Databases Reference
In-Depth Information
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A distributed database is a single logical database that is physically divided among computers at several sites
on a network. A user at any site can access data at any other site. A DDBMS is a DBMS capable of support-
ing and manipulating distributed databases.
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Computers in a network communicate through messages. Minimizing the number of messages is important
for rapid access to distributed databases.
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A homogenous DDBMS is one that has the same local DBMS at each site, whereas a heterogeneous DDBMS
is one that does not.
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Location transparency, replication transparency, and fragmentation transparency are important characteris-
tics of DDBMSs.
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DDBMSs permit local control of data, increased database capacity, improved system availability, and added
efficiency.
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DDBMSs are more complicated than non-DDBMSs in the areas of updating replicated data, processing que-
ries, treating concurrent update, providing measures for recovery, managing the data dictionary, designing
databases, and managing security and backup requirements.
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The two-phase commit usually uses a coordinator to manage concurrent update.
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C. J. Date presented 12 rules that serve as a benchmark against which you can measure DDBMSs. These
rules are local autonomy, no reliance on a central site, continuous operation, location transparency, fragmen-
tation transparency, replication transparency, distributed query processing, distributed transaction manage-
ment, hardware independence, operating system independence, network independence, and DBMS
independence.
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A file server stores the files required by users and sends entire files to the users.
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In a two-tier client/server architecture, a DBMS runs on a file server and the server sends only the requested
data to the clients. The server performs database functions, and the clients perform presentation functions.
A fat client can perform the business functions, or the server can perform the business functions in a thin cli-
ent arrangement.
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In a three-tier client/server architecture, the clients perform the presentation functions, database servers per-
form the database functions, and application servers perform business functions. A three-tier architecture is
more scalable than a two-tier architecture.
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The advantages of client/server systems are lower network traffic; improved processing distribution; thinner
clients; greater processing transparency; increased network, hardware, and software transparency; improved
security; decreased costs; and increased scalability.
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Web servers interact with Web clients using HTTP and TCP/IP to display HTML Web pages on Web clients'
screens.
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Dynamic Web pages, not static Web pages, are used in e-commerce; and server-side and client-side exten-
sions provide the dynamic capability, including the capability to interact with databases.
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Cookies and session management techniques are used to counteract the stateless nature of HTTP.
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XML was developed in response to the need for data exchange between organizations and due to the inabil-
ity of HTML to specify the structure and meaning of its data. XML is a metalanguage designed for the
exchange of data on the Web.
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The W3C has developed recommendations for other languages related to XML. These languages include
XHTML, a markup language based on XML and a stricter version of HTML; DTD and XML schema, both used
to specify the structure and meaning of data in an XML document; XSL, a language for creating stylesheets;
XSLT, which transforms an XML document into another document; and XQuery, which is an XML query
language.
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OLTP is used with relational database management systems, and OLAP is used with data warehouses.
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A data warehouse is a subject-oriented, integrated, time-variant, nonvolatile collection of data in support of
management's decision-making process.
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A typical data warehouse data structure is a star schema consisting of a central fact table surrounded by
dimension tables.
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