Legacy Systems

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?

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

3
Historical Perspective (ca. 1992)

• The only GUI environments were Windows 3.11,
  Macintosh OSs, and IBMs 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)

4
The legacy

• An IBM OS/2 platform. (still, not true GUI)
• Used IBMs OS/2 network solution
• Loaded the network software high, so 604K was
  available for the system in that session

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

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)

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!

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

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

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

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

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

13
PECOs 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)