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OLE DB Programming


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OLE DB Programming
A D V E R T I S E M E N T

This home page is your starting point for information about the Microsoft OLE DB database technology and the OLE DB Template Library.
It is important to know that Microsoft provides several implementations of OLE DB. OLE DB is a set of COM interfaces that provide uniform access to data in diverse information sources and formats.

The OLE DB templates are C++ templates that make the high-performance OLE DB database technology easier to use by providing classes that implement many commonly used OLE DB interfaces. This template library is divided into consumer templates and provider templates.

Visual C++ also has wizard support for creating OLE DB starter applications.

In addition, you can use attributes to implement the OLE DB consumer templates.


To learn more about See
Using the OLE DB consumer templates (conceptual topics) OLE DB Consumer Templates
Using the OLE DB provider templates (conceptual topics) OLE DB Provider Templates
OLE DB templates classes and macros OLE DB Templates Reference (Visual C++)
OLE DB consumer attributes OLE DB Consumer Attributes
OLE DB interfaces OLE DB programmer's reference (in the Windows SDK)
OLE DB templates samples OLE DB Templates Samples
Data access programming overview (Visual C++) Data Access Programming
ODBC vs. DAO Should I Use DAO or ODBC?
ODBC conceptual topics Open Database Connectivity (ODBC)
Data-bound controls (ADO and RDO) conceptual topics Data-Bound Controls (ADO and RDO)



OLE DB Programming Overview

What is OLE DB, and what makes it distinct from other database technologies? OLE DB is a high-performance, COM-based database technology produced by Microsoft. What sets OLE DB apart from Microsoft's other database technologies is how it provides Universal Data Access.



Universal Data Access

Universal Data Access provides a common way to access data regardless of the form in which it is stored. In a typical business situation, a vast amount of information is stored outside of corporate databases. This information is found in file systems (such as FAT or NTFS), indexed-sequential files, personal databases (such as Access), spreadsheets (such as Excel), project planning applications (such as Project), and email (such as Outlook).

Accessing this data using the various associated applications presents a major bottleneck in workflow or at least an annoyance. Most companies find themselves in this situation and deal with the problem by consolidating the information in a database management system (DBMS). However, such a move is expensive, time consuming, and in many cases not practical.

The alternative is to develop a Universal Data Access solution. OLE DB and ADO provide Universal Data Access capability. Of the two, OLE DB is the more performance intensive and is recommended for use with Visual C++ applications.

Universal Data Access implies two capabilities: the first is distributed query or uniform access to multiple (distributed) data sources and the second is the ability to make non-DBMS data sources accessible to database applications.

  • Distributed query

    The ability to access data uniformly on multiple (that is, distributed) data sources. The data sources can be of the same type (such as two separate Access databases) or of different types (such as a SQL Server database and an Access database). Uniformly means that you can meaningfully run the same query on all data sources.

  • Non-DBMS access

    The ability to make non-DBMS data sources accessible to database applications. Examples of DBMS data sources include IMS, DB2, Oracle, SQL Server, Access, and Paradox. Examples of non-DBMS data sources include information in file systems, email, spreadsheets, and project management tools.

Consider a scenario in which a sales department needs to find all email messages received within a one-week period from customers in a certain area. This query might require a search on an email application's mailbox file and a search on an Access table of customers to specify the customers' names. While Access is a DBMS application, Outlook is not.

OLE DB allows you to develop applications that access diverse data sources, whether they are DBMS or not. OLE DB makes universal access possible by using COM interfaces that support the appropriate DBMS functionality for a given data source. COM reduces unnecessary duplication of services and maximized interoperability not only among data sources but also among other applications.




Benefits of COM

This is where COM comes in. OLE DB is a set of COM interfaces. By accessing data through a uniform set of interfaces, you can organize a database into a matrix of cooperating components.

Based on the COM specification, OLE DB defines an extensible and maintainable collection of interfaces that factor and encapsulate consistent, reusable portions of DBMS functionality. These interfaces define the boundaries of DBMS components such as row containers, query processors, and transaction coordinators, which enable uniform transactional access to diverse information sources.

