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[作者 高健@博客园 luckyjackgao@gmail.com]
EXEC SQL CONTEXT... 是Oracle Pro*C的特有的语法，详细信息参见
http://docs.oracle.com/cd/B28359_01/appdev.111/b28427/pc_11thr.htm#i997959
EXEC SQL ENABLE THREADS;
EXEC SQL CONTEXT ALLOCATE :context_var;
EXEC SQL CONTEXT USE { :context_var | DEFAULT};
EXEC SQL CONTEXT FREE :context_var;
所谓context ，被称为runtime context，其实质就是保留连接数据库的信息，保留连接数据库的通道。
官方解释中，使用 exec sql context ，有几种方式：
1 各个进程之间不共享context
事实上，Oracle的官方例子---- Thread_example1.pc，采用的就是这种方式。
各个进程共通处理一堆转账数据，所以它们之间需要考虑对共同数据(文本格式的转账记录)读取时加mutex。
但是，由于每个进程可以自己拥有一个context---数据库通道，故此基本互相不干涉。
我认为这是合理的，也可能是比较高效率的方式。
2 各个进程之间共享一个context
此时，瓶颈出现在此context之上。可以想像一下，通过多线程来处理数据库之外的数据之后，
还是要回到一个共通的数据库通道来排队等候，是否效率并未发挥出来呢。
3 各个进程之间共享多个context
这个是最复杂的，可能也是最没有道理的。1的方式应该更好些。
而且，上述这些，都没有考虑到连接池的作用，也许是这种技术出现的年代是很早的。
为了备忘，记录oracle官方例子如下：
/*
* Name: Thread_example1.pc
*
* Description: This program illustrates how to use threading in
* conjunction with precompilers. The program creates as many
* sessions as there are threads. Each thread executes zero or
* more transactions, that are specified in a transient
* structure called 'records'.
* Requirements:
* The program requires a table 'ACCOUNTS' to be in the schema
* scott/tiger. The description of ACCOUNTS is:
* SQL> desc accounts
* Name Null? Type
* ------------------------------- ------- ------
* ACCOUNT NUMBER(36)
* BALANCE NUMBER(36,2)
*
* For proper execution, the table should be filled with the accounts
* 10001 to 10008.
*
*
*/
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sqlca.h>
#define _EXC_OS_ _EXC__UNIX
#define _CMA_OS_ _CMA__UNIX
#ifdef DCE_THREADS
#include <pthread.h>
#else
#include <thread.h>
#endif
/* Function prototypes */
void err_report();
#ifdef DCE_THREADS
void do_transaction();
#else
void *do_transaction();
#endif
void get_transaction();
void logon();
void logoff();
#define CONNINFO "scott/tiger"
#define THREADS 3
struct parameters
{ sql_context * ctx;
int thread_id;
};
typedef struct parameters parameters;
struct record_log
{
char action;
unsigned int from_account;
unsigned int to_account;
float amount;
};
typedef struct record_log record_log;
record_log records[]= { { 'M', 10001, 10002, 12.50 },
{ 'M', 10001, 10003, 25.00 },
{ 'M', 10001, 10003, 123.00 },
{ 'M', 10001, 10003, 125.00 },
{ 'M', 10002, 10006, 12.23 },
{ 'M', 10007, 10008, 225.23 },
{ 'M', 10002, 10008, 0.70 },
{ 'M', 10001, 10003, 11.30 },
{ 'M', 10003, 10002, 47.50 },
{ 'M', 10002, 10006, 125.00 },
{ 'M', 10007, 10008, 225.00 },
{ 'M', 10002, 10008, 0.70 },
{ 'M', 10001, 10003, 11.00 },
{ 'M', 10003, 10002, 47.50 },
{ 'M', 10002, 10006, 125.00 },
{ 'M', 10007, 10008, 225.00 },
{ 'M', 10002, 10008, 0.70 },
{ 'M', 10001, 10003, 11.00 },
{ 'M', 10003, 10002, 47.50 },
{ 'M', 10008, 10001, 1034.54}};
static unsigned int trx_nr=0;
#ifdef DCE_THREADS
pthread_mutex_t mutex;
#else
mutex_t mutex;
#endif
/*********************************************************************
* Main
********************************************************************/
main()
{
sql_context ctx[THREADS];
#ifdef DCE_THREADS
pthread_t thread_id[THREADS];
pthread_addr_t status;
#else
thread_t thread_id[THREADS];
int status;
#endif
parameters params[THREADS];
int i;
EXEC SQL ENABLE THREADS;
EXEC SQL WHENEVER SQLERROR DO err_report(sqlca);
/* Create THREADS sessions by connecting THREADS times */
for(i=0;i<THREADS;i++)
{
printf("Start Session %d....",i);
EXEC SQL CONTEXT ALLOCATE :ctx[i];
logon(ctx[i],CONNINFO);
}
/*Create mutex for transaction retrieval */
#ifdef DCE_THREADS
if (pthread_mutex_init(&mutex,pthread_mutexattr_default))
#else
if (mutex_init(&mutex, USYNC_THREAD, NULL))
#endif
{
printf("Can't initialize mutex
");
exit(1);
}
/*Spawn threads*/
for(i=0;i<THREADS;i++)
{
params[i].ctx=ctx[i];
params[i].thread_id=i;
printf("Thread %d... ",i);
#ifdef DCE_THREADS
if (pthread_create(&thread_id[i],pthread_attr_default,
(pthread_startroutine_t)do_transaction,
(pthread_addr_t) ¶ms[i]))
#else
if (status = thr_create
(NULL, 0, do_transaction, ¶ms[i], 0, &thread_id[i]))
#endif
printf("Cant create thread %d
",i);
else
printf("Created
");
}
/* Logoff sessions....*/
for(i=0;i<THREADS;i++)
{
/*wait for thread to end */
printf("Thread %d ....",i);
#ifdef DCE_THREADS
if (pthread_join(thread_id[i],&status))
printf("Error when waiting for thread % to terminate
", i);
else
printf("stopped
");
printf("Detach thread...");
if (pthread_detach(&thread_id[i]))
printf("Error detaching thread!
