Microsoft Jet Database Engine
The Microsoft Jet Database Engine is a database engine on which several Microsoft products have been 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 and Visual Basic use or have used Jet as their underlying database engine. It has since been superseded for general use, however, first by Microsoft Desktop Engine (MSDE), then later by SQL Server Express. Jet is now part of Microsoft Windows and is no longer a component of Microsoft Data Access Components (MDAC). For larger database needs, Jet databases can be upgraded (or, in Microsoft parlance, "up-sized") to Microsoft's flagship database product, SQL Server.
Over the years, Jet has become almost synonymous with Microsoft Access, to the extent where many people refer to a Jet database as an "Access database".
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Jet allows the manipulation of a relational database and is part of a Relational Database Management System (RDBMS). It offers a single interface that other software can use to access Microsoft databases and provides support for security, referential integrity, transaction processing, indexing, record and page locking, and data replication. In later versions, the engine has been extended to be able to run SQL queries, store character data in Unicode format, create database views and allow bi-directional replication with Microsoft SQL Server.
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 Indexed Sequential Access Method (ISAM). Another one of the modules contains the ISAM Drivers, DLLs that allow access to a variety of ISAM databases, among them Xbase, Paradox, Btrieve and FoxPro, depending on the version of Jet. The final module is the Data Access Objects (DAO) DLL. DAO provides an API that allows programmers to access JET databases using any programming language.
Jet allows multiple users to access the database concurrently. 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. Any single user can only modify those database records (that is, items in the database) to which they have applied a lock that gives them exclusive access to the record until the lock is released. In Jet versions before version 4, a page locking model is used, and in Jet 4, a record locking model is employed. Microsoft databases are organized into data "pages", which are fixed length (2 kB before Jet 4, 4 kB in Jet 4) data structures that divide up the database. Data is stored in "records" of variable length that may take up less or more than one page. The page locking model works by locking the pages, instead of individual records, which though less resource intensive also means that when a user intentionally locks one record, any and all other records on the same page are collaterally locked. 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. In Jet 4, the record locking model eliminates collateral locks, so that every record that is not in use is available.
There are two mechanisms that Microsoft uses 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 locking is delayed until the edited record is saved. Conflicts are less likely to occur with optimistic locking, since the record is locked only for a short period of time. However, with optimistic locking one cannot be certain that the update will succeed because another user could lock the record first. With pessimistic locking, the update is guaranteed to succeed once the lock is obtained. Other users must wait until the lock is released in order to make their changes. 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.
Transaction processing 
Jet supports transaction processing for database systems that have this capability (ODBC systems have one level transaction processing, while several ISAM systems like Paradox do not have transaction processing capability). A transaction is 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.
Implicit transactions were supported in Jet 3.0. These are transactions that are started automatically after the last transaction was committed to the database. Implicit transactions in Jet occurred when an SQL DML statement was issued. However, it was found that this had a negative performance impact in 32-bit Windows (Windows 95, Windows 98), so in Jet 3.5 Microsoft removed implicit transactions when SQL DML statements were made.
Data integrity 
Jet enforces entity integrity and referential integrity. Entity integrity is one of the key concepts of relational databases, and ensures that no two records can be identical, that is, have fields containing identical values. It also ensures that no key field including (the primary key) can contain a NULL value. Jet enforces referential integrity by ensuring that foreign keys always reference primary keys in other tables in that database. If a foreign key value exists that does not have a corresponding primary key in the referenced table, then the referential integrity is broken and the data between tables will no longer be synchronised.
For instance, a music lover may have a database that stores information about a record collection, and need to store data about an artist and his/her music. In this example, the artist can record many albums, but the album is only recorded by one artist, so two database tables are created: Artist and Album. The Artist table uses the field artist_id as its primary key, and the Album table uses album_id. The album table references the artist table using artist_id as a foreign key. If, for some reason, an artist is deleted and there is an album in the system that contains a reference to that artist then the referential integrity of this record would be broken. Jet will by default prevent this from happening. Jet is also capable of doing cascading updates and deletes. With cascading deletes enabled for the Album table, if the artist in the previous example were deleted, then all the artists' albums would also be deleted.
