCON.”
`
`SCOPE AND USAGE
`
`The configuration systems provide full product cover-
`age for Digital’s current product set. This product set
`today consists of 42 different families of central proces-
`sor types and their supporting peripherals and software.
`In order to be useful business tools, released versions of
`these systems must include configuration knowledge
`of Digital’s newest products by the time of product
`announcement. In practice, this means that CSDG
`provides major releases of these systems once each
`
`quarter, with at least one interim upgrade to insure
`adherence to the time-of—announcement requirement.
`Hardware and software configuration is at the core of
`Digital’s business. The configuration systems are used
`worldwide, throughout the corporation, by a broad set
`of users across the company’s major functions: sales,
`manufacturing, field service, and engineering. The
`users of these systems perform functions which span
`Digital’s complete order flow and manufacturing cycle,
`and, thus, are involved with many different business
`processes. This is a large and varied constituency to
`support—each has different needs and takes a different
`perspective on the configuration information provided:
`
`0 Sales uses the configuration systems as an integral
`part of the automated process to generate quota-
`tions for customers, and to insure that every order
`is technically valid.
`0 Manufacturing uses the information to verify build-
`ability of all incoming orders, to understand phys-
`ical partitioning of an order into various sub-
`assemblies to determine which plants should build
`which segments of an order, to guide the assembly
`of all orders, to determine the optimal set of diag-
`nostics to run on each order.
`
`0 Field service has the perspective of consolidation
`and assembly of the order in the customer’s unique
`environment and possibly with existing equipment
`already installed.
`0 Manufacturing and engineering benefit from the
`configuration systems’ focus on system integration,
`as analysis of product knowledge for inclusion in
`the configuration systems identifies potential prob-
`lems in system-level design and manufacturability.
`
`This user profile has expanded dramatically over the
`years (see timeline). For example, the initial purpose of
`XCON was to assist manufacturing plant personnel in
`validating the technical correctness of system orders
`about to be built. Since then, this technical validation
`function has changed in response to business needs and
`is now performed in the field as well. In addition, the
`technicians in the plants who build the computers now
`use XCON diagrams to see how to put the systems
`together, and they include those diagrams in the ship-
`ment to the customer for use by field service installers
`at the customer site. XSEL was originally designed for
`use by sales representatives, and is now used by our
`OEM customers as well. Implementation of XNET will
`add specialized field support personnel to the user list
`in the near future. There are additional “indirect” users
`
`of these systems through automated linkages to other
`software systems [both traditional and expert systems)
`which depend on the configuration information
`supplied.
`The configuration systems user base not only repre-
`sents over 10 distinct business functions, but is also
`spread across the world, and has varied geography—
`specific needs. Overall, CSDG supports over 50 produc-
`tion installation sites, and the number is growing. In
`sum, then, the configuration systems are firmly em-
`
`March 1989 Volume 32 Numbers
`
`Communications of the ACM
`
`299
`
`2
`
`

`

`Case Study
`
`
`
`III| I
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`RUN DATE/TIME:
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`
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`OMPONENTS ORDERED
`
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`
`FIGURE 1. Selected Pages from a Sample XCON Diagram
`
`3
`
`

