软件体系结构经典问题——KWIC的分析和解决

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

Elements of Main/Subroutine Architectural style are:

• Components: Functions
• Interactions: Function calls
• Pattern: Main function controls the calling sequence

This is the primary organization of many software systems. This style reflects structural/procedural programming language (e.g. C programming language) in which these system are written.

 

 

•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

•Intermediate data stored in shared memory.

–Arrays of characters with indexes
带索引的字符数组

–Relies on sequential processing
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

•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

–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

 

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：

http://coronet.iicm.edu/sa/swp_how.htm

http://www.cs.cmu.edu/~ModProb/index.html

http://hi.baidu.com/snowxshy/item/f8c5957430788145ee1e532f