Typically, OLE DB applications are written as DLLs, but its COM implementation overcomes the disadvantages of DLLs (such as naming and version problems) by using componentized code. In OLE DB, you call interfaces or access other components using their globally unique identifiers (GUIDs).

Finally, COM keeps track of component usage by using reference counting. When you call a method on an interface, the reference count is incremented; when the method returns, the reference count is decremented. When the count equals zero, the component to which the method belongs is released.




OLE DB Consumers and Providers

The OLE DB architecture uses the model of consumers and providers. A consumer makes requests for data. A provider responds to these requests by placing data in a tabular format and returning it to the consumer. Any call that the consumer can make must be implemented in the provider.
Technically defined, a consumer is any system or application code (not necessarily an OLE DB component) that accesses data through OLE DB interfaces. The interfaces are implemented in a provider. Thus, a provider is any software component that implements OLE DB interfaces to encapsulate access to data and expose it to other objects (that is, consumers).

In terms of roles, a consumer calls methods on OLE DB interfaces; an OLE DB provider implements the needed OLE DB interfaces.

OLE DB avoids the terms client and server because these roles do not always make sense, especially in an n-tier situation. Because a consumer could be a component on a tier that serves another component, to call it a client component would be confusing. Also, a provider sometimes acts more like a database driver than a server.




OLE DB Object Model

The OLE DB object model comprises the following objects or components. The first four objects or components listed (data sources, sessions, commands, and rowsets) allow you to connect to a data source and view it. The rest, starting with accessors, relate to working with the data when it is displayed.



Data Sources

Data source objects allow you to connect to a data source such as a file or DBMS. A data source object creates and manages the connection and contains permissions and authentications information (such as login name and password). A data source object can create one or more sessions.




Sessions

A session manages a particular interaction with the data source to query and retrieve data. Each session is a single transaction. A transaction is an indivisible work unit defined by the ACID test. For a definition of ACID, see Transactions.

Sessions perform important tasks such as opening rowsets and returning the data source object that created it. Sessions can also return metadata, or information about the data source itself (such as table information).

A session can create one or more commands.




Commands

Commands execute a text command such as a SQL statement. If the text command specifies a rowset, such as a SQL SELECT statement, the command creates the rowset.

A command is simply a container for a text command, which is a string (such as a SQL statement) passed from a consumer to a data source object for execution by the provider's underlying data store. Typically, the text command is a SQL SELECT statement (in which case, because SQL SELECT specifies a rowset, the command automatically creates a rowset).




Rowsets

Rowsets expose data in tabular format. An index is a special case of a rowset. You can create rowsets from the session or the command.

Schema Rowsets

Schemas contain metadata (structural information) about a database. Schema rowsets are rowsets that contain schema information. Some OLE DB providers for DBMS support schema rowset objects. For more information about schema rowsets, see Obtaining Metadata with Schema Rowsets and Schema Rowset Classes and Typedef Classes.

View Objects

A view object defines a subset of the rows and columns from a rowset. It contains no data of its own. View objects cannot combine data from multiple rowsets.



Accessors

Only OLE DB uses the concept of accessors. An accessor describes how data is stored in a consumer. It contains a set of bindings (called a column map) between rowset fields (columns) and data members that you declare in the consumer.




Transactions

Transaction objects are used when committing or aborting nested transactions at other than the lowest level. A transaction is an indivisible work unit defined by the ACID test. ACID stands for:

  • Atomicity: cannot be divided into smaller work units.
  • Concurrency: more than one transaction can occur at a time.
  • Isolation: one transaction has limited knowledge about changes made by another.
  • Durability: the transaction makes persistent changes.



Enumerators

Enumerators search for available data sources and other enumerators. Consumers that are not customized for a particular data source use enumerators to search for a data source to use.

A root enumerator, shipped in the Microsoft Data Access SDK, traverses the registry looking for data sources and other enumerators. Other enumerators traverse the registry or search in a provider-specific manner.




Errors

Any interface on any OLE DB object can generate errors. Errors contain additional information about an error, including an optional custom error object. This information is contained in an HRESULT.




