Microsoft Office 14.0 Access Database Engine Object Library
Flocka Bilodeau
Libraries are components that provide functionality. Access itself and its programming language (VBA) are two essential libraries in every project. If Access does not provide something you need (such as a calendar or tree-view), you may be able to find a library and add it. However, adding extra libraries increases the number of things that can go wrong.
microsoft office 14.0 access database engine object library
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If a library is marked MISSING, click the Browse button, and locate the correct file for the library. The table below lists the files for the crucial libraries of your version of Access.
Occasionally, the problem is not solved until you unregister the library and re-register it. Uncheck the missing library in Access. Close Access. Issue this command, and then the one above to re-register it:
regsvr32 -u "c:\program files\common files\microsoft shared\dao\dao360.dll"
Re-register it (as above). Open Access, and select the library reference again.
Ambiguities arise if libraries have objects with the same name. For example, the DAO and ADO libraries both have a Recordset object, so Access 2000 and 2002 often choke when a DAO recordset was intended. Any of these ideas will solve the clash:
Inconsistencies and data corruption can result from referencing the wrong library for your version of Access. For example, the DAO 3.5 library was designed for Access 97, whereas DAO 3.6 is designed for Access 2000, 2002, and 2003. Access 2007 uses the ACEDAO library to give you the new functionality in the private JET engine. Use the table below to ensure you reference the correct files for your version of Access.
Unnecessary libraries waste memory, slow loading, increase the chance of problems, and hamper debugging. The three essentials are VBA, Access, DAO. The VBA and Access libraries are built in, and will always have highest priority. Access will not let you remove these libraries from your database.
DAO stands for Data Access Objects. It is the object model written specifically for Access, so it's no surprise that it gives the best power and performance for data stored in Access tables.
ADO stands for ActiveX Data Objects. It is a more generic library, designed to handle data from sources other than Access tables (SQL Server) and interfaces other than Access (e.g. web-based.) If you are working on these enterprise databases, you don't need an explanation of ADO here.
Unfortunately, Microsoft made ADO the default library in Access 2000 and 2002. DAO is back by default in Access 2003 and 2007. Michael Kaplan illustrates why you need DAO in his blog posting, What does DAO have that ADO/ADOX/JRO do not?
Include other libraries when you have a reason to do so. For example, referencing the Microsoft Office 10.0 Object Library in Access 2002 gives you the FileDialog object so you can show the user a File Open dialog without resorting to API calls. (Note: FileDialog does not work in MDEs or the runtime version, and the msoFileDialogSaveAs option doesn't work at all.)
JET stands for Joint Engine Technology. Microsoft Access and Visual Basic use or have used Jet as their underlying database engine. However, it has been superseded for general use, first by Microsoft Desktop Engine (MSDE), then later by SQL Server Express. For larger database needs, Jet databases can be upgraded (or, in Microsoft parlance, "up-sized") to Microsoft's flagship SQL Server database product.
A five billion record MS Jet (Red) database with compression and encryption turned on requires about one terabyte of disk storage space[citation needed]. It comprises typically hundreds of *.mdb files.
Jet, being part of a relational database management system (RDBMS), allows the manipulation of relational databases.[1] 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 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 a (random access) file system API. 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. The final module is the Data Access Objects (DAO) DLL.[2] 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 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. 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. 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: 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.
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.
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. 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.[3] When Jet 2 was released, security could also be set programmatically through DAO.
Jet passes the data retrieved for the query in a dynaset. This is a set of data that is linked dynamically 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, the user would only need to update the data in the dynaset, which would automatically update the student's database record without the need for the user 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."
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