Microsoft Data Access Components (MDAC): Обзор и Архитектура

Framework

Microsoft Data Access Components (MDAC; also known as Windows DAC) is a framework of interrelated Microsoft technologies that allows programmers a uniform and comprehensive way of developing applications that can access almost any data store. Its components include: ActiveX Data Objects (ADO), OLE DB, and Open Database Connectivity (ODBC). There have been several deprecated components as well, such as the Jet Database Engine, MSDASQL (the OLE DB provider for ODBC), and Remote Data Services (RDS). Some components have also become obsolete, such as the former Data Access Objects API and Remote Data Objects. The first version of MDAC was released in August 1996. At that time Microsoft stated MDAC was more a concept than a stand alone program and had no widespread distribution method. Later Microsoft released upgrades to MDAC as web based redistributable packages. Eventually, later versions were integrated with Microsoft Windows and Internet Explorer, and in MDAC 2.8 SP1 they ceased offering MDAC as a redistributable package. Throughout its history, MDAC has been the subject of several security flaws, which led to attacks such as an escalated privileges attack, although the vulnerabilities were generally fixed in later versions and fairly promptly. The current version is 2.8 service pack 1, but the product has had many different versions and many of its components have been deprecated and replaced by newer Microsoft technologies. MDAC is now known as Windows DAC in Windows Vista.

The latest version of MDAC (2.8) consists of several interacting components, all of which are Windows specific except for ODBC (which is available on several platforms). MDAC architecture may be viewed as three layers: a programming interface layer, consisting of ADO and ADO. NET, a database access layer developed by database vendors such as Oracle and Microsoft (OLE DB, NET managed providers and ODBC drivers), and the database itself. These component layers are all made available to applications through the MDAC API. The Microsoft SQL Server Network Library, a proprietary access method specific to Microsoft SQL Server, is also included in the MDAC. Developers of Windows applications are encouraged to use ADO or ADO. NET for data access, the benefit being that users of the application program are not constrained in their choice of database architecture except that it should be supported by MDAC. Naturally, developers still have the choice of writing applications which directly access OLE DB and ODBC.

The Microsoft SQL Server Network Library (also known as Net Lib) is used by the Microsoft SQL Server to read and write data using many different network protocols. Though Net Lib is specific to the SQL Server, Microsoft includes it with MDAC. The SQL Server uses the Open Data Services (ODS) library to communicate with Net Lib, which interfaces directly with the Windows NT operating system line's Win32 subsystem. The SQL Server Network Library is controlled through the use of a Client Network Utility, which is bundled with the SQL Server. Each Net Lib supported network protocol has a separate driver (not to be confused with a device driver), and has support for a session layer in its protocol stack. There are two general types of Net Lib: the primary and the secondary. The primary Net Lib consists of a Super Socket Net Lib and the Shared Memory Net Lib, while there are numerous secondary Net Libs, including TCP/IP and named pipes network libraries (named pipes are a method of communicating with other processes via a system persistent pipeline that is given an identity). The Microsoft OLE DB Provider for SQL Server (SQLOLEDB) communicates via primary Net Libs. The Super Socket Net Lib deals with inter computer communications and coordinates the secondary Net Libs – though the TCP/IP secondary Net Lib is an exception in that it calls on the Windows Sockets 2 API directly. The Banyan VINES, AppleTalk, ServerNet, IPX/SPX, Giganet, and RPC Net Libs were dropped from MDAC 2.5 onwards. The Network Library router had the job of managing all these protocols, however now only the named pipes secondary Net Lib is managed by the router. The Super Socket Net Lib also handles data encryption via the use of the Windows SSL API. The Shared Memory Net Lib, on the other hand, manages connections between multiple instances of SQL Server that exist on one computer. It uses a shared memory area to communicate between the processes. This is inherently secure; there is no need for data encryption between instances of SQL Server that exist on one computer as the operating system does not allow any other process access to the instances' area of shared memory. Net Lib is also able to support the impersonation of a logged in user's security context for protocols that support authenticated connections (called trusted connections). This allows Net Lib to provide an integrated logon authentication mechanism via the use of Windows Authentication. Windows Authentication is not supported on Windows 98 or Windows Me.

