V_LOCK_METHOD

This variable selects which locking method Vision uses to control simultaneous access to indexed files. It affects only the Vision file system, and only files directly accessed by the runtime (it does not apply to files accessed via AcuServer).

The default setting of "0" causes Vision to lock the first byte of the file for every access to the file (both reads and updates). This ensures that the process is not interfered with by another process. This locking method is always used by Vision Version 2 files.

Setting this variable to "1" (one) causes Vision to lock the first byte of the file for all operations except random READs or READ NEXTs. These two operations proceed without the lock. Instead they perform some additional reads of the file, to ensure that they get consistent results. If they get inconsistent results, they are retried, this time locking the first byte as other operations do. This locking method is available only for Vision Version 3 and Version 4 files.

Note that this variable must have the same setting for all the runtimes accessing a file, whether they are reading or writing to it. For example, if a runtime set with V_LOCK_METHOD=1 is reading from a file, any runtimes that are writing to that same file must also have V_LOCK_METHOD set to 1.

Lock method "1" can produce better performance on some machines. These machines fall into two categories:

• Machines that take a long time to place a lock.
• Machines that do not queue lock requests, and are very busy. In this case, some users typically get good performance, while others get poor performance.

If you are experiencing poor performance on Vision version 3 or 4 files, setting V_LOCK_METHOD to "1" may help.

Setting this variable to "2" (two) enables "asynchronous reads" of Vision files. This can reduce the number of file locks required to perform READs and READ NEXTs. V_LOCK_METHOD=2 works only for Vision 4 files.

If any process reading a particular file is using V_LOCK_METHOD=2, all other processes updating that file must be Version 5.0.0 or greater. This is because Version 5.0.0 contains changes that affect the way Vision updates the tree structure of its files. This requirement is not enforced by Vision, however, so it is important for users to pay careful attention to the versions of programs accessing the files to avoid receiving erroneous data.