Q114355: INFO: Single-Tier Drivers Don’t Support Asynchronous Processing

Article: Q114355
Product(s): Microsoft FoxPro
Version(s): 2.5,2.5a,2.5b,2.6,3.0
Operating System(s): 
Keyword(s): kbcode kbvfp300 kbGrpDSFox kbDSupport kbvfp250
Last Modified: 28-MAR-2001

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The information in this article applies to:

- Microsoft Visual FoxPro for Windows, version 3.0 
- Microsoft FoxPro for Windows, versions 2.5, 2.5a, 2.5b, 2.6 
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SUMMARY
=======

After you have set the connection option to Asynchronous using the Connectivity
Kit's DBSetOpt() command, DBExec() commands are still processed synchronously
when you are using a single-tier driver. The asynchronous connect option works
only with two-tier drivers; it is ignored when you are using a single-tier
driver.

MORE INFORMATION
================

A single-tier driver is a driver that directly reads from and writes to a
database such as the FoxPro, Btrieve, dBASE, Microsoft Excel, and text file
drivers. A two-tier driver is a driver that connects to another DBMS such as SQL
Server and ORACLE.

NOTE: dBASE is manufactured by a vendor independent of Microsoft; we make no
warranty, implied or otherwise, regarding this product's performance or
reliability.

The Asynchronous connection option works with two-tier drivers only if a
connection is actually made. When a DBExec() command is processed with the
connection option set to Asynchronous, the DBExec() command has to be
continually called until all the results are returned.

If the code below is run through a trace, it demonstrates that the single- tier
driver processes the SQL statement synchronously. Note that the DO WHILE
statement is never executed.

NOTE: Depending on your version of FoxPro, use one of the following two code
examples:

FoxPro 2.x Code:
----------------

     SET LIBRARY TO SYS(2004)+"fpsql.fll"

     PUBLIC handle
     dsource="foxtest"
     errval=0
     errmsg=""
     * foxtest is a single-tier FoxPro database driver.
     handle=DBConnect(dsource,"","")
     =DBSetOpt(handle,"Asynchronous",1)

     sql="select * from customer"

     waittime=2
     start=SECONDS()
     retcode=DBExec(handle,sql)

     * This DO WHILE code below is never executed because retcode=1....
     * i.e., the DBExec() above returns all the results synchronously.
     DO WHILE ((retcode=0) AND (SECONDS() - start < waittime))
        retcode=DBExec(handle,sql)
     ENDDO

     IF (retcode= 0)
         retcode=DBCancel(handle)
     ENDIF

     IF (retcode != 1)
        error=DBError(handle,@errmsg,@errval)
        CLEAR
        @ 2,2 SAY errval
        @ 3,2 SAY errmsg
     ENDIF

Visual FoxPro Code:
-------------------

     PUBLIC handle
     dsource="foxtest"
     errval=0
     errmsg=""
  **** foxtest is a datasource name for a single-tier FoxPro database driver.
     handle=SQLConnect(dsource,"","")
     =SQLSetProp(handle,"Asynchronous",.T.)

  **** The customer table must be in the
     sql="select * from customer"

     waittime=2
     start=SECONDS()
     retcode=SQLExec(handle,sql)

  **** This DO WHILE code below is never executed because retcode=1....
  ****  i.e., the DBExec() above returns all the results synchronously.
     DO WHILE ((retcode=0) AND (SECONDS() - start < waittime))
        retcode=SQLExec(handle,sql)
     ENDDO

     IF (retcode= 0)
         retcode=SQLCancel(handle)
     ENDIF

     IF (retcode < 0)
        val=AERROR(atmp)
        errval=atmp(1,1)
        errmsg=atmp(1,2)
        WAIT WINDOW STR(errval)+': '+errmsg
        CLEAR
     ENDIF

   ***** Remove the connection
     =SQLDisconnect(handle)

REFERENCES
==========

FoxPro Connectivity Kit "User's Guide," version 2.5, pages 26-27, 31-32, 35,
38-39, and 46-47

Additional query words: VFoxWin FoxWin 2.50 ck 3.00 2.50a

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Keywords          : kbcode kbvfp300 kbGrpDSFox kbDSupport kbvfp250 
Technology        : kbVFPsearch kbAudDeveloper kbFoxproSearch kbFoxPro260 kbFoxPro250 kbFoxPro250a kbFoxPro250b kbVFP300
Version           : :2.5,2.5a,2.5b,2.6,3.0
Issue type        : kbinfo

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