Presentation is loading. Please wait.

Presentation is loading. Please wait.

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 26Slide 1 Chapter 26 Legacy Systems.

Similar presentations


Presentation on theme: "©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 26Slide 1 Chapter 26 Legacy Systems."— Presentation transcript:

1 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 26Slide 1 Chapter 26 Legacy Systems

2 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 26Slide 2 Input-process-output l Input components read and validate data from a terminal or file l Processing components carry out some transformations on that data l Output components format and print the results of the computation l Input, process and output can all be represented as functions with data flowing between them

3 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 26Slide 3 Functional design process l Data-flow design l Model the data processing in the system using data-flow diagrams l Structural decomposition l Model how functions are decomposed to sub-functions using graphical structure charts that reflect the input/process/output structure l Detailed design l The functions in the design and their interfaces are described in detail.

4 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 26Slide 4 Data flow diagrams l Show how an input data item is functionally transformed by a system into an output data item l Are an integral part of many design methods and are supported by many CASE systems l May be translated into either a sequential or parallel design. In a sequential design, processing elements are functions or procedures; in a parallel design, processing elements are tasks or processes

5 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 26Slide 5 Payroll system DFD

6 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 26Slide 6 Payroll batch processing l The functions on the left of the DFD are input functions l Read employee record, Read monthly pay data, Validate employee data l The central function - Compute salary - carries out the processing l The functions to the right are output functions l Write tax transaction, Write pension data, Print payslip, Write bank transaction, Write social security data

7 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 26Slide 7 Transaction processing l A ban ATM system is an example of a transaction processing system l Transactions are stateless in that they do not rely on the result of previous transactions. Therefore, a functional approach is a natural way to implement transaction processing

8 Design description of an ATM

9 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 26Slide 9 Using function-oriented design l For some classes of system, such as some transaction processing systems, a function- oriented approach may be a better approach to design than an object-oriented approach l Companies may have invested in CASE tools and methods for function-oriented design and may not wish to incur the costs and risks of moving to an object-oriented approach

10 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 26Slide 10 Legacy system assessment l Organisations that rely on legacy systems must choose a strategy for evolving these systems l Scrap the system completely and modify business processes so that it is no longer required l Continue maintaining the system l Transform the system by re-engineering to improve its maintainability l Replace the system with a new system l The strategy chosen should depend on the system quality and its business value

11 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 26Slide 11 System quality and business value

12 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 26Slide 12 Legacy system categories l Low quality, low business value l These systems should be scrapped l Low-quality, high-business value l These make an important business contribution but are expensive to maintain. Should be re-engineered or replaced if a suitable system is available l High-quality, low-business value l Replace with COTS, scrap completely or maintain l High-quality, high business value l Continue in operation using normal system maintenance

13 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 26Slide 13 Business value assessment l Assessment should take different viewpoints into account l System end-users l Business customers l Line managers l IT managers l Senior managers l Interview different stakeholders and collate results

14 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 26Slide 14 System quality assessment l Business process assessment l How well does the business process support the current goals of the business? l Environment assessment l How effective is the systems environment and how expensive is it to maintain l Application assessment l What is the quality of the application software system

15 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 26Slide 15 Business process assessment l Use a viewpoint-oriented approach and seek answers from system stakeholders l Is there a defined process model and is it followed? l Do different parts of the organisation use different processes for the same function? l How has the process been adapted? l What are the relationships with other business processes and are these necessary? l Is the process effectively supported by the legacy application software?

16 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 26Slide 16 Environment assessment

17 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 26Slide 17 Application assessment

18 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 26Slide 18 System measurement l You may collect quantitative data to make an assessment of the quality of the application system l The number of system change requests l The number of different user interfaces used by the system l The volume of data used by the system

19 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 26Slide 19 Key points l A legacy system is an old system that still provides essential business services l Legacy systems are not just application software but also include business processes, support software and hardware l Most legacy systems are made up of several different programs and shared data l A function-oriented approach has been used in the design of most legacy systems

20 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 26Slide 20 Key points l The structure of legacy business systems normally follows an input-process-output model l The business value of a system and its quality should be used to choose an evolution strategy l The business value reflects the systems effectiveness in supporting business goals l System quality depends on business processes, the systems environment and the application software


Download ppt "©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 26Slide 1 Chapter 26 Legacy Systems."

Similar presentations


Ads by Google