Jet also supports "business rules" (also known as "constraints"), or rules that apply to any column to enforce what data might be placed into the table or column. For example, a rule might be applied that does not allow a date to be entered into a date_logged column that is earlier than the current date and time, or a rule might be applied that forces people to enter a positive value into a numeric only field.
Access to Jet databases is done on a per user-level. The user information is kept in a separate system database, and access is controlled on each object in the system (for instance by table or by query). In Jet 4, Microsoft implemented functionality that allows database administrators to set security via the SQL commands CREATE, ADD, ALTER, DROP USER and DROP GROUP. These commands are a subset of ANSI SQL 92 standard, and they also apply to the GRANT/REVOKE commands. When Jet 2 was released, security could also be set programmatically through DAO.
Queries are the mechanisms that Jet uses to retrieve data from the database. They can be defined in Microsoft QBE (Query By Example), through the Microsoft Access SQL Window or through Access Basic's Data Access Objects (DAO) language. These are then converted to an SQL SELECT statement. The query is then compiled — this involves parsing the query (involves syntax checking and determining the columns to query in the database table), then converted into an internal Jet query object format, which is then tokenized and organised into a tree like structure. In Jet 3.0 onwards these are then optimised using the Microsoft Rushmore query optimisation technology. The query is then executed and the results passed back to the application or user who requested the data.
Jet passes the data retrieved for the query in a dynaset. This is a set of data that is dynamically linked back to the database. Instead of having the query result stored in a temporary table, where the data cannot be updated directly by the user, the dynaset allows the user to view and update the data contained in the dynaset. Thus, if a university lecturer queries all students who received a distinction in their assignment and finds an error in that student's record, they would only need to update the data in the dynaset, which would automatically update the student's database record without the need for them to send a specific update query after storing the query results in a temporary table.
Jet originally started in 1992 as an underlying data access technology that came from a Microsoft internal database product development project, code named Cirrus. Cirrus was developed from a pre-release version of Visual Basic code and was used as the database engine of Microsoft Access. Tony Goodhew, who worked for Microsoft at the time, says
"It would be reasonably accurate to say that up until that stage Jet was more the name of the team that was assigned to work on the DB engine modules of Access rather than a component team. For VB [Visual Basic] 3.0 they basically had to tear it out of Access and graft it onto VB. That's why they've had all those Jet/ODBC problems in VB 3.0."
Jet became more componentised when Access 2.0 was released because the Access ODBC developers used parts of the Jet code to produce the ODBC driver. A retrofit was provided that allowed Visual Basic 3.0 users to use the updated Jet issued in Access 2.0.
Jet 2.0 was released as several dynamic linked libraries (DLLs) that were utilised by application software, such as Microsoft's Access database. DLLs in Windows are "libraries" of common code that can be used by more than one application—by keeping code that more than one application uses under a common library which each of these applications can use independently code maintenance is reduced and the functionality of applications increases, with less development effort. Jet 2.0 comprised three DLLs: the Jet DLL, the Data Access Objects (DAO) DLL and several external ISAM DLLs. The Jet DLL determined what sort of database it was accessing, and how to perform what was requested of it. If the data source was an MDB file (a Microsoft Access format) then it would directly read and write the data to the file. If the data source was external, then it would call on the correct ODBC driver to perform its request. The DAO DLL was a component that programmers could use to interface with the Jet engine, and was mainly used by Visual Basic and Access Basic programmers. The ISAM DLLs were a set of modules that allowed Jet to access three ISAM based databases: Xbase, Paradox and Btrieve.. Jet 2.0 was replaced with Jet 2.1, which used the same database structure but different locking strategies, making it incompatible with Jet 2.0
Jet 3.0 included many enhancements, including a new index structure that reduced storage size and the time that was taken to create indices that were highly duplicated, the removal of read locks on index pages, a new mechanism for page reuse, a new compacting method for which compacting the database resulted in the indices being stored in a clustered-index format, a new page allocation mechanism to improve Jet's read-ahead capabilities, improved delete operations that speeded processing, multithreading (three threads were used to perform read ahead, write behind, and cache maintenance), implicit transactions (users did not have to instruct the engine to start manually and commit transactions to the database), a new sort engine, long values (such as memos or binary data types) were stored in separate tables, and dynamic buffering (whereby Jet's cache was dynamically allocated at start up and had no limit and which changed from a first in, first out (FIFO) buffer replacement policy to a least recently used (LRU) buffer replacement policy). Jet 3.0 also allowed for database replication. Jet 3.0 was replaced by Jet 3.5, which uses the same database structure, but different locking strategies, making it incompatible with Jet 3.0.
