Games - Microsoft Jet Database Engine

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	Games - Microsoft Jet Database Engine

The Microsoft Jet Database Engine is a database engine on which several Microsoft products were built. A database engine is the underlying component of a database, a collection of information stored on a computer in a systematic way. The first version of Jet was developed in 1992, consisting of three modules which could be used to manipulate a database.

JET stands for Joint Engine Technology, sometimes being referred to as Microsoft JET Engine or simply Jet. Microsoft Access, Microsoft Exchange Server and Visual Basic use or have used Jet as their underlying database engine. It has since been superseded, however, by Microsoft Desktop Engine (MSDE) and no longer exists as a component of Microsoft Data Access Components (MDAC). Jet databases can be upgraded (or in Microsoft parlance, "up-sized") to an MSDE database.

Architecture

Jet allowed the manipulation of relational database and was part of a Relational Database Management System (RDBMS). It offered a single interface that other software could use to access Microsoft databases and provided support for security, referential integrity, transaction processing, indexing, record, page locking and data replication. In later versions, the engine was extended to be able to run SQL queries, store character data in Unicode format, create database views and allowed bi-directional replication with Microsoft SQL Server. There were three modules to Jet. One was the Native Jet ISAM Driver, a dynamic link library (DLL) that could directly manipulate Microsoft Access database files (MDB), which was a modified form of an Indexed Sequential Access Method (ISAM) database. Another one of the modules were the ISAM Drivers, DLLs that allowed access to ISAM databases, among them being Xbase, Paradox, Btrieve and FoxPro files. The final module was the Data Access Objects (DAO) DLL, DAO allowed programmers access to the Jet engine. It was basically an object-oriented data language used by Access Basic and Visual Basic programmers to access Jet.

Locking

Jet allowed multiple users to access the database concurrently. To prevent that data from being corrupted or invalidated when multiple users tried to write to the database, Jet employed a data write locking policy. Any single user could only modify those database records (that is, items in the database) to which they had applied a lock that gave them exclusive access to the record until the lock was released. Up to Jet 4, a page locking model was used, and in Jet 4 a record locking model was employed. Microsoft databases are organised into data "pages", which are fixed length (2 kB) data structures that divide up the database. Data is stored in "records", but these are of variable length and so may take up less or more than one page. The page locking model worked by locking the pages, instead of individual records, which though less resource intensive also meant that more than one record might be locked at any one time. Record locking was introduced in Jet 4.

There were two mechanisms that Microsoft used for locking: pessimistic locking, and optimistic locking. With pessimistic locking, the record or page is locked immediately when the lock is requested, while with optimistic locking, the synchronization is delayed for transactions until the operations are actually performed. Conflicts are less likely to occur with optimistic locking; since the record is locked for a shorter duration of time, there is a lesser chance of someone needing to access it while it is locked. However, it cannot be certain that the update will succeed because another user could potentially update the record first. With pessimistic locking, the update is guaranteed to succeed once the lock is obtained, but other users are unable to make their own changes until the lock is released. Lock conflicts, which either require the user to wait, or cause the request to fail (usually after a timeout) are more common with this policy.

Transaction processing

Jet supported transaction processing for database systems that had this capability (ODBC systems had one level transaction processing, while several ISAM systems like Paradox did not have transaction processing capability). Transactions are a series of operations performed on a database that must be done together — this is known as atomicity and is a part of ACID (Atomicity, Consistency, Isolation, and Durability), concepts considered to be the key transaction processing features of a database management system. For transaction processing to work (until Jet 3.0), the programmer needed to begin the transaction manually, perform the operations needed to be performed in the transaction, and then commit (save) the transaction. Until the transaction is committed, changes are made only in memory and not actually written to disk . Transactions have a number of advantages over independent database updates. One of the main advantages is that transactions can be abandoned if a problem occurs during the transaction. This is called rolling back the transaction, or just rollback, and it restores the state of the database records to precisely the state before the transaction began. Transactions also permit the state of the database to remain consistent if a system failure occurs in the middle of a sequence of updates required to be atomic. There is no chance that only some of the updates will end up written to the database; either all will succeed, or the changes will be discarded when the database system restarts. With ODBC's in-memory policy, transactions also allow for many updates to a record to occur entirely within memory, with only one expensive disk write at the end.

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