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Acknowledgements

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Ince, D. Developing Internet Applica
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References

[1] E. Freeman, S. Hupfer and A.K. Arnald, JavaSpaces Principles, Patterns and Practice. Reading, MA: Addison-Wesley, 1999.
[2] G. Coulouris, J. Dollimore and T. Kindberg, Distributed Systems Concepts and Design. Harlow: Addison-Wesley, 2001.
[3] N. Lynch, Distributed Algorithms. New York: Morgan Kauffman, 1996.
[4
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6.1 Further reading

There is not a lot published on distributed development paradigms. The book by Coulouris et al. [2] indirectly introduces some of the paradigms introduced in this unit. Lynch's book [3] on distributed algorithms is full of algorithms which are message passing based. The book by Patzer and 14 others [4] is a good practical introduction to many of the technologies detailed in this unit. One of the few current books on JavaSpaces has been written by Freeman et al. [1].


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4.1 Introduction

Many of you will already be familiar with event processing if you have developed visual interfaces with the later versions of Java. Developing such an interface consists of a number of steps:

  • A visual object such as a button is placed in a container such as an applet or a object.
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3.1 Distributed objects technology

This technology virtually hides the network from the designer and programmer. A distributed object is an object which is resident on one computer and for which methods can be invoked associated with code resident on other computers. A good distributed objects technology should totally hide the underlying communication details from the programmer, for example when a programmer wants to invoke the method Author(s): The Open University

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2.2 Fixed and adaptive protocols

The protocol described above for a simple naming service is an example of a fixed protocol. This is a protocol whose vocabulary is fixed: it is embedded in the client and server's code and data and does not change. An adaptive protocol is one where the protocol changes. A fixed protocol could change over a period of time because the functionality provided by a server changes. However, this change will be over months or years rather than over seconds.

There are some instances wher
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Introduction

In this unit I look at a number of different programming and design styles associated with distributed system development. The unit first examines message passing and the role of protocols – both fixed and adaptive protocols. Two styles of message passing are also examined: synchronous and asynchronous message passing. The next part of the unit introduces distributed object technology. Event-based development relies on listener objects listening to events which are propagated along a bus; t
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8 Further reading

There are a large number of books that have been written on e-commerce, many of which are of varying quality. I have found three useful. The first is by Kalakota and Whinstone (1997). It is an excellent introduction to both the technologies and applications involved in electronic commerce. Kalakota has also written a book on electronic commerce which avoids many of the clichés and which concentrates on unglamorous areas such as supply chain automation (Kalakota, 1999) . If you ignore the jar
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7.2.15 E-learning

This term is used to describe companies or organisations who offer educational courses via the web. The quality and features found in sites which can be described by this business model can vary. At its simplest such sites offer students the ability to download conventional texts. More complex instantiations of the model offer the students facilities to read individual lessons, try out online multiple choice questions and experience simulations relevant to the topic being taught.


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7.2.5 Virtual communities

A virtual community is a website which sells some product or service. In this respect there is no difference from an e-shop. The feature which distinguishes a virtual community is that the operator of the website provides facilities whereby the customers for a product or a service interact with each other, for example by pointing out ways a product can be improved. Technologies used for this interaction include mailing lists, bulletin boards and FAQ lists. The theory behind virtual communitie
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7.2.1 E-shop

This is the most ubiquitous form of commerce on the World Wide Web. It involves a company presenting a catalogue of its wares to internet users and providing facilities whereby such customers can purchase these products. Almost invariably such a site will contain facilities for ordering and paying for products by means of credit cards. The sophistication of sites described by this business model range from just the simple presentation of a static catalogue to the presentation of an interactiv
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7.3.2 Subtraction

As I indicated at the start of this section, subtraction is converted to addition by replacing the number to be subtracted by its additive inverse, which in the case of binary arithmetic is its 2's complement. An example should make this clear.

Example 9

Subtract the signed integer 1010 10
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2.7 Other commercial websites

So far I have detailed e-commerce applications which are connected with very large organisations; to conclude this section it is worth looking at a number of smaller applications, many of which are distinguished by the fact that they are novel. They are in contrast to the applications discussed in previous subsections which mainly consist of standard functions such as order processing.

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1.7 Testing

The aim of testing is to uncover errors in the design and implementation of the database, its structure, constraints and associated user and management support. Testing is usually considered to involve two main tasks – validation and verification. Without adequate testing users will have little confidence in their data processing.

Validation answers the question: has the right database been developed to meet the requirements? It attempts to confirm that the right database has been co
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1.6.2 Populating the database

After a database has been created, there are two ways of populating the tables – either from existing data, or through the use of the user applications developed for the database.

For some tables, there may be existing data from another database or data files. For example, in establishing a database for a hospital you would expect that there are already some records of all the staff that have to be included in the database. Data might also be bought in from an outside agency (address
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5.2.1 Transforming the natural to the designed

The artist Christine Martell lives in Oregon in the United States and works with beads and visual images. I asked her to describe how she makes use of a computer to create her visual images of flowers and trees. She writes of her work:

I start by finding flowers that are compelling in some way, most often in form and colour. I take photographs with a 35 mm camera having a macro lens.

I'm usually looking for a
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5.2 Art and the common computer

Art is difficult to define. But all art involves the Exercise of human skill. A natural object, such as a piece of driftwood, a flower, a bird song, can move us to admire it as beautiful or intriguing or comforting, but it isn't art. Artists (be they photographers, painters, sculptors, actors, musicians, authors or dancers) use their skill to transform natural objects, materials or signs (paint, clay, their own body or voice, the sounds of musical instrument, words) into somethi
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5.1.2 The human genome

All life is ‘encoded’ chemically in genes. What this means is that the structure of an organism, the organs it possesses, its colouring, and so on are all determined by different genes. A very simple organism may have just a few genes, and a complex one tens of thousands. The ‘map’ of an organism's genes is referred to as its genome. It shows, in essence, which genes give rise to which characteristics or traits of the organism. The word ‘template’ would describe the
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4.1 Where am I and how do I get to … ?

Computers can be used to find things and the obvious thing they can find is information. The World Wide Web (WWW or just the web) is just one example of a vast store of information which can be searched to find what you want using computers (The web consists of linked data which is accessed via the internet using a browser). But computers can also ‘find’ things in the sense of locating them geographically, either by generating maps that can be used for navigation or by locat
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3.4.1 A computer system is the combination of:

  • the computer (with its processor and storage);

  • other equipment such as a scanner or printer,

  • the software programs that make it all work (software programs that are designed to help with some human task are often referred to as applications).


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