Architecture of Web-based systems

Client Server and 2 tier Web Architectures

Typically, when you are browsing the Internet, you will be using Web Browser software such as Internet Explorer or Mozilla Firefox. The computer which is running a browser is called a client, whilst the machine which is providing Web pages is called a server.

When you dial up to an Internet Service Provider ( ISP ) e.g. Blue Yonder, Aol, your computer is forming a network connection to a Web server. In this situation, your computer is in effect a client, which is linked to an ISP Web server. The web server, as the name suggests, serves your browser with Web pages (e.g. HTML, ASPX, JSP pages etc).

This simple scenario, where the Web server is connected to one or more clients is known as a 2 tier architecture model.

Figure B below demonstrates how Web pages are accessed via a browser, using a 2 tier architecture.

Figure B - A simple diagram of 2 tier Client/server architecture

3-Tier architectures

Generally computing applications consist of three different and distinct types of functionalities.

Data Management

Functional Logic

Presentation Services

• Presentation Services: These manifest themselves in the form of information display and user data input facilities. Generally the front-end for user interaction. For example logging in requires interaction in the form of collecting username and password information using a HTML-form.
• Functional logic: Every application includes some data processing and this may also involve database interactivity. For example user authentication requires the logic unit to read username-password combinations from a database and compare until a good comparison (hopefully) is arrived at.
• Data Management: Data, its storage, insertion and retrieval, its management and alteration is central to computing applications. For example a database management system (DBMS) is required for the management of usernames and associated passwords, their owners, etc.

Figure C - An illustration of a 3 tier architecture model

1st tier- Web browser 2nd tier- Web server 3rd tier- Data storage

n-Tier Architectures

It is also important to note that Web application architecture sometimes is referred to as multi-tier. In effect we could have more than 3-tiers in circumstances when the Web server requires to access one or more application servers for specialised services. This is known as n-Tier architecture

Each layer within an N-tier architecture could be thought of as 'logical components' interacting with the layer above or below. Layers provide a means of grouping functionality within the application structure.

Some benefits of this approach include flexibility of component location - each layer may be held on a different server, this facilitates scalable applications capable of handling heavier server loads. Additionally each layer is encapsulated making it possible to change one layer without affecting another.

Activities; Learning Checkpoints

	Learning Checkpoint 1
	What are the differences between a static website and a Web application? Explain your thinking (create a list of differences if you wish)
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	Learning Checkpoint 2
	Generally computing applications consist of three different and distinct types of functionalities. What are these?
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	Learning Checkpoint 3
	There maybe many variations of web-based systems architecture. What are the different types (the various ways we can distribute) of client-server architecture?
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	Learning Checkpoint 4
	How many tiers may a static and dynamic Web site have. What are the limitations of each approach? explain your thinking.
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	Learning Checkpoint 5
	If we have a multi-tier application, how do you decide on the functionality of the various tiers? How do we distribute functionalities across the tiers? What are the factors to consider in designing the architecture?
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	Learning Checkpoint 6
	What are the key benefits to adopting a 3 tier architecture for our Web applications? Name at least 4 examples
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