Microsoft KB Archive/136128

= ACC: Introduction to .ldb Files (95/97) =

Article ID: 136128

Article Last Modified on 1/19/2007

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APPLIES TO


 * Microsoft Access 95 Standard Edition
 * Microsoft Access 97 Standard Edition

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This article was previously published under Q136128



Moderate: Requires basic macro, coding, and interoperability skills.

This article applies only to a Microsoft Access database (.mdb).



SUMMARY
The .ldb file plays an important role in the multiuser scheme of the Microsoft Jet database engine versions 3.0 and 3.5. The .ldb file is used to determine which records are locked in a shared database and by whom.



Automatic .ldb File Creation and Deletion
For every database opened for shared use, an .ldb file is created to store computer and security names and to place extended byte range locks. The .ldb file always has the same name and is located in the same folder as the opened database (.mdb). For example, if you open (for shared use) the Northwind.mdb sample database in the C:\MIcrosoft Office\Office\Access\Samples folder, then a file called Northwind.ldb is automatically created in the same folder.

Whenever the last user closes a shared database, the .ldb file is deleted. The only exception is if a user does not have delete rights or when the database is marked as corrupted; then, the .ldb file is not deleted because it contains information about who was using the database at the time the database was marked as corrupted.

Required Folder Permissions
If you plan to share a database, the .mdb file should be located in a folder where users have read, write, create, and delete (or full control) permissions. Even if you want users to have different file permissions (for example, some read-only and some read-write), all users sharing a database must have read, write, and create permissions to the folder. You can, however, assign read-only permissions to the .mdb file for individual users while still allowing full permissions to the folder.

Note If a user opens a database with exclusive access (by selecting the Exclusive check box in the Open dialog box), record locking is not used; therefore, Microsoft Access does not attempt to open or create an .ldb file. If the database is always opened for exclusive use, a user needs only read and write permissions to the folder.

The .ldb File Contents
For each person who opens a shared database, the Jet database engine writes an entry in the database's .ldb file. The size of each .ldb entry is 64 bytes. The first 32 bytes contains the computer name (such as JohnDoe). The second 32 bytes contains the security name (such as Admin). The maximum number of concurrent users that the Jet database engine supports is 255; therefore, the .ldb file size is never larger than 16 kilobytes.

Note Although a file-server solution can support up to 255 simultaneous users, if the users of your solution will be frequently adding data and updating data, it is a good idea for an Access file-server solution to support no more than 25 to 50 users.

When a user closes a shared database, the user's entry is not removed from the .ldb file. However, it may be overwritten when another user opens the database. This means, you cannot use the .ldb file alone to determine who is currently using the database.

The .ldb File Usage
The Jet database engine uses .ldb file information to prevent users from writing data to pages that other users have locked and to determine who has other pages locked. If the Jet database engine detects a lock conflict with another user, it reads the .ldb file to get the computer and security name of the user who has the file or record locked.

In most lock conflict situations, you receive a generic "Write conflict" error message that allows you to save the record, copy it to the Clipboard, or drop the changes you made. In some circumstances, however, you receive the following error message:

Couldn't lock table ; currently in use by user on computer.

Note The state of the information in the .ldb file has no bearing on the state of the database. If an .ldb file becomes corrupted, everything in the database should still work correctly. However, you may see garbled text instead of user names in any lock conflict messages.

