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2.4 The message passing idea

Figure 1 shows the central idea behind the message passing paradigm. It involves an architecture in which clients and servers communicate using communication lines. In this model, in contrast with the others that are to be presented in this course, the underlying structure of the network is visible via the communication media used to connect servers and clients and devices such as sockets, ports and server sockets which are involved in the transfer of a message from one computer to another.
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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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2.1 Protocols

Message passing is the simplest form of development paradigm. For example, the way that a client running a browser communicates with a web server is via message passing.

Message passing is based on the idea of a protocol: a language which embodies the functions required by one entity in a distributed system (usually a client) which another entity provides (usually a server). As an example of a protocol consider Table 1. It shows the protocol associated with a naming servic
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1 Distributed models

Before looking in detail at some of the technologies it is worth examining some of the implementation and design models that can be used for distributed system development ranging from the familiar, message passing, to the unfamiliar, tuple-based technology. In describing these I have two aims: first, to detail the variety of methods and architectures that are available and the degree of closeness they have to Internet technologies such as TCP/IP, for example message passing is close in conce
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Acknowledgements

The following material is Proprietary (not subject to Creative Commons) and used under licence (see terms and conditions).

Grateful acknowledgement is made to the following for permission to reproduce material:

Course image: Seika in Flickr made available under Creative Comm
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5.2 The application

The first application I shall describe is that of an online bookseller. Such a book sales system would carry out a number of functions:

  • It would allow the user to browse through a catalogue of books.

  • It would allow the user to browse through a list of the most popular books, with the list being updated every hour.

  • It would provide the facility whereby a user can buy books and add them to a notional shopping basket.


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3.3 Email

This is one of the most ubiquitous technologies on the internet and, along with the World Wide Web, is the most used. When you write an email you use a program known as a mailer. When the email is completed it is sent via a number of computers known as email servers and via a number of other intermediate computers before it reaches its destination where it is read. In e-commerce applications email is a subsidiary, but important technology. It is used as the transport medium for
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2.5.5 Email providers

These are sites which provide free email facilities; often they provide other facilities such as sending anonymous mail and constructing mailing lists. Such sites are valuable to users who are too impecunious to be able to afford conventional mailing software and to frequent travellers who can access such sites anywhere in the world. Their main disadvantage is that they tend to be slow compared with conventional mailing utilities such as Microsoft Outlook and Eudora.


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2.5.2 Link checking sites

The World Wide Web contains millions of web pages. Many of these pages are impossible to read, even though many existing web pages will reference them: your browser will usually return with some message such as ‘Error 404 Page not Found’ when you try to access them. Error 404 is a standard message returned by web servers when a non-existent page is accessed. It is also the telephone area code for Atlanta in the United States; you will occasionally hear technical staff referring to non-exi
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2.2 E-tailing

The next example is probably the one that you expected me to introduce first: that of selling goods over the internet. However, I deliberately introduced supply chain management first since it is an area where companies are making huge savings in their investment in internet technology. The investments in retailing using the internet (e-tailing) are only gradually being realised.

The archetypal e-tailing application Amazon is renowned for the fact that it only sells books over th
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