Careercup - Microsoft面试题 - 6366101810184192

2014-05-10 22:30

题目链接

原题：

Design database locks to allow r/w concurrency and data consistency.

题目：设计一个支持并行读写并保证数据一致性的数据库锁。

解法：这是什么范畴的面试题？对于我这种完全没有相关项目经验的人，还真是无从下手。翻了书之后顺便重新复习了一下共享锁、互斥锁的概念。说白了，这些都是课本上的基础知识，可是毕业没多久就忘光了。看来知识不用，保质期比水果还短。然后我琢磨着锁的模型，写了个模拟的代码。这个代码和SQL毫无关系，可能也和正确答案相去甚远。姑妄言之，姑妄听之吧。

代码：

 // http://www.careercup.com/question?id=6366101810184192
 import java.util.concurrent.Semaphore;

 public class FooBar {
     public static final int MAX_CONCURRENT_READ = 100;
     private Semaphore sharedSemaphore;
     private Semaphore exclusiveSemaphore;

     public FooBar() {
         // TODO Auto-generated constructor stub
         this.sharedSemaphore = new Semaphore(MAX_CONCURRENT_READ);
         this.exclusiveSemaphore = new Semaphore(1);
     }

     public void reader() {
         // The reader here is not to return a value, but to perform read()
         // action. Thus it is 'void reader()'.
         while (exclusiveSemaphore.availablePermits() < 1) {
             try {
                 Thread.sleep(50);
             } catch (InterruptedException e) {
                 // TODO Auto-generated catch block
                 e.printStackTrace();
             }
         }

         try {
             sharedSemaphore.acquire();
             System.out.println("Performing read() operation.");
             sharedSemaphore.release();
         } catch (InterruptedException e) {
             // TODO Auto-generated catch block
             e.printStackTrace();
         }
     }

     public void writer() {
         while (exclusiveSemaphore.availablePermits() < 1
                 && sharedSemaphore.availablePermits() < MAX_CONCURRENT_READ) {
             try {
                 Thread.sleep(50);
             } catch (InterruptedException e) {
                 // TODO Auto-generated catch block
                 e.printStackTrace();
             }
         }

         try {
             exclusiveSemaphore.acquire();
             System.out.println("Performing write() operation.");
             exclusiveSemaphore.release();
         } catch (InterruptedException e) {
             // TODO Auto-generated catch block
             e.printStackTrace();
         }
     }

     public static void main(String[] args) {
         FooBar fooBar = new FooBar();

         fooBar.reader();
         fooBar.writer();
     }
 }