Jet supports transaction processing for database systems that have this capability. Lock conflicts, which either require the user to wait, or cause the request to fail (usually after a timeout) are more common with pessimistic locking. Other users must wait until the lock is released in order to make their changes.
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With pessimistic locking, the update is guaranteed to succeed once the lock is obtained. However, with optimistic locking one cannot be certain that the update will succeed because another user could lock the record first. Conflicts are less likely to occur with optimistic locking, since the record is locked only for a short period of time. With pessimistic locking, the record or page is locked immediately when the lock is requested, while with optimistic locking, the locking is delayed until the edited record is saved. There are two mechanisms that Microsoft uses for locking: pessimistic locking, and optimistic locking. In Jet 4, the record locking model eliminates collateral locks, so that every record that is not in use is available. As a result, no other user can access the collaterally locked records, even though no user is accessing them and there is no need for them to be locked. The page locking model works by locking the pages, instead of individual records, which though less resource-intensive also means that when a user locks one record, all other records on the same page are collaterally locked. Data is stored in "records" of variable length that may take up less or more than one page. Microsoft databases are organized into data "pages", which are fixed-length (2 kB before Jet 4, 4 kB in Jet 4) data structures. In Jet versions before version 4, a page locking model is used, and in Jet 4, a record locking model is employed. Any single user can modify only those database records (that is, items in the database) to which the user has applied a lock, which gives exclusive access to the record until the lock is released. To prevent that data from being corrupted or invalidated when multiple users try to edit the same record or page of the database, Jet employs a locking policy. Jet allows multiple users to access the database concurrently.
DAO provides an API that allows programmers to access JET databases using any programming language. The final module is the Data Access Objects (DAO) DLL. Another one of the modules contains the ISAM Drivers, DLLs that allow access to a variety of Indexed Sequential Access Method ISAM databases, among them xBase, Paradox, Btrieve and FoxPro, depending on the version of Jet. There are three modules to Jet: One is the Native Jet ISAM Driver, a dynamic link library (DLL) that can directly manipulate Microsoft Access database files (MDB) using a (random access) file system API.
It comprises typically hundreds of *.mdb files. For larger database needs, Jet databases can be upgraded (or, in Microsoft parlance, "up-sized") to Microsoft's flagship SQL Server database product.Ī five billion record MS Jet (Red) database with compression and encryption turned on requires about one terabyte of disk storage space. However, it has been superseded for general use, first by Microsoft Desktop Engine (MSDE), then later by SQL Server Express. Microsoft Access and Visual Basic use or have used Jet as their underlying database engine. The first version of Jet was developed in 1992, consisting of three modules which could be used to manipulate a database. The Access Database Engine (also Office Access Connectivity Engine or ACE and formerly Microsoft Jet Database Engine, Microsoft JET Engine or simply Jet) is a database engine on which several Microsoft products have been built.
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