OLE DB (also called OLEDB or OLE DB) allows MDAC applications access to different types of data stores in a uniform manner. Microsoft has used this technology to separate the application from data can store in the website the data store that it needs to access. This was done because different applications need access to different types and sources of data, and do not necessarily need to know how to access technology specific functionality. The technology is conceptually divided into consumers and providers. The consumers are the applications that need access to the data, and the provider is the software component that exposes an OLE DB interface through the use of the Component Object Model (or COM). OLE DB is the database access interface technology used by MDAC. OLE DB providers can be created to access such simple data stores as a text file or spreadsheet, through to such complex databases as Oracle and SQL Server. However, because different data store technology can have different capabilities, OLE DB providers may not implement every possible interface available. The capabilities that are available are implemented through the use of COM objects an OLE DB provider will map the data store technology's functionality to a particular COM interface. Microsoft calls the availability of an interface to be "provider specific" as it may not be applicable depending on the database technology involved. Additionally, however, providers may also augment the capabilities of a data store; these capabilities are known as services in Microsoft parlance. The Microsoft OLE DB Provider for SQL Server (SQLOLEDB) is the OLE DB provider that Microsoft provides for the Microsoft SQL Server from version 6.5 upwards.

Universal data link files (or '. udl files') provide a common user interface for specifying connection attributes. A user can use a Data Link Properties dialog box to save connection information in a udl file as an alternative to directly specifying them by hand in a connection string. Consequently, these files provide a convenient level of indirection. Additionally, the dialog box specifies a number of alternate OLE DB data providers for a variety of target applications.

Open Database Connectivity (ODBC) is a native interface that is accessed through a programming language (usually C) that can make calls into a native library. In MDAC this interface is defined as a DLL. A separate module or driver is needed for each database that must be accessed. The functions in the ODBC API are implemented by these DBMS specific drivers. The driver that Microsoft provides in MDAC is called the SQL Server ODBC Driver (SQLODBC), and (as the name implies) is designed for Microsoft's SQL Server. It supports SQL Server v6.5 and upwards. ODBC allows programs to use SQL requests that will access databases without having to know the proprietary interfaces to the databases. It handles the SQL request and converts it into a request that the individual database system understands. According to Microsoft, "After SQL Server 2012, the ODBC driver will be updated for the most recent server features, including Microsoft Windows Azure SQL Database, and released as the Microsoft ODBC Driver for SQL Server."

ADO. NET is the latest version of ADO (after ADO 2.8, now often referred to as ADO Classic) and is part of the MDAC 2.8 stack alongside classic ADO. It is built around Microsoft NET. Though sometimes seen as an evolutionary step up from ADO, some fundamental structural changes were made by Microsoft. ADO. NET runs through a NET Managed Provider, a modified version of an OLE DB provider specifically designed for NET. The object structure is no longer built around a Recordset object. Instead a Dataset object is used to contain data gathered from multiple sources. This is transparent to the programmer. Unlike the old ADO Recordset, the Dataset's design promotes the use of disconnected data. Conceptually, a Dataset object can be seen as a small in memory relational database in its own right that allows for manipulation of data in any direction. In order to propagate changes back into the database, a DataAdapter object is used that transfers data from between the data source and the DataSet object. Cursors were also deprecated in ADO. NET, being replaced with a DataReader object, which is used to efficiently process a large list of results one record at a time without storing them.

MDAC is a continually evolving component framework. As such, there have been several components that were previously part of it but have since been deprecated or removed entirely from the framework.

Jet stands for Joint Engine Technology and was a database engine used for Microsoft Access, Microsoft Exchange Server and Visual Basic. Jet was part of a Relational Database Management System (RDBMS) and offered a single interface that other software could use to access Microsoft databases. Jet also provided support for security, referential integrity, transaction processing, indexing, record and page locking, and data replication. In later versions of Jet, the engine was extended to run SQL queries, store character data in Unicode format, create views, and allowed bi directional replication with the Microsoft SQL Server. It was superseded by MSDE which was superseded SQL Server Express. There were three modules to Jet. One was the Native Jet ISAM Driver, a Jet 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 application developers to access Jet. Similarly, the Microsoft Jet OLE DB Provider and Replication Objects (JRO) which allowed replication between Jet data sources was removed from MDAC 2.6

Remote Data Services (RDS) allowed the retrieval of a set of data from the server, which the client then altered in some way and then sent back to the server for further processing. With the popular adoption of Transact SQL, which extends SQL with such programming constructs as loops and conditional statements, this became less necessary and it was eventually deprecated in MDAC 2.7. Microsoft produced SOAP Toolkit 2.0, which allows clients to do this via an open XML based standard.

