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Legacy Systems

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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  1. 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

  2. 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

  3. 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

  4. 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

  5. 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

  6. Implementation • 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

  7. 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

  8. 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

  9. 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

  10. 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

  11. 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

  12. 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

  13. 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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