PRODUCT  :  Paradox Engine                        NUMBER  :  1002
  VERSION  :  All
       OS  :  All
     DATE  :  October 19, 1993                         PAGE  :  1/1

    TITLE  :  Lock File Contention




       Lock file contention is caused when many users attempt to
  use a single table at the same time in a tight and repetitive
  loop.  When the Engine opens a table, it attempts to place a PFL
  (Prevent Full Lock) on the table.  To do this, the Engine must
  access the table's lock file.  The Engine detects a Lock by
  opening the lock file and scanning it from beginning to end.
  Each entry in the lock file is checked for potential conflicts
  with the lock that the Engine is attempting to place.

       A problem arises when the lock count for a table drops to
  zero.  When the count reaches zero, the Engine deletes the lock
  file for the table.  However, under MS-DOS, in order to delete a
  file, the file must be closed.  So the Engine closes the table's
  lock file and then attemps to delete the file.  However, another
  user can (re)open the file in the meantime ( i.e. after our user
  closes the lock file but before the file can be deleted ).  Thus,
  our user is on hold waiting for the file to be closed before the
  deletion can succeed.  If the lock file is not closed for a long
  time, the user is effectively locked up and unable to do
  anything.  To avoid the stalled situation, tables should not be
  opened and closed in a tight loop.

       Under Windows, the Paradox Engine optimizes the locking
  necessary between applications running in different windows but
  sharing the same DLL.  If two or more such applications share the
  same table, and the table is on the local disk (not stored on a
  networked disk), the lock files are kept entirely in memory and
  are never created on disk.  If there is memory corruption, the
  state of the locks might be disturbed.  The problem should
  disappear if the optimization is turned off by selecting "Share
  Local Tables" in the configuration utility, PXEngCfg.EXE.


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