MORE INFORMATION
Microsoft ODBC version 3.0 introduced the concept of connection pooling.
Connection pooling enables an ODBC application to reuse an existing
connection from a pool, so the ODBC application does not have to
go
through the complete connection process for any subsequent connection.
When an ODBC application disconnects a connection, the connection is saved
into a pool instead of actually disconnected. How long this connection
remains
in the pool depends on the CPTimeout property of the ODBC driver.
When the
timeout expires, the connection is closed and removed from
the pool. The ODBC
application can use SQLConfigDriver to change the value
of CPTimeout, and this
value applies to all the ODBC applications that are
using the specified ODBC driver.
The default value for the CPTimeout is 60
seconds.
Connection pooling is very useful when an ODBC application like Microsoft
Internet
Information Server (for example) connects and disconnects
frequently. Microsoft
Internet Information Server (IIS) version 3.0 with
Active Server Pages (ASP)
takes advantage of connection pooling. You can
enable connection pooling for
IIS users by changing the value of
StartConnectionPool to 1.
StartConnectionPool is located under:
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\W3SVC\ASP\Parameters
An ODBC application can use SQLSetEnvAttr to enable connection pooling. Following is an example of how to enable connection pooling in an ODBC
application:
#include <windows.h>
#include <stdio.h>
#include <sql.h>
#include <sqlext.h>
void main()
{
SQLHENV henv;
SQLHDBC hdbc;
int i=0;
if (!SQL_SUCCEEDED(SQLSetEnvAttr(
NULL, // make process level cursor pooling
SQL_ATTR_CONNECTION_POOLING,
(SQLPOINTER)SQL_CP_ONE_PER_DRIVER,
SQL_IS_INTEGER)))
printf("SQLSetEnvAttr/SQL_ATTR_CONNECTION_POOLING error\n");
if (!SQL_SUCCEEDED(SQLAllocHandle(SQL_HANDLE_ENV, NULL, &henv)))
printf("SQLAllocHandle error\n");
// set the ODBC behavior version.
if (!SQL_SUCCEEDED(SQLSetEnvAttr(henv, SQL_ATTR_ODBC_VERSION,
(SQLPOINTER) SQL_OV_ODBC2, SQL_IS_INTEGER)))
printf("SQLSetEnvAttr/SQL_ATTR_ODBC_VERSION error\n");
//set the matching condition for using an existing connection in the
pool
if (!SQL_SUCCEEDED(SQLSetEnvAttr(henv, SQL_ATTR_CP_MATCH,
(SQLPOINTER) SQL_CP_RELAXED_MATCH, SQL_IS_INTEGER)))
printf("SQLSetEnvAttr/SQL_ATTR_CP_MATCH error\n");
while (i < 10) {
if (!SQL_SUCCEEDED(SQLAllocHandle(SQL_HANDLE_DBC,
henv, &hdbc)))
printf("SQLAllocHandle error\n");
if (!SQL_SUCCEEDED(SQLConnect(hdbc,
(unsigned char*)"testing\0", SQL_NTS,
(unsigned char*)"sa\0", SQL_NTS,
(unsigned char*)"\0", SQL_NTS)))
printf("SQLConnect error\n");
else
printf("Connect successfully %d times\n", i);
//the first time, the application calls SQLDisconenct, it will return
//the connection to the //pool
SQLDisconnect(hdbc);
if (!SQL_SUCCEEDED(SQLFreeHandle(SQL_HANDLE_DBC, hdbc)))
printf("SQLFreeHandle error\n");
i++;
}
SQLFreeHandle(SQL_HANDLE_ENV, henv);
}
When the ODBC application calls
SQLDisconnect the first time, the
connection is saved to the pool. Any subsequent SQLConnect/SQLDisconnect
that matches the required condition will reuse the first connection.