`

`Case Study
`
`bedded in Digital's most critical business processes. The
`existence of these expert systems has significantly im-
`proved these processes and continues to provide insight
`into opportunities for the future.
`
`BENEFITS
`
`Only a modest amount of attention has been given to
`documenting the benefits of applying expert systems
`technology [3, 8]. The configuration systems are a suc-
`cess and there is a major dependency on them within
`the corporation, worldwide. They benefit Digital in a
`number of ways, contributing to customer satisfaction,
`lower costs, and higher productivity. These systems
`are recognized as a critical factor in Digital’s ability to
`maintain its highly successful a la carte, build-to-order
`marketing strategy (customized configurations to fit
`each customer’s specific needs]; this is one of the com-
`pany’s key competitive advantages. Some of the bene-
`fits are difficult to quantify, but overall the net return
`to Digital is estimated to be in excess of $40 million per
`year.
`The use of the configuration systems insures that
`complete, consistently configured systems are shipped
`to the customer. Incomplete orders do not get through
`the process. In addition, XCON generates configurations
`which optimize system performance, so customers con-
`sistently get the best View of our products. Before the
`configuration systems, we would often ship the same
`parts configured differently. (There are multiple ways
`to configure the same set of parts to create a working
`system.) This was a major source of customer com-
`plaints and confusion, especially for OEM’s who order
`large numbers of the same system and in turn recon-
`figure them all in the same way as part of their market-
`specific value added process.
`The process of new product introduction is enhanced
`greatly by the focus on configuration information pro-
`vided by the configuration systems. The existence of a
`single source of configuration information by the time
`of product announcement for use on initial customer
`orders simplifies field and manufacturing training
`needs and avoids confusion about new products which
`can delay time-to-market significantly. This is of criti-
`cal importance given the volume of new products Digi—
`tal continues to introduce each year. This single source
`of configuration information also increases manufactur-
`ing’s flexibility by enabling product manufacture (and
`the knowledge of how it is done) to be moved from one
`plant to another without costly training or disruptive
`re-assignment of people. The use of XCON has facili-
`tated this re-alignment of manufacturing capacity sev-
`eral times.
`
`The use of the configuration systems has significantly
`increased the technical accuracy of orders entering
`manufacturing. Straightening out problem orders is a
`costly and time-consuming activity which disrupts the
`normal processes and increases order cycle time (i.e.,
`delays order shipment). Overall, the additional disci-
`pline of both field and manufacturing use of these sys-
`
`terns has shortened cycle times, contributed to more
`flexible, smoother-running processes, and lowered the
`number of people who would otherwise be needed for a
`given volume of orders.
`XCON is seen as a critical component of Digital’s
`current process of shipping segments of an order from
`various component plants, to consolidation points, and
`from there to the customer site. The use of XCON
`
`throughout the manufacturing process assures that
`when the components of the order come together for
`the first time at the customer site the system will work.
`Before the use of XCON, there were special manufac-
`turing plants where every customer order was com-
`pletely assembled, tested, disassembled, and repacked
`prior to shipping in order to insure the system would
`work when installed at the customer site. The elimina-
`
`tion of this step in the manufacturing process (called
`Final Assembly and Test) has had a major positive im-
`pact on cycle times, inventory levels, and manufactur-
`ing costs, especially in light of Digital’s four-fold in-
`crease in systems product volume since 1980.
`Thus, the configuration systems are key in Digital’s
`ability to handle the product complexity that technol-
`ogy advances are enabling and that our customers are
`demanding.
`
`THE CONFIGURATION SYSTEMS EXPERIENCE
`
`At Digital, we believe that our success in the applica-
`tion of expert systems technology is attributable to a
`conscious recognition of and careful balance among
`three important perspectives: strategic/business, tech-
`nical, and human resource/organizational.
`
`STRATEGIC/BUSINESS ISSUES
`Digital has reaped and continues to reap enormous ben-
`efits from the configuration systems. It was, of course,
`an experimental effort at first but it was nurtured and
`allowed to progress and turned out to be a resounding
`success. Now Digital considers its use of expert systems
`technology a strategic investment.
`What are the characteristics of an appropriate busi-
`ness problem for this type of strategic investment in a
`new technology? The problem must be real and sys-
`temic to the enterprise to justify the right kind of nur-
`turing and supportive business environment. The solu-
`tion will probably impact multiple organizations or
`functions and, hopefully, will bring about significant
`and positive changes in the way the enterprise oper-
`ates. The configuration problem at Digital certainly
`qualifies. Digital’s strategy of selling customized solu-
`tions is one of its critical competitive advantages, and
`gives rise to an indefinite number of valid configura-
`tions of its product set. In the 19705, problems with
`handling this were starting to surface, and it was
`understood that automated support would be critically
`important as sales volumes increased and products
`became more varied and complex.
`One of the main challenges we have encountered
`in developing systems which support strategic, cross-
`
`March 1989 Volume 32 Number 3
`
`Communications of the ACM
`
`301
`
`4
`
`

