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July Learn how and when to remove this template message Jet, being part of a relational database management system RDBMSallows the manipulation of relational databases.
It offers a single interface that other software can use to access Microsoft databases and provides support for security, referential integritytransaction processingindexingrecord and page locking, and data replication.
In later versions, the engine has been extended to run SQL queries, store character data in Unicode format, create database views and allow bi-directional replication with Microsoft SQL Server.
Locking[ edit ] 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 modify only those database records that is, items in the database to which the user has applied a lockwhich gives 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.
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 locks one record, 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: 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[ edit ] Jet supports transaction processing for database systems that have this capability.
A transaction is a series of operations performed on a database that must be done together — this is known as atomicity and is one of the ACID Atomicity, Consistency, Isolation, and Durabilityconcepts considered to be the key transaction processing features of a database management system.
For transaction processing to work until Jet 3. Until the transaction is committed, changes are made only in memory and not actually written to disk. One of the main advantages is that transactions can be abandoned if a problem occurs during the transaction.
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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. These are transactions that are started automatically after the last transaction was committed to the database.
However, it was found that this had a negative performance impact in bit Windows Windows 95, Windows 98so in Jet 3. Data integrity[ edit ] Jet enforces entity integrity and referential integrity. Jet will by default prevent any change to a record that breaks referential integrity, but Jet databases can instead use propagation constraints cascading updates and cascading deletes to maintain referential integrity.
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. Security[ edit ] 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.
Queries[ edit ] Queries are the mechanisms that Jet uses to retrieve data from the database.
The query is then compiled — this involves parsing the query involves syntax checking and determining the columns to query in the database tablethen converted into an internal Jet query object format, which is then tokenized and organised into a tree like structure. The query is then executed and the results passed back to the application or user who requested the data.
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