How to: Set Data Access
In a shared environment, you can access data in two ways: from exclusive files or from shared files. If you open a table for shared access, other users also have access to the file. If you open a table for exclusive access, no other user can read or write to that file. Because exclusive use defeats many of the benefits of sharing data on a network, it should be used sparingly.
Using a Table with Exclusive Access
The most restrictive way to open a file is to open it exclusively. When you open a table through the interface, it opens for exclusive use by default. You can also explicitly open a table for exclusive use by using Visual FoxPro commands.
To open a table for exclusive use
Type the following commands in the Command window:
SET EXCLUSIVE ON USE cMyTable-or-
Type the following command in the Command window:
USE cMyTable EXCLUSIVE
The following commands require you to open a table for exclusive use:
INDEX Command when creating, adding, or deleting a compound index tag.
MODIFY STRUCTURE Command To use this command to change a table structure, you must open the table exclusively. You can, however, use this command in read-only mode when you open the table for shared use.
Visual FoxPro returns the error, "Exclusive open of file is required," if you try to execute one of these commands on a shared table.
You can restrict access to a table by using the FLOCK( ) Function. If you use FLOCK( ) to lock the table, other users cannot write to the table but they can read it.
Using a Table with Shared Access
When you open a table for shared use, more than one workstation can use the same table at the same time. When you open a table through the interface, you can override the default ON setting for the SET EXCLUSIVE Command. You can explicitly open a table for shared use by using Visual FoxPro commands.
To open a table for shared use
Type the following commands in the Command window:
SET EXCLUSIVE OFF USE cMyTable-or-
Type the following command in the Command window:
USE cMyTable SHARED
When you add or change data in a shared table, you must first lock the affected record or the entire table. You can lock a record or a table opened for shared use in the following ways:
Use a command that performs an automatic record or table lock.
Manually lock one or more records or an entire table with the record and table locking functions.
Initiate buffering with the CURSORSETPROP( ) Function.
Associated memo and index files always open with the same share status as their table.
If your application uses a table for lookup purposes only and all users of the application access it, then you can improve performance by marking the table as read-only.