INF: How the dBASE Driver Performs Locking

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

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 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.