`

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`

`

`Case Study
`
`functional business needs, is sorting out how the tech-
`nologists connect with the business. As with any soft—
`ware development effort, it is necessary to form a solid
`partnership with the business being supported. Usually,
`that means with the end users and their immediate,
`operational management. In the case of strategic, cross-
`functional systems, we have discovered that the devel-
`opment effort must also be tightly connected at a more
`strategic level of management. This is simply a reflec-
`tion of the fact that the problem being addressed has
`strategic impact and the solution may include and/or
`provide opportunities for far-reaching changes in the
`business [6] of which the tactically focused end user
`groups may be unaware.
`CSDG is tapped into business planning at several lev-
`els to insure adequate awareness and connectivity and
`support. This has been and continues to be of critical
`importance to the ongoing success of the configuration
`effort at Digital. Much of the strategic connectivity over
`the years has been informal for the configuration sys-
`tems effort: based on personal relationships with key
`individuals. Recently, we have initiated some efforts to
`formalize some of these processes. For example, we
`have set up a Configuration Systems Steering Commit-
`tee made up of strategically focused managers repre—
`senting our various business constituencies. This forum
`provides guidance on planning and priority-setting for
`the configuration systems in the context of a cross-
`functional vision looking out several years. In addition,
`a more formal corporate-level focus on system integra-
`tion issues is evolving, which should provide formal
`long-term direction.
`One of the most important factors in our success has
`been the existence of an enabling business /develop-
`ment environment for the introduction of new technol-
`
`ogies like expert systems. During the early 1980s, Digi-
`tal’s manufacturing operations were open to embracing
`new ideas and technologies that would help them
`change the way work was done, affording higher pro—
`ductivity and shorter cycle times in response to cus-
`tomer requirements. Further, it was believed that this
`expert system technology could help shape the way
`Digital did business, allow the company functions to
`manage complexity, preserve the knowledge of its ex-
`perts, and provide an approach to exception manage—
`ment more consistent with its growth demands. An
`environment has been provided which has nurtured
`the emerging technology and recognized that the peo—
`ple involved are managing change, evolution, and sig-
`nificant learning. It is understood that there will be
`new and changing operating norms and success criteria
`over time. Thus, at Digital there has been real manage-
`ment commitment to helping the technology succeed in
`helping the business progress.
`
`TECHNICAL ISSUES
`
`Technologies develop iteratively through a number of
`phases:
`
`0 Investigation and experimentation,
`0 Stabilization with limitations,
`
`0 Identification of opportunities for optimization, and
`o Maturity.
`
`No aspects of expert systems technology have really
`reached maturity yet. This need not be an impediment
`to its successful use, but it must be understood and
`proper expectations must be set. In a new and emerging
`technology, there are many opportunities for technical
`creativity as new discoveries and capabilities surface.
`There is also the likelihood that a serious application
`development effort will push at the limits of what the
`technology has to offer at a particular point in time, and
`in so doing, can contribute greatly to the further devel-
`opment of that technology.
`
`Technology Challenges
`The main challenges stemming from the emerging
`nature of this technology are:
`
`1. Dealing creatively with the current limitations of the
`specialized languages and tools, and
`2. Formulating an appropriate software engineering
`methodology (this is the thrust of the RIME develop-
`ment effort)
`
`to produce efficient, maintainable, extensible expert
`systems.
`Today’s expert systems languages (and shells) offer
`varied but somewhat limited functionality. Most do not
`provide true extensibility to a production environment.
`There are very few knowledge engineering productivity
`tools available as yet on the market. The OPS5 lan-
`guage was originally chosen for the development of R1,
`the forebear of XCON. As XCON and the other configu-
`ration systems have evolved using VAX OPSS,G its
`strong pattern—matching capability and its speed have
`maintained it as our language of choice over all others
`on the market today. CSDG has directly influenced the
`development of VAX OPSS, based on our experience,
`and continues to do so.
`
`Bachant and Soloway introduce a software engineer-
`ing methodology appropriate for the development of ex-
`pert systems. They make two primary points: (1) A task
`such as XCON’s, where thousands of highly context-
`dependent decisions must be made in order to correctly
`configure a system, can take advantage of a language,
`like OPSS, that presupposes “situation recognition”
`rather than “algorithmic” control. (2) But, as the num-
`ber of alternatives that an expert system must decide
`among becomes large, there is strong motivation to aug-
`ment situation recognition with other kinds of control
`mechanisms. RIME, a software engineering methodol-
`ogy developed within CSDG, identifies three mecha-
`nisms that an OPS5 program can use to determine what
`to do and when to do it. This methodology, which was
`first used to reimplement XCON, has proven beneficial
`in the management of complexity and facilitation of
`change.
`
`6VAX OPSS is a registered trademark of Digital Equipment Corporation.
`
`March 1989 Volume 32 Number 3
`
`Communications of the ACM
`
`303
`
`6
`
`