SQLXML was designed for SQL Server 2000, but was deprecated with MDAC 2.6. It allowed Microsoft's relational database to be viewed by XPath and allowed data to viewable as an XML file. It has not actually been deprecated but has been removed from later versions of MDAC, though Microsoft does provide it as a downloadable component and will support it on their 64 bit operating systems.

Several components have been completely removed from MDAC by Microsoft and are no longer supported. They are:
ESQL/C: Embedded SQL (also known as E SQL or ESQL/C) is a way of using SQL when programming in Visual C. Microsoft dropped support for this after SQL Server 6.5 was released, though they did license some of the ESQL/C run time environment to a company called Micro Focus, who develops COBOL compilers and tools
DAO: DAO, or Data Access Objects were an object oriented interface created by Microsoft which allowed early versions of Microsoft Access and Visual Basic to access the Jet Database Engine. Later (in version 3.5) it was able to bypass the Jet engine altogether and directly access ODBC data sources. RDO: Remote Data Objects, or RDO, was a Microsoft technology that allowed for the creation of interfaces that directly called on ODBC. RDO version 2.0 was the final version developed by Microsoft. DB Library: a C based API that allowed an application to interact with SQL Server. It will not be supported on any product after SQL Server 2000, and no features were added after SQL Server 6.5.

Microsoft has released several versions of MDAC over time. The distribution method has varied and the feature set is different for each version.

MDAC 1.0 was first released in August 1996. According to Microsoft, "MDAC 1.0 existed more as concept than a coordinated, stand alone setup program." The MDAC 1.0 stack consisted of ODBC 3.0, OLE DB 1.1, ADO 1.0, and the Advanced Data Connector (ADC) 1.0 – which according to Microsoft was the precursor to the Remote Data Service of MDAC 1.5. It also included ODBC drivers for Access/Jet, SQL Server and Oracle databases. MDAC 1.0 was released via several mechanisms: the Advanced Data Connector shipped with Internet Information Server (IIS) 3.0 and as a downloadable cab file; OLE DB 1.1 and ADO 1.0 shipped with the OLE DB 1.1 SDK, which came with Visual Studio 97 and was also downloadable. MDAC 1.0 came with Active Server Pages, that itself came in IIS 3.0, and also came with Visual InterDev 1.0.

MDAC 1.5 was released between September 1997 and March 1998, and involved a more centralised distribution mechanism than MDAC 1.0. It was released with Microsoft Internet Explorer 4.0, the Internet Client SDK 4.0 and through a CD ROM given out at the 1997 Professional Developers Conference (PDC). There were five versions of MDAC 1.5:
MDAC 1.5 (initial release): included with Internet Explorer 4.0 and the Internet Client SDK. MDAC 1.5a: downloadable from Microsoft's website
MDAC 1.5b: came with Windows NT 4.0 Option Pack & Office 97
MDAC 1.5c: fixed issues with ADO threading and ODBC Connection Pooling and was distributed via the Microsoft website. It only came with the ADO/MDAC runtime components. MDAC 1.5d: came included with Windows 98 and Internet Explorer 4.01 service pack 1. MDAC 1.5 was the last data access component release supported under Windows NT 3.51 SP5.

MDAC 2.0 was distributed with the Data Access 2.0 SDK and included the contents of MDAC 1.5, the ODBC 3.5 SDK and the OLE DB 1.5 SDK, and the OLE DB for OLAP Specification. It also had included many updates to the core product, This version came included in Windows NT 4.0 SP4, and also with Visual Studio 6.0, which came with the full Data Access SDK. This version had security vulnerabilities whereby an unchecked buffer could allow an elevated privileges attack. This was found some time later and it affected MDAC 2.1, 2.5 and 2.6 and was addressed in a later patch

MDAC 2.6 was released in September 2000 and was distributed through the web and with Microsoft SQL Server 2000 MDAC 2.6 RTM, SP1 (released June 20, 2001), and SP2 (released June 11, 2002) were distributed in parallel with the Microsoft SQL Server 2000 service packs, and could also be downloaded from the Microsoft website. Beginning with this version of MDAC, Microsoft Jet, Microsoft Jet OLE DB Provider, and the ODBC Desktop Database Drivers were not included. Instead, these could be installed manually. Microsoft also released an alert warning that MDAC 2.6 should not be installed on an SQL Server 7.0 Cluster, because "if you install MDAC 2.6 or later on any node in the cluster, directly or through the installation of another program, it may cause a catastrophic failure of the SQL Server Agent or other SQL Server services." This issue affected Veritas Software's Backup Exec 9.0 for Windows Servers, because it installs Microsoft SQL Server 2000 Desktop Engine (MSDE 2000) as its database. Revision 4367 installed MDAC version 2.6 SP2 while revision 4454 installed MDAC version 2.7 SP1, which did not have the problem

