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software engineering ian sommerville 8th edition phần 7

khi chúng tôi nhận được giá trị dữ liệu của chúng tôi từ một nơi xa xôi, chúng tôi chỉ cần để đảm bảo nó được gọi là x và y. Một khi chúng ta đã làm điều đó, chúng tôi đã có các biến. Biến cho phép chúng tôi cung cấp cho một phần dữ liệu tên | 21.3 Evolution processes 501 Figure 21.10 Forward engineering and re-engineering System specification Il Design and implementation Forward engineering Software re-engineering 21.3.1 System re-engineering As discussed in the previous section the process of system evolution involves understanding the program that has to be changed then implementing these changes. However many systems especially older legacy systems discussed in Chapter 2 are difficult to understand and change. The programs may have been originally optimised for performance or space utilisation at the expense of understandability or over lime the initial program structure may have been corrupted by a series of changes. To simplify the problems of changing its legacy systems a company may decide to re-engineer these systems to improve their structure and understandability. Software re-engineering is concerned with re-implementing legacy systems to make them more maintainable. Re-engineering may involve re-documenting the system. organising and restructuring the system translating the system to a more modem programming language and modifying and updating the structure and values of the system s data. The functionality of the software is not changed and normally the system architecture also remains the same. Re-engineering a software system has two key advantages over more radical approaches to system evolution 1. Reduced risk There is a high risk in re-developing business-critical software. Errors may be made in the system specification or there may be development problems. Delays in introducing the new software may mean that business is lost and extra costs are incurred. For example in 1999 a large US food company encountered delays in introducing a new ordering system which led to delays in delivering 100 million worth of goods during a peak sales season. 2. Reduced cost The cost of re-engineering is significantly less than the cost of developing new software. Ulrich Ulrich 1990 quotes an example of a