");
else
printf("Detached!
");
#else
if (thr_join(thread_id[i], NULL, NULL))
printf("Error waiting for thread to terminate
");
#endif
printf("Stop Session %d....",i);
logoff(ctx[i]);
EXEC SQL CONTEXT FREE :ctx[i];
}
/*Destroys mutex*/
#ifdef DCE_THREADS
if (pthread_mutex_destroy(&mutex))
#else
if (mutex_destroy(&mutex))
#endif
{
printf("Can't destroy mutex
");
exit(1);
}
}
/*********************************************************************
* Function: do_transaction
*
* Description: This functions executes one transaction out of the
* records array. The records array is 'managed' by
* the get_transaction function.
*
*
********************************************************************/
#ifdef DCE_THREADS
void do_transaction(params)
#else
void *do_transaction(params)
#endif
parameters *params;
{
struct sqlca sqlca;
record_log *trx;
sql_context ctx=params->ctx;
/* Done all transactions ? */
while (trx_nr < (sizeof(records)/sizeof(record_log)))
{
get_transaction(&trx);
EXEC SQL WHENEVER SQLERROR DO err_report(sqlca);
EXEC SQL CONTEXT USE :ctx;
printf("Thread %d executing transaction
",params->thread_id);
switch(trx->action)
{
case 'M': EXEC SQL UPDATE ACCOUNTS
SET BALANCE=BALANCE+:trx->amount
WHERE ACCOUNT=:trx->to_account;
EXEC SQL UPDATE ACCOUNTS
SET BALANCE=BALANCE-:trx->amount
WHERE ACCOUNT=:trx->from_account;
break;
default: break;
}
EXEC SQL COMMIT;
}
}
/*****************************************************************
* Function: err_report
*
* Description: This routine prints out the most recent error
*
****************************************************************/
void err_report(sqlca)
struct sqlca sqlca;
{
if (sqlca.sqlcode < 0)
printf("
%.*s
",sqlca.sqlerrm.sqlerrml,sqlca.sqlerrm.sqlerrmc);
exit(1);
}
/*****************************************************************
* Function: logon
*
* Description: Logs on to the database as USERNAME/PASSWORD
*
*****************************************************************/
void logon(ctx,connect_info)
sql_context ctx;
char * connect_info;
{
EXEC SQL WHENEVER SQLERROR DO err_report(sqlca);
EXEC SQL CONTEXT USE :ctx;
EXEC SQL CONNECT :connect_info;
printf("Connected!
");
}
/******************************************************************
* Function: logoff
*
* Description: This routine logs off the database
*
******************************************************************/
void logoff(ctx)
sql_context ctx;
{
EXEC SQL WHENEVER SQLERROR DO err_report(sqlca);
EXEC SQL CONTEXT USE :ctx;
EXEC SQL COMMIT WORK RELEASE;
printf("Logged off!
");
}
/******************************************************************
* Function: get_transaction
*
* Description: This routine returns the next transaction to process
*
******************************************************************/
void get_transaction(trx)
record_log ** trx;
{
#ifdef DCE_THREADS
if (pthread_mutex_lock(&mutex))
#else
if (mutex_lock(&mutex))
#endif
printf("Can't lock mutex
");
*trx = &records[trx_nr];
trx_nr++;
#ifdef DCE_THREADS
if (pthread_mutex_unlock(&mutex))
#else
if (mutex_unlock(&mutex))
#endif
printf("Can't unlock mutex
");
}
[作者 高健@博客园 luckyjackgao@gmail.com]
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