How to Use the SYS(3052) Function to Avoid Scrolling Error

MORE INFORMATION

The "Record not available, please wait" message is displayed when a table is updated but the index or memo file cannot be locked. There is not a way to escape or cancel this warning message until the contention has been resolved. By modifying the locking order of files, the SYS(3052) function can help suppress the scrolling message.

Currently, for performance optimization, Visual FoxPro performs record locks on a table and file locks on a memo and index file in the following order: When the index and memo attributes of the SYS(3052) function are set to True (.T.), the locking and updating order is as follows: As a result of using the SYS(3052) function, the scrolling "Record not available, please wait" message is less likely to occur. If this message is displayed, an abnormal shutdown would cause less discrepancy because neither the table nor the memo or index files have been updated.

The following example illustrates how to use the SYS(3052) function. To run it:

1. Copy and paste the two sample programs into two program files, Server.prg and Client.prg.
2. Start two instances of Visual FoxPro and run Server.prg.
3. When the programs have run, issue an END TRANSACTION command in the instance of Visual FoxPro that ran Server.prg.

Sample Programs

*:*************************************************************************
*:*: Procedure File Server.prg
*:*************************************************************************
*:   SERVER
   SET SAFETY OFF
   SET EXCLUSIVE OFF

   CLOSE DATA ALL
   IF FILE('TSTSRVER.dbc')
       DELETE DATA TSTSRVER DELETETABLES
   ENDIF

   =SYS(3052,1,.T.)
   =SYS(3052,2,.T.)

   CREATE DATA TSTSRVER  && Creates a database and a table
   CREATE TABLE xxx (dbkey i PRIMARY KEY, field2 c(5), field3 m)

   INSERT INTO xxx VALUES (1, 'aaaaa', 'long string 1')
   INSERT INTO xxx VALUES (2, 'bbbbb', 'long string 2')
   INSERT INTO xxx VALUES (3, 'ccccc', 'long string 3')
   INSERT INTO xxx VALUES (4, 'ddddd', 'long string 4')
   INSERT INTO xxx VALUES (5, 'eeeee', 'long string 5')

   CLOSE DATA ALL
   OPEN DATA TSTSRVER SHARED
   =MESSAGEBOX('Start the Client program on the second instance now.')
   USE xxx SHARED  && Executes after the Client program is started

   BEGIN TRANSACTION
   GOTO 2
   REPLACE field3 WITH 'Changed String'
   SKIP
   ?'Return to client and press ok.'

*:*************************************************************************
*: Procedure File Client.prg
*:*************************************************************************
*: CLIENT
   SET SAFETY OFF
   SET EXCLUSIVE OFF
   CLOSE DATA ALL

   =SYS(3052,1,.T.)  && Changes the lock and write order
   =SYS(3052,2,.T.)

   USE XXX SHARED
   =MESSAGEBOX('Switch to server and press OK')

   GOTO 1
   REPLACE field3 WITH 'Changed String from client'
   SKIP
				

When these programs are run, the message "Attempting To Lock. Press Cancel" is displayed. When you press the ESC key, the error "Record is in Use by another user" appears, or your ON ERROR routine is triggered.

If you change the settings of SYS(3052) to false (.F.) in Client.prg, the scrolling message "Record not available, please wait" is displayed.