OCCI线程安全

线程是任务调度的基本单位，一个进程中能够有多个线程，每一个线程有本身的堆栈空间，
进程中的代码段、数据段和堆栈对进程中的线程是可见的。在使用线程时一般都要考虑数据的安全访问。
经常使用的线程同步方法有:

• 互斥变量
• 读写锁
• 条件变量
• 屏障
• 自旋锁(用于CPU，锁住后CPU将不执行其余事情，即一直等待)

在OCCI程序中使能线程安全，则应指定THREADED_MUTEXED选项建立运行环境

例如：
    Environment *env =
         Environment::createEnvironment(Environment::THREADED_MUTEXED)

注意：

• 若是应用程序是单线程，则使用默认值建立环境便可；
  若是指定THREADED_MUTEXED选项建立将会影响应用程序性能。
• OCCI中的线程安全仅仅指的是Environment、Map、ConnectionPool、
  StatelessConnectionPool和Connection对象。对于Statement、ResultSet、
  SQLException、Stream等不是线程安全的，所以不该该在多个线程中共享。

---

如下例子建立一个只有一个链接的无状态链接池，建立两个线程去竞争从池中获得链接
当以线程安全建立环境时程序运行正常，当使用默认选项建立环境时，程序将抛出异常：

what():  ORA-03117: two-task save area overflow

#include <iostream>
#include <cstdlib>

#include <pthread.h>
#include <unistd.h>
#include <occi.h>


#define USERNAME    "scott"
#define PASSWORD    "scott"
#define DBNAME      "//192.168.42.135:1521/orcl"
#define MAXCON  1
#define MINCON  0
#define INCCON  1   


using namespace oracle::occi;
using namespace std;

void *displayAllRows(void *arg);
void *updateRow(void *arg);


int main(void)
{
    Environment *env = Environment::createEnvironment(Environment::THREADED_MUTEXED);

    //建立无状态链接池
    StatelessConnectionPool *scp = 
        env->createStatelessConnectionPool(
            USERNAME, PASSWORD, DBNAME, MAXCON, MINCON, INCCON,
            StatelessConnectionPool::HOMOGENEOUS);
    
    //设置池中的链接空闲超时时间，超时后OCCI会自动释放此链接，须要时在建立
    scp->setTimeOut(10);
    

    cout << "*****************Information**************************" << endl;
    cout << "Open Connection : " << scp->getOpenConnections() << endl;
    cout << "Busy Connection : " << scp->getBusyConnections() << endl;
    cout << "Time Out Connection : " << scp->getTimeOut() << endl;
    cout << "******************************************************" << endl;


    pthread_t tid_1, tid_2;
    int err = 0;
    err = pthread_create(&tid_1, NULL, displayAllRows, (void *)scp);
    if (err != 0) {
        cout << "create thread failure for tid_1." << endl;
    } else {
        cout << ">> create thread successful for tid_1." << endl;
    }

    err = pthread_create(&tid_2, NULL, updateRow, (void *)scp);
    if (err != 0) {
        cout << "create thread failure for tid_2." << endl;
    } else {
        cout << ">> create thread successful for tid_2." << endl;
    }

    pthread_join(tid_1, (void **)NULL);
    pthread_join(tid_2, (void **)NULL);


    env->terminateStatelessConnectionPool(scp);
    Environment::terminateEnvironment(env);

    return 0;
}


void *displayAllRows(void *arg)
{
    StatelessConnectionPool *scp = (StatelessConnectionPool *)arg;


    for ( ; ; )
    {
        std::cout << ">>>> displayAllRows thread runing [" << long(pthread_self()) << "]" << std::endl;

        Connection *conn = scp->getConnection("");
        
        string sqlStmt = "SELECT * FROM DEPT WHERE DEPTNO=60";
        
        Statement *stmt = conn->createStatement(sqlStmt);
        
        ResultSet *rset = stmt->executeQuery();
    
        try {
            while (rset->next()) {
                cout << int(rset->getNumber(1)) << "    ";
                cout << rset->getString(2) << " ";
                cout << rset->getString(3) << " ";
                cout << endl;
            }
        }catch(SQLException &ex) {
            cout << "display all rows failure." << endl;
            cout << ex.getMessage();
            cout << endl;
        }
    
        sleep(5);

        stmt->closeResultSet(rset);
        conn->terminateStatement(stmt);
        scp->releaseConnection(conn, "");
    }

    pthread_exit((void *)0);
}

void *updateRow(void *arg)
{
    StatelessConnectionPool *scp = (StatelessConnectionPool*)arg;


    for ( ; ; )
    {
        std::cout << ">>> updateRow thread running [" << long(pthread_self()) << "]" << std::endl;

        Connection *conn = scp->getConnection("");
        
        string sqlStmt = "UPDATE DEPT SET LOC=:deptLocal WHERE DEPTNO=51";

        Statement *stmt = conn->createStatement(sqlStmt);

        try {
            stmt->setString(1, "ShangHai");
            stmt->executeUpdate();
            conn->commit();
        } catch(SQLException &ex) {
            cout << "update row failure." << endl;
            cout << ex.getMessage();
            cout << endl;
        }

        conn->terminateStatement(stmt);
        scp->releaseConnection(conn, "");

        sleep(1);
    }

    pthread_exit((void *)0);
}