XML Databases

Definition

            An XML database is a data persistence software system that allows data to be stored in XML format

Data-Centric Documents:

            Data-centric documents are documents that use XML as a data transport. Examples of data-centric documents are sales orders, flight schedules, scientific data, and stock quotes. Data-centric documents are characterized by fairly regular structure, fine-grained data (that is, the smallest independent unit of data is at the level of a PCDATA-only element or an attribute), and little or no mixed content. The order in which sibling elements and PCDATA occurs is generally not significant, except when validating the document.

Document-Centric Documents:

            Document-centric documents are (usually) documents that are designed for human consumption. Examples are books, email, advertisements, and almost any hand-written XHTML document. They are characterized by less regular or irregular structure, larger grained data (that is, the smallest independent unit of data might be at the level of an element with mixed content or the entire document itself), and lots of mixed content.

XML-QL: Querying XML Data

The motivation for XQuery is that increasing amounts of information are stored, exchanged, and presented using XML. An XQuery is an expression that:

Reads a number of XML documents or fragments

Returns a sequence of well-formed XML fragments

Application Servers

CGI does not scale well because of the drawback mentioned above. This performance problem led to the development of specialized programs called application servers. An application server has pre- forked threads or processes and thus avoids the start-up cost of creating a new process for each request. In addition to eliminating the process-creation

overhead:

Integration of heterogeneous data sources: Most companies have data in many different database systems, from legacy systems to modern object-relational systems. Electronic commerce applications require integrated access to all these data sources.

Transactions involving several data sources: In electronic commerce applications, a user transaction might involve updates at several data sources. An application server can ensure transactional semantics across data sources by providing atomicity, isolation, and durability. The transaction boundary is the point at which the application server provides transactional semantics. If the transaction boundary is at the application server, very simple client programs are possible.

Security: Since the users of a Web application usually include the general population, database access is performed using a general-purpose user identifier that is known to the application server. While communication between the server and the application at the server side is usually not a security risk, communication between the client (Web browser) and the Web server could be a security hazard. Encryption is usually performed at the Web server, where a secure protocol (in most cases the Secure Sockets Layer (SSL) protocol) is used to communicate with the client.

Session management: Often users engage in business processes that take several steps to complete. Users expect the system to maintain continuity during a session, and several session identifiers such as cookies, URL extensions, and hidden fields in HTML forms can be used to identify a session. Application servers provide functionality to detect when a session starts and ends and to keep track of the sessions of individual users.

CGI (Common Gateway Interface):

Web-to-database connecting technologies

To build a bridge between web and enterprise database, a number of alternative technologies and architectures have been available. These include:

CGI (Common Gateway Interface):

CGI is a Web standard for accessing external programs, to integrate databases with Web servers. CGI Dynamically generates HTML documents from back-end databases. CGI programs can be written in any scripting language (interpreted) or programming language available to be executed on a web server, including C, C++, Fortran, PERL, TCL, VB and others.

WEB DATABASES

What is a Web Database?

A web database is an organized listing of web pages. It's like the card catalogue that you might find in the library. The database holds a "surrogate" (or selected pieces like the title, the headings, etc.) for each web page. The creation of these surrogates is called "indexing", and each web database does it in a different way. Web databases hold surrogates for anywhere from 1 million to several billion web pages.

Why Web Database?

The current web is largely based on file system technology, which can deal well with the resources that are primarily static. However, with unprecedented growth of resources, it is no longer adequate to rely on this conventional file technology for organising, storing and accessing large amount of information on the web. Thus, many websites turn towards database technology to keep track of the increasing amount of data.

Role of databases in Web applications

Many organizations offer products through their Web sites, and customers can place orders by visiting a Web site. For such applications a URL must identify more than just a file, however rich the contents of the file; a URL must provide an entry point to services available on the Web site

The Semi structured Data Model

Consider a set of documents on the Web that contain hyperlinks to other documents. These documents, although not completely unstructured, cannot be modelled naturally in the relational data model because the pattern of hyperlinks is not regular across documents.

While some data is completely unstructured -- for example video streams, audio streams, and image data -- lot of data is neither completely unstructured nor completely structured. We refer to data with partial structure as semi structured data.

There are many reasons why data might be semi structured. First, the structure of data might be implicit, hidden, unknown, or the user might choose to ignore it. Second, consider the problem of integrating data from several heterogeneous sources where data exchange and transformation are important problems. Third, we cannot query a structured database without knowing the schema, but sometimes we want to query the data without full knowledge of the schema.