oracle db组面试 复习数据库

很多台DB server, 怎么确保其中一台挂了，里边的东西不会消失？ 有多台server做replication
怎么知道刚刚消失的data在replication的哪边？ 用多个pointer指每个server目前data的位址
很多人要读，偶尔写，要怎么保护？ Reader-writer lock
如果一直不能轮到write怎么处理？ 设一个duration，超过没轮到write，就禁止上读锁
cache用什么机制? lru, lfu
如果是做影音串流，cache用lru还是lfu好? lfu
lfu怎么做？

 

 

implement一个spin lock，
spin lock自旋锁：表示线程忙的锁

https://leetcode.com/discuss/interview-question/operating-system/134290/Implement-your-own-spinlock

public class SpinLock {
    private AtomicBoolean lock;
    
    public SpinLock() {
        lock = new AtomicBoolean(false);
    }

    public void lock() {
        while(!tryLock()){};    
    }

    public boolean tryLock() {
        return lock.compareAndSet(false, true);
    }

    public void unlock() {
        lock.set(true);
    }
}

 

实现wait&notify

看着其实好像挺简单的

参考：https://howtodoinjava.com/java/multi-threading/wait-notify-and-notifyall-methods/

wait()

synchronized( lockObject )
{ 
    while( ! condition )
    { 
        lockObject.wait();
    }
     
    //take the action here;
}

 

notify()

synchronized(lockObject) 
{
    //establish_the_condition;
 
    lockObject.notify();
     
    //any additional code if needed
}

notifyAll()

synchronized(lockObject) 
{
    establish_the_condition;
 
    lockObject.notifyAll();
}

然后创建Producer Thread，Consumer Thread

 

使用mutex（其实就是锁吧）

参考：https://crunchify.com/what-is-java-semaphore-and-mutex-java-concurrency-multithread-explained-with-example/

public class CrunchifySemaphoreMutexTutorial {
    static Object crunchifyLock = new Object();
    static LinkedList<String> crunchifyList = new LinkedList<String>();
    
    // Semaphore maintains a set of permits.
    // Each acquire blocks if necessary until a permit is available, and then takes it.
    // Each release adds a permit, potentially releasing a blocking acquirer.
    static Semaphore semaphore = new Semaphore(0);
    static Semaphore mutex = new Semaphore(1);
    
    // I'll producing new Integer every time
    static class CrunchifyProducer extends Thread {
        public void run() {
            
            int counter = 1;
            try {
                while (true) {
                    String threadName = Thread.currentThread().getName() + counter++;
                    
                    mutex.acquire();
                    crunchifyList.add(threadName);
                    System.out.println("Producer is prdoucing new value: " + threadName);
                    mutex.release();
                    
                    // release lock
                    semaphore.release();
                    Thread.sleep(500);
                }
            } catch (Exception x) {
                x.printStackTrace();
            }
        }
    }

 

 如何用纯c语言连接mongodb数据库进行读写操作？

参考：https://docs.mongodb.com/drivers/c/

#include <mongoc/mongoc.h>
int
main (int argc, char *argv[])
{
   mongoc_database_t *database;
   mongoc_client_t *client;
   mongoc_init ();
   client = mongoc_client_new(
      "mongodb+srv://<username>:<password>@<cluster-url>/test?retryWrites=true&w=majority"
   );
   database = mongoc_client_get_database (client, "test");
   mongoc_database_destroy (database);
   mongoc_client_destroy (client);
   mongoc_cleanup ();
   return 0;
}