Notifications

Notifications are used by groups of cooperating consumers sharing a rowset (where sharing means that the consumers are assumed to be working within the same transaction). Notifications enable cooperating consumers sharing a rowset to be informed about actions on the rowset performed by their peers.




OLE DB Templates, Attributes, and Other Implementations




ATL OLE DB Templates

The OLE DB Templates, which are part of ATL (Active Template Library), make the high-performance OLE DB database technology easier to use by providing classes that implement many of the commonly used OLE DB interfaces. Along with this template library comes wizard support for creating OLE DB starter applications.

This template library contains two parts:

  • OLE DB Consumer Templates Used to implement an OLE DB client (consumer) application.
  • OLE DB Provider Templates Used to implement an OLE DB server (provider) application.

To use the OLE DB Templates, you should be familiar with C++ templates, COM, and the OLE DB interfaces. If you are not familiar with OLE DB, see OLE DB Programmer's Reference.

For more information, you can:

  • Read the topics about the OLE DB Consumer Templates or OLE DB Provider Templates.
  • Create an OLE DB consumer or OLE DB provider.
  • See the list of OLE DB consumer classes or OLE DB provider classes.
  • See the list of OLE DB templates samples.
  • See OLE DB Programmer's Reference (in the Windows SDK).



OLE DB Attributes

The OLE DB consumer attributes provide a convenient way to create OLE DB consumers. The OLE DB attributes inject code based on the OLE DB consumer templates to create working OLE DB consumers and providers. If you need to specify functionality not supported by the attributes, you can use the OLE DB Templates in conjunction with attributes in your code.




MFC OLE DB Classes

The MFC library has one class, COleDBRecordView, that displays database records in controls. The view is a form view directly connected to a CRowset object and displays the fields of the CRowset object in the dialog template's controls. It also supplies a default implementation for moving to the first, next, previous, or last record and an interface for updating the record currently on view. For more information, see COleDBRecordView.




OLE DB SDK Interfaces

In the cases where the OLE DB Templates do not support OLE DB functionality, you need to use the OLE DB interfaces themselves. For more information, see OLE DB Programmer's Reference in the Windows SDK.




OLE DB Architectural Design Issues

You should consider the following issues before starting your OLE DB application:
What programming implementation will you use to write your OLE DB application?
Microsoft offers several libraries to accomplish this: an OLE DB Template library, OLE DB attributes, and the raw OLE DB interfaces in the OLE DB SDK. In addition, there are wizards that help you write your program. These implementations are described in OLE DB Templates, Attributes, and Other Implementations.
Do you need to write your own provider?
Most developers do not need to write their own provider. Microsoft provides several providers. Whenever you create a data connection (for example, when you add a consumer to your project using the ATL OLE DB Consumer Wizard), the Data Link Properties dialog box lists all the available providers registered on your system. If one of these providers is appropriate for your own data store and data access application, the easiest thing to do is use one of these. However, if your data store does not fit one of these categories, you have to create your own provider. For information about creating providers, see OLE DB Provider Templates.
What level of support do you need for your consumer?
Some consumers can be very basic; while others can be very complex. The functionality of OLE DB objects is specified by properties. When you use the ATL OLE DB Consumer Wizard to create a consumer or the Database Provider Wizard to create a provider, it sets the appropriate object properties for you to give you a standard set of functionalities. However, if the wizard-generated consumer or provider classes do not support everything you need them to do, you need to refer to the interfaces for those classes in the OLE DB Templates Library. These interfaces wrap the raw OLE DB interfaces, providing extra implementation to make using them easier for you.

For example, if you want to update data in a rowset, but forgot to specify this when you created the consumer with the wizard, you can specify the functionality after the fact by setting the DBPROP_IRowsetChange and DBPROP_UPDATABILITY properties on the command object. Then, when the rowset is created, it has the IRowsetChange interface.

Do you have legacy code using another data access technology (ADO, ODBC, or DAO)?
Given the possible combinations of technologies (such as using ADO components with OLE DB components and migrating ODBC code to OLE DB), covering all situations is beyond the scope of the Visual C++ documentation. However, many articles covering various scenarios are available on the following Microsoft Web sites:


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