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Legacy Systems. What is meant by a legacy system? What it is / what it is not Why is it critical to business operations? Function-oriented design Assessing legacy systems. discard it? maintain it? re-engineer it? replace it?. An actual business example.

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Legacy Systems

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Legacy systems l.jpg

Legacy Systems

  • What is meant by a legacy system?

  • What it is / what it is not

  • Why is it critical to business operations?

  • Function-oriented design

  • Assessing legacy systems

  • discard it?

  • maintain it?

  • re-engineer it?

  • replace it?

Ch.26 - Legacy Systems

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An actual business example

  • From Fall, 1992 to Summer, 1996: Working on the TMS/AMS System for PECO Energy

  • TMS – Task Managements System

    • Tracking Labor and Materials for projects

  • AMS – Asset Management System

    • PC Based System that was re-written from a previous legacy system

      • Mainframe NOMAD Database

Ch.26 - Legacy Systems

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Historical Perspective (ca. 1992)

  • The only GUI environments were Windows 3.11, Macintosh OS’s, and IBM’s OS/2.

  • The only network lion-share leaders: Novell, Banyan

  • Computers of the day:

    • 20MB (up to 1GB for a server)

    • 386 or 486 processing

    • 640K Conventional memory (RAM)

Ch.26 - Legacy Systems

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The “legacy”

  • An IBM OS/2 platform. (still, not true GUI)

  • Used IBM’s OS/2 network solution

  • Loaded the network software high, so 604K was available for the system “in that session”

Ch.26 - Legacy Systems

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From Somerville p. 582:

  • Replacing a legacy system is a risky strategy for a number of reasons:

  • There is rarely a complete specification of the legacy system.

    • OJT, User testing of specific code, etc.

  • Business processes and legacy systems are often entwined

    • For PECO: It was a necessary part of operations (it tracked hours spent on a job, billing for the job, materials, miscellaneous labor, and miscellaneous overhead.)

  • Business rules may be embedded in the software of the legacy system

    • The PECO way

  • There may be unexpected problems with the new development of software to replace a legacy system

  • Ch.26 - Legacy Systems

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    • Different parts of the system implemented by different parts of the team (Dynamic nature permitted me to add new reports to the reporting library)

    • Part or all of the system may be implemented using an obsolete programming language (Clipper 5.0)

    • System documentation is often inadequate or obsolete. (or non-existent)

    • Years of maintenance may have corrupted the system structure. (ad-hoc requests were rampant)

    Ch.26 - Legacy Systems

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    Implementation (cont.)

    • The system may have been optimized for space utilization or execution speed (as previously outlined)

    • The data processed by the system may be maintained in different files that have incompatible structures (data subsets, flat files, etc.)

    • A potential work-around as TMS/AMS got bigger: divorce the two, as two separate programs

      • The thought was that this would minimize the 604K requirements; not at all!

    Ch.26 - Legacy Systems

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    What made up the legacy?

    • As a legacy system, TMS needed to be networked, and was benchmarked at using 604K memory:

    • Written in:

      • Clipper 5.0 (95% Clipper, 5% C)

    • Database of choice:

      • dBASE

    • External Reporting:

      • Report Writer 5.0

    Ch.26 - Legacy Systems

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    Legacy System Structures (26.1)

    • System hardware (Legacy systems written for hardware systems not available anymore)

      • IBM PS/2 Series

    • Support software (Legacy systems may rely on supporting software)

      • RR and OS/2

    • Application software (The application relies on a series of interconnected programs)

      • Clipper & C, RR, dBASE

    Ch.26 - Legacy Systems

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    Legacy System Structures (26.1) (cont.)

    • Application data (The data which are processed by the application system)

      • Terminal data entry

      • Files grew quickly, and wastefully

    • Business Process (The business objective)

      • Tracking the labor, materials and assets

    • Business policies and rules (How business was done at PECO)

      • Billing codes, constraints, overrides

    Ch.26 - Legacy Systems

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    Legacy System Design (26.2)

    • Most are non- or pre-OO Development

      • (TMS tried to go OOP with class(y))

    • Conforms to a function-oriented design (broken down into reusable components)

      • this can cause problems: too many programmers dilute the code. Integrity is violated

      • it is only successful when information sharing is explicit

    Ch.26 - Legacy Systems

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    Legacy System Assessment (26.3)

    • Organizations, which depend on these systems, must decide (realistically) on the course to follow on their system

    • Assessment:

      • Scrap the system completely

      • Continue maintaining the system

      • Transform the system in some way to improve its maintainability

      • Replace the system with a new system

    Ch.26 - Legacy Systems

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    PECO’s Solution

    • Scrap the system?

      • Auditors (PECO & the subcontractors) needed the data

    • Continue maintaining the system?

      • The company was moving away from OS/2

    • Transform the system in some way to improve its maintainability?

      • Had been tried, unsuccessfully

    • Replace the system with a new system?

      • Spring 1997, a new team came in to re-write TMS using VBA (Visual Basic for Applications)

    Ch.26 - Legacy Systems

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