MDAC 2.7 was released in October 2001 through Microsoft's website. A refresh release was issued in April 2002 through the release of Windows XP and through Microsoft's website. Version 2.7 was available in U. S. English, Chinese (Traditional and Simplified), German, Japanese, Korean, Brazilian Portuguese, Czech, Danish, Greek, Slovak, Slovenian, Spanish, Finnish, French, Hungarian, Italian, Dutch, Norwegian, Polish, Portuguese, Russian, Swedish, and Turkish. Hebrew and Arabic were only available through Windows XP. The main feature change was support for Microsoft's 64 bit operating system, however support for Banyan VINES was also dropped from this version of MDAC. There were several known issues: MDAC 2.7 continued causing connectivity problems on clustered servers running Microsoft SQL Server 6.5 or SQL Server 7.0, with no workaround provided by Microsoft. When creating or configuring ODBC data source names (DSNs) using the Microsoft SQL Server ODBC driver the network library protocol might unexpectedly switch to TCP/IP, even if the DSN was configured to use named pipes. This issue was found by InfoWorld reporter Randall C. Kennedy, who identified that the change was actually made in MDAC 2.6 but was never documented. It was discovered when testing client/server database workloads on a Windows XP computer; InfoWorld claims that although overall server CPU utilization rose by only 8 percent using TCP/IP, context switches per second dropped by more than 150 percent (which is of course impossible because you would then have a negative context switch rate the drop is either 33% or 60% depending on which planet the author was on at the time of writing) for a 10 user workload. They were unimpressed that a fundamental functional change to the default behaviour of Net Lib occurred without more than a passing mention in an unrelated document. Windows XP users also sometimes experienced problems connecting to SQL Server because SQL Server attempts to use certificates it finds on the local computer, however if there is more than one certificate available it did not know which one to use. When attempting to use Microsoft Analysis Services 2000 RTM, an error would sometimes appear when trying to browse cubes. Microsoft also discovered a problem in Windows 95, Windows 98, and Windows Me's setup program which prevented the MDAC installation program from rolling back when it encountered an installation error. Several security issues were resolved by Microsoft for MDAC 2.7. David Litchfield of Next Generation Security Software Ltd reported a security vulnerability that results because one of the ODBC functions in MDAC that is used to connect to data sources contained an unchecked buffer. Another vulnerability that was fixed was one whereby an attacker could respond to an SQL Server discovery message broadcast by clients with a specially crafted packet that could cause a buffer overflow. Another flaw was found whereby code could be executed remotely when the attacker responded to the broadcast with another specially crafted packet.

MDAC 2.8 was released in August 2003 and distributed with Microsoft Windows Server 2003, as well as on Microsoft's Data Access Technologies website. It did not introduce any new features to the product but fixed a number of bugs and security issues – a reg file (automates changes to the registry) was removed that made the server run in an "unsafe" mode whereby the RDS could be exploited to gain unauthorized access to the system and a new restriction was imposed on the length of the Shape query string. There were also several ODBC Administrator changes. On May 23, 2005 Brad Rhodes (Lead Program Manager of Microsoft Data Access Technologies) announced that MDAC 2.8 SP1 was the last stand alone redistributable of MDAC that Microsoft will ship. MDAC is now an official component of the Microsoft's operating system, though they will be providing ongoing bug and security fixes to previously released versions of the web distributable version. However, Microsoft have created a new component called the SQL Native Client (SQLNCLI), which is a stand alone data access API that has combined the OLE DB and ODBC libraries into one DLL. It was formed to be independent of MDAC, which is now reliant on the state the operating system is in – a developer now links to this library and avoids situations where an update of the operating system which updates MDAC breaks applications built to a different version of MDAC. Windows 7 SP1 has broken forward compatibility of MDAC 2.8. Software compiled on Windows 7 SP1 that relies on MDAC ADO will not work on Windows versions prior to Windows 7 SP1 (including Windows 7 RTM, Vista, XP). Microsoft has provided solutions to work around this issue for some applications but VBA applications remain affected. The fix for this issue has been release in February 2012.

Windows Vista will no longer use MDAC, but instead use Windows DAC, which consists of updated versions of ADO, OLE DB, and ODBC components. According to Microsoft, "Windows DAC includes some changes to work with Windows Vista, but is almost entirely functionally equivalent to MDAC 2.8."