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

The information in this article applies to:

Microsoft Access versions 7.0, 97

SUMMARY

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

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.

MORE INFORMATION

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:\Msoffice\Access 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 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.

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 <table name>; currently in use by user <security name> on computer <computer name>.

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.

REFERENCES

For more detailed information about the .ldb file and extended byte range locks, please see the Jet Locking white paper. For details about how to obtain this white paper, please see the following article in the Microsoft Knowledge Base:

Q176670 ACC: Microsoft Jet Utilities Available on MSL

Keywords          : kbusage GnlMu 
Version           : 7.0 97
Platform          : WINDOWS 
Issue type        : kbinfo