软件体系结构经典问题——KWIC的分析和解决
2014-01-10 23:15 youxin 阅读(7301) 评论(0) 编辑 收藏 举报KWIC作为一个早年间在ACM的Paper提出的一个问题，被全世界各个大学的软件设计课程奉为课堂讲义或者作业的经典。（From Wiki，FYI，D. L. Parnas uses a KWIC Index as an example on how to perform modular design in his paper "On the Criteria To Be Used in Decomposing Systems into Modules" - Available as ACM Classic Paper）
问题陈述：KWIC(Key Word In Context)，Parnas (1972)
KWIC索引系统接受一些行，每行有若干字，每个字由若干字符组成；每行都可以循环移位，亦即重复地把第一个字删除，然后接到行末； KWIC把所有行的各种移位情况按照字母表顺序输出
•目的：考察不同的体系结构对变化的适应能力（modifiability）
•评价准则
–处理算法的改变：例如，行的移位可在每行读入后、在所有行读入后、或当排序要求一组移位的行时執行；
–数据表示的改变：例如，行、字、字符可以不同的方式存储；类似地，循环移位后的行可以显式或隐式存储（索引和偏移量）；
–系统功能的增强：例如，限制以某些“修饰词”(a, an, and等)打头的移位结果；支持交互，允许用户从原始输入表中删除一些行等；
–效率：时间和空间；
–复用：构件被复用的潜力。
Solution 1：
Main Program/Subroutine with Shared Data
Main Program/Subroutine with Shared Data
Elements of Main/Subroutine Architectural style are:
- Components: Functions
- Interactions: Function calls
- Pattern: Main function controls the calling sequence
•Decompose the overall processing into a sequence of processing steps.
–Read lines; Make shifts; Alphabetize; Print results
•Each step transforms the data completely.
每一步完全转换数据
每一步完全转换数据
•Intermediate data stored in shared memory.
–Arrays of characters with indexes
带索引的字符数组
带索引的字符数组
–Relies on sequential processing
串行处理
串行处理
Solution 1：Modularization
•Module 1: Input
–Reads data lines and stores them in “core”.
–Storage format: 4 chars/machine word; array of pointers to start of each line.
–
•Module 2: Circular Shift
–Called after Input is done.
–Reads line storage to produce new array of pairs: (index of 1st char of each circular shift, index of original line)
–
•Module 3: Alphabetize
–Called after Circular Shift.
–Reads the two arrays and produces new index.
•Module 4: Output
–Called after alphabetization and prints nicely formatted output of shifts
–Reads arrays produced by Modules 1 & 3
–
•Module 5: Master Control
–Handles sequencing of the first 4 modules
–Handles errors
Properties of Solution 1
•Batch sequential processing.
•Uses shared data to get good performance.
用共享数据获得性能
用共享数据获得性能
•Processing phases handled by control module.
–So has some characteristics of main program – subroutine organization.
–Depends critically on single thread of control.
•Shared data structures exposed as inter-module knowledge.
共享数据的结构是所有模块必须知道的
共享数据的结构是所有模块必须知道的
–Design of these structures must be worked out before work can begin on those modules.
数据结构的设计必须在其他模块设计开始之前进行
数据结构的设计必须在其他模块设计开始之前进行
方案优缺点：
+系统自然分解，符合人的处理习惯
+数据共享，处理效率高
+
–难以适应数据存储格式和整体处理算法的变化——爲什麽？
–系统构件难以支持复用——爲什麽？
仔细参考：
