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Introduction to .ldb files
Article ID: 299373 - View products that this article applies to.
This article was previously published under Q299373
Moderate: Requires basic macro, coding, and interoperability skills.
This article applies only to a Microsoft Access database (.mdb).
For a Microsoft Access 2000 version of this article, see 208778
For a Microsoft Access 97 version of this article, see 136128
The .ldb file plays an important role in the multiuser scheme of the Microsoft Jet database engine version 4.0. The .ldb file is used to determine which records are locked in a shared database and by whom.
Automatic .ldb file creation and deletionFor every database that is 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 as the opened database (.mdb), and it is located in the same folder as the opened database. For example, if you open (for shared use) the Northwind.mdb sample database in the C:\Program Files\Microsoft Office\Office 10\Samples folder, a file named Northwind.ldb is automatically created in the same folder.
Whenever the last user closes a shared database, the .ldb file is deleted. The only exceptions are when 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 privilegesIf you plan to share a database, the .mdb file should be located in a folder where users have read, write, create, and delete privileges. Even if you want users to have different file privileges (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 clicking the arrow to the right of the Open button, and then clicking Open Exclusive), 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 to have only read and write privileges to the folder.
The .ldb file contentsFor each person who opens a shared database, the Jet database engine writes an entry in the .ldb file of the database. The size of each .ldb entry is 64 bytes. The first 32 bytes contain the computer name (such as JohnDoe). The second 32 bytes contain 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. For more information, visit the following Microsoft Web site:
When a user closes a shared database, the user's entry is not removed from the .ldb file. However, the user's entry may be overwritten when another user opens the database. This means that you cannot use the .ldb file alone to determine who is currently using the database.
The .ldb file usageThe Jet database engine uses .ldb file information to prevent users from writing data to pages or records that other users have locked and to determine who has other pages or records 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" message that allows you to save the record, copy it to the Clipboard, or drop the changes that you made. In some circumstances, however, you receive the following error message:
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 scrambled text instead of user names in any lock conflict messages.
Couldn't lock table <table name>; currently in use by user <security name> on computer <computer name>.
With Microsoft Visual Basic for Applications in Access 2002 or later, you can output a list of users who are logged into a specific database.
For additional information about how to do this and sample code, click the following article number to view the article in the Microsoft Knowledge Base:
(http://support.microsoft.com/kb/285822/ )How to determine who is logged on to a database by using Microsoft Jet UserRoster in Access 2002 or in Access 2003