The essential components that affect the implementation of VDB Engines are, High-Level Data Access Interfaces, Low-Level Data Access Interfaces and Traditional Database Functionality.

A VDB Engine's capabilities are exposed via High Level Data Access interfaces. For the purpose of this document, a high level data access interface is an interface utilized predominantly by applications, as opposed to middleware developers for achieving application database independence. A high level data access interface sits atop Low-Level data access interfaces, providing an abstraction layer that serves to simplifying the process of database independent application development.

A number of High Level Data Access standards exist today, the more prevalent being:

Data Access Objects (DAO)
Remote Data Objects (RDO)
ActiveX Data Objects (ADO)

It is important to note that low-level Data access interfaces such as ODBC, UDBC, JDBC and OLE-DB transparently serve the high-level interfaces mentioned in the section above. Thus, in most cases VDB vendors will treat ODBC, UDBC, JDBC, and OLE-DB as high-level interfaces by providing VDB data access drivers conforming to these standards as part of the VDB deliverable.

A VDB Engine's data I/O occurs via low-level data access interfaces to underlying database engines or data sources. In recent times the Open Database Connectivity (ODBC ) API and the X/Open SQL Call Level Interface (CLI) have emerged as the dominant industry wide Low-Level Data Access Standards. OLE-DB from Microsoft is also emerging as a new low-level data access standard for relational and non-relational data in the Microsoft Component Object Model (COM) world. While JDBC is emerging like wise as the low-level data access standard for the burgeoning Java world.

A VDB may also be a Native Database Interface Client, making use of database engine vendor provided data access interfaces. Native interfaces are based upon Embedded SQL, an older format Low-Level data access interface that preceded the X/Open SQL CLI. It is important to note that ODBC from Microsoft, JDBC from JavaSoft, and UDBC from OpenLink Software are all derived from the X/Open SQL CLI.

The degree to which a VDB implements a traditional database engine's functionality has a direct bearing on the intrinsic value of a VDB engine. Traditional database functionality is extensive, but for the purposes of this document, a core set of functionality common to all commercial database engines has been assembled. The functionality list includes:

Query Language Support.  standard syntax for interrogating, manipulating, describing, and securing data contained within a database. Examples include the Structured Query Language (SQL) for relational databases and the Object Query Language (OQL) for Object and Object-Relational Databases.

Query Processor.  the mechanism used by a database engine to convert Query Language Statements into actual data retrieval instructions. In addition, this database component is responsible for ensuring Query Language syntax conformance, Query Execution Plan Assembly and Query Fulfillment.

Standard Data Types Support.  data contained within a database must be describable using standard data types e.g. Character, Number, Date, etc.

VIEW Support.  pre constructed query statements stored within a database, for the purpose of query simplification, or content and structural security.

Stored Procedure Support.  Stored Procedures facilitate the embedding of application programming logic within a database. Their pre-compiled nature enhances data access performance by reducing message hops between database servers and database clients.

Scrollable Cursor Support.  the process by which the result of a database query (known as a result-set) is traversed. Traversal occurs in either direction, backwards or forwards, using result-set chunks (known as row-sets). Resultset scrolling occurs when database engines exchange data with database clients.

Concurrency Control.  the process through which a database engine supports multiple sessions running concurrently, across multiple database users and database client applications without compromising underlying data integrity or introducing quantum increases in application response times.

Transaction Support.  ensures that database instructions can be grouped into logical units of execution that are Atomic, Consistent, Isolated from the effect of other units of execution affecting the same underlying data, and Durable.

Transaction Isolation.  describes the ability of a database engine to provide transaction process partitioning options called Isolation Levels, that offer different ways of managing the effects of multiple and concurrent transactions affecting the same underlying data.

Distributed Transaction Support.  describes the ability to preserve transaction atomicity, consistency, integrity, and durability across database servers hosted on the same or different database server machines within a networked environment. This involves supporting transaction Commits and Rollbacks using a 2-phase commit protocol.

User Definable Type Support.  this is how a database engine allows end-users extend its base functionality. This is achieved by providing interfaces that allow end-users create new ways in which a database engine's data is described and manipulated.

Federated Database Support.  data access and manipulation across database servers resident on the same machine.

Distributed Database Support.  data access, and manipulation across database servers resident on the different machines within a networked environment.

Security.  the process by which data, and data transmission is protected using a combination of database and operating system privileges, roles and roles hierarchies. It also includes the ability of a database engine to protect data transmitted to its clients using data encryption.