Jet 4.0 gained numerous additional features and enhancements.
- Unicode character storage support, along with an NT sorting method that was also implemented in the Windows 95 version;
- Changes to data types to be more like SQL Server's (LongText or Memo; Binary; LongBinary; Date/Time; Real; Float4; IEEESingle; Double; Byte or Tinyint; Integer or Integer synonyms Smallint, Integer2, and Short; LongInteger or LongInteger synonyms Int, Integer, and Counter; Currency or Money; Boolean and GUID); a new decimal data type
- Memo fields could now be indexed
- Compressible data types
- SQL enhancements to make Jet conform more closely to ANSI SQL-92
- Finer grained security; views support; procedure support
- Invocation and termination (committing or rolling back) of transactions
- Enhanced table creation and modification
- Referential integrity support
- Connection control (connected users remain connected, but once disconnected they cannot reconnect, and new connections cannot be made. This is useful for database administrators to gain control of the database)
- A user list, which allows administrators to determine who is connected to the database
- Record-level locking (previous versions only supported page-locking)
- Bi-directional replication with MS SQL Server.
Jet 4 databases can be "upsized" (upgraded) to "an equivalent database on SQL Server with the same table structure, data, and many other attributes of the original database" if the developer has a copy of Microsoft Office 2000 Professional Edition via a Microsoft Access Upsizing Wizard utility. Reports, queries, macros and security are not handled by this tool, meaning that some manual modifications may need to be done if the developer has been heavily reliant on these Jet features.
Jet has been included in every version of Windows from Windows 2000 to Windows 7, and therefore is no longer distributed separately with the Microsoft Data Access Components (MDAC). Access 2003 relied on the Jet engine component of the operating system for its data storage and query processing.
With version 2007 onwards, Access includes an Office-specific version of Jet, initially called the Office Access Connectivity Engine (ACE), but which is now called the Access Database Engine. This engine is fully backward-compatible with previous versions of the Jet engine, so it reads and writes (.mdb) files from earlier Access versions. It introduces a new default file format, (.accdb), that brings several improvements to Access, including complex data types such as multivalue fields, the attachment data type and history tracking in memo fields. It also brings security and encryption improvements and enables integration with Microsoft Windows SharePoint Services 3.0 and Microsoft Office Outlook 2007.
The engine in Microsoft Access 2010 discontinued support for Access 1.0, Access 2.0, Lotus 1-2-3 and Paradox files. A 64-bit version of the ACE Driver/Provider has been introduced, which in essence provides a 64-bit version of Jet. The driver is not part of the Windows operating system, but is available as a redistributable. Previously the Jet Database Engine was only 32-bit and did not run natively under 64-bit versions of Windows. This meant that native 64-bit applications (such as the 64-bit versions of SQL Server) could not access data stored in MDB files through ODBC, OLE DB, or any other means, except through intermediate 32-bit software (running in WoW64) that acted as a proxy for the 64-bit client.
From a data access technology standpoint, Jet is considered a deprecated technology by Microsoft, but it is an intrinsic part of Windows, and therefore Microsoft continues to support it.
Microsoft provides the JET drivers only for Microsoft Windows. Therefore, third party software support for JET databases is almost exclusively found on Windows. There is an Open Source project that attempts to enable working with JET databases on other platforms, the MDB Tools and its much extended Java port named Jackcess.
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