`

`Case Study
`
`Testing, by traditional standards, is a major problem
`for expert systems. Can you test an expert system the
`way you evaluate a human expert? Since the under-
`standing of the business problem is often enhanced
`over time by the use of the expert system, how can you
`define a set of tests that are sure to be relevant from
`
`one release to the next? Since there are, typically, an
`indeterminate number of paths through an expert sys-
`tem, it is not possible to exhaustively test all scenarios.
`What is correctness? Consider the relationship between
`the optimal solution and an accurate solution. An error
`can be difficult to distinguish from a wish-list item. The
`expert system may come up with the right answer, but
`for the wrong reasons. What about the cases where the
`experts disagree? There are other related testing issues
`as well, all of which cry out for new views of design-
`for-testability and new testing concepts, methods, and
`tools. This is assuredly an area which needs more at-
`tention, and we hope to devote more energy to this in
`the future.
`
`Application Challenges
`There are three application characteristics which are
`major contributors to the challenges faced in expert
`systems development:
`
`1. Volatile subject domain,
`2. Expanding functional scope, and
`3. Large system size and complexity.
`
`These are primary characteristics of the configuration
`systems, and have presented us with our toughest chal-
`lenges over the years: [1) computer configuration is a
`dynamic subject domain; (2] once the systems were
`viewed as successful business support tools, users have
`wanted more and more from them; and (3] the configu-
`ration systems have become very large and are dealing
`with increasingly complex information relationships.
`Success has come only from our ability to respond to
`these challenges without disrupting or degrading the
`accuracy of the systems or our predictability in provid-
`ing regilar production releases.
`
`Volatility
`Each year, about 40 percent of the rules in the configu-
`ration systems knowledge base change [this includes
`rule additions and deletions, as well as rule modifica-
`tions). This rate of change in the engineering, manufac-
`turing, and marketing rules is due to a number of fac-
`tors. Digital offers several hundred major new products
`and many more “minor” ones each year. Configuration
`information about them all must be included in the
`
`configuration systems knowledge base. In addition,
`sometimes knowledge about how to configure existing
`products must be changed to incorporate their configur-
`ability with new ones. Also, existing products are often
`re-packaged, as dictated by engineering, marketing, or
`government regulation. And, finally, CSDG periodically
`re-writes focused sets of rules as more is learned about
`
`the configuration of a particular product set and how to
`represent the knowledge more effectively.
`There is another dimension of volatility in the con-
`figuration domain; the scope of the configuration task
`itself has changed significantly. The complexity and
`flexibility of Digital’s product strategies has increased
`steadily since XCON was first designed. A single com-
`puter system can no longer be configured in isolation,
`but must be considered in relation to its role in a clus-
`ter or network. Clusters and networks offer multiple
`ways of configuring computers and connecting devices.
`Cluster and network connections are coming to be
`thought of as the main bus, with systems and devices
`providing functionality for one large “solution system."
`Moreover, distinctions between systems and devices
`are becoming less clear. The configuration-related in-
`terdependencies between hardware and software are
`much more numerous and complex today. In summary,
`the basic concepts and components of the configuration
`domain are being redefined continuously.
`
`Expansion
`The configuration systems we have noted serve in-
`creasingly varied user groups from different business
`functions and geographies. New types of users with
`new perspectives generate new requirements which
`frequently enlarge the functional scope of the systems.
`For example, when the plant technicians began to use
`the XCON diagram as the official document from
`which to construct the computer, a whole new set of
`requirements was generated. CSDG made an architec-
`tural change to the system in order to support these
`needs (altering some of the knowledge representation,
`transitioning functionality between the OPSS rules and
`non-OPSS code to create the diagrams, and developing
`a specialized database).
`As their businesses change, existing users have dis-
`covered new and different ways to use the configura-
`tion systems. For example, as order volumes have in-
`creased and the technical validation function has
`changed, those users want to interact with the tool in
`different ways and need different kinds of information
`from the systems. They have asked for more stream-
`lined ways to access the XCON results, as their pro—
`cesses have changed to preclude the need for steps pre-
`viously performed and reflected in the XCON menu
`access scheme.
`
`When XCON started feeding information to other
`systems, there have been instances where the rules
`have had to be changed to accommodate a new per-
`spective on the configuration information. For example,
`there is another expe