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Q95081: INF: How the dBASE Driver Performs Locking

Article: Q95081
Product(s): Open Database Connectivity (ODBC)
Version(s): 
Operating System(s): 
Keyword(s): 
Last Modified: 30-JUL-2001

1.00

MS-DOS

kbusage

SUMMARY
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This article discusses the locking mechanisms of the dBASE driver in single and
multiuser modes.

MORE INFORMATION
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The ODBC dBASE driver is capable of using a dBASE data source in single and
multiuser modes. The data source is in single user mode by default, after setup
of the data source is done. There is an entry called

  SingleUser=TRUE

in the ODBC.INI file, under the section for that data source. Changing the value
of this entry to FALSE will put the data source in multiuser mode.

As far as the dBASE driver is concerned, a "dBASE data source" is a directory
containing dBASE files (data files, index files, and so on). In single user
mode, all the tables in the data source (that is, all the .DBF files in the
directory) are exclusively locked by the dBASE driver. Also, no other user can
connect to the data source. In multiuser mode, multiple users can access the
same data source and the same table (file) simultaneously. If a user wants to
update a record(s), then it is exclusively locked. Whether other users can read
the record depends on the whether the table is a dBASE III or a dBASE IV table.
In a dBASE III table, other users cannot read the record. In a dBASE IV table,
other users can read this record ("dirty reads" are permitted).

The fact that record locking is done in multiple user mode does not imply that
any kind of transaction serializability is available. The dBASE driver is not
transaction capable and does not provide any kind of transaction isolation
options. So, if you are browsing through a table and want to update a record,
there is no guarantee that the record will not be updated behind your back.

Additional query words: 1.00

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