by Vivek Viswanathan
09/28/2001
http://www.onjava.com/pub/a/onjava/2001/09/26/load.html?page=1
A cluster is a group of servers running a Web application simultaneously, appearing to the world as if it were a single server. To balance server load, the system distributes requests to different nodes within the server cluster, with the goal of optimizing system performance. This results in higher availability and scalability -- necessities in an enterprise, Web-based application. High availability provided by redundant resources. Scalability is an application's ability to support a growing number of users.
DNS round robin and Hardwaer load balancers are the two main methods available to balance a server load.
For DNS round robin, duplicate servers are required to form a cluster, each server
within the cluster takes their turn to answer incoming request. It is simple, easy to setup and inexpensive. However, it cannot support server affinity. This method balance the number of connection to each server not the actual loading.
For hardware load balancer: It use a virutal IP to represent the the IP of all servers in the cluster. When a request comes to the load balancer, it rewrites the request's header to point to other machines in the cluster. If a machine is removed from the cluster, the request doesn't run the risk of hitting a dead server, since all of the machines in the cluster appear to have the same IP address. This address remains the same even if a node in the cluster is down. It provides server affinity and high availability throught failover.
Friday, February 29, 2008
Tiered Software Architectures
http://www.sei.cmu.edu/str/descriptions/clientserver_body.html
Client/server architecture (2 tier architectures).
This approach split the processing management between the user system interface environment and the database management server environment. The client initiate the request (Remote procedure call or SQL ) sent to the server, and the servers respond to client with the answer.
This is good at providing distributed computer within a small size network, may be upto 100 simultaneous connections. If more than that, performance will deteriorate due to process of sending the "keep-alive" message. Furthermore, using proprietary databse restricts flexibility and choice of DBMS for application, it is not portable between different paltforms.
Three tier architectures is also referred to as the multi-tier architecture, a middle tier was added between the user system interface client environment and the database management server environment to perform queuing, application execution, and database staging. In addition the middle layer adds scheduling and prioritization for work in progress. There are a variety of ways of implementing this middle tier, such as transaction processing monitors, message servers, or application servers. The three tier client/server architecture has been shown to improve performance for groups with a large number of users (in the thousands) and improves flexibility when compared to the two tier approach. Flexibility in partitioning can be a simple as "dragging and dropping" application code modules onto different computers in some three tier architectures. A limitation with three tier architectures is that the development environment is reportedly more difficult to use than the visually-oriented development of two tier applications.
(see more on Three Tier Software Architectures)
Author: Darleen Sadoski, GTE
Copyright 2007 by Carnegie Mellon University
Client/server architecture (2 tier architectures).
This approach split the processing management between the user system interface environment and the database management server environment. The client initiate the request (Remote procedure call or SQL ) sent to the server, and the servers respond to client with the answer.
This is good at providing distributed computer within a small size network, may be upto 100 simultaneous connections. If more than that, performance will deteriorate due to process of sending the "keep-alive" message. Furthermore, using proprietary databse restricts flexibility and choice of DBMS for application, it is not portable between different paltforms.
Three tier architectures is also referred to as the multi-tier architecture, a middle tier was added between the user system interface client environment and the database management server environment to perform queuing, application execution, and database staging. In addition the middle layer adds scheduling and prioritization for work in progress. There are a variety of ways of implementing this middle tier, such as transaction processing monitors, message servers, or application servers. The three tier client/server architecture has been shown to improve performance for groups with a large number of users (in the thousands) and improves flexibility when compared to the two tier approach. Flexibility in partitioning can be a simple as "dragging and dropping" application code modules onto different computers in some three tier architectures. A limitation with three tier architectures is that the development environment is reportedly more difficult to use than the visually-oriented development of two tier applications.
(see more on Three Tier Software Architectures)
Author: Darleen Sadoski, GTE
Copyright 2007 by Carnegie Mellon University
Tuesday, February 5, 2008
Why Learn CGI?
http://www.oreillynet.com/pub/a/oreilly/web/news/cgi_0700.html
by Scott Guelich
07/01/2000
The advantages of using CGI
1. CGI is the most popular way to develop dynamic content
2. CGI is support by all major web servers
3. CGI is language independent
4. CGI was intentionally designed as a lightweight interface over HTTP
5. Numerous ISPs and homepage services allow CGI scripts.
6. CGI are easy to learn and used by programmers
7. There are a lot of on-line CGI resources.
by Scott Guelich
07/01/2000
The advantages of using CGI
1. CGI is the most popular way to develop dynamic content
2. CGI is support by all major web servers
3. CGI is language independent
4. CGI was intentionally designed as a lightweight interface over HTTP
5. Numerous ISPs and homepage services allow CGI scripts.
6. CGI are easy to learn and used by programmers
7. There are a lot of on-line CGI resources.
Tuesday, January 29, 2008
Design Consideration for Web-based Applications
This paper was published in the 2001 Human Factors and Ergonomics Society's Annual Meeting proceedings. By Luke Wroblewski & Esa M. Rantanen
http://www.lukew.com/resources/articles/web_applications.html
While developing web applications we also need to follow standards to provide quality applications for all end users.
This document provides guidelines for effective design and implementation for web application including web services. "weblications"—a term first coined by Bruce Tognazzini (Norman, 2000)—are defined as serious software being delivered as a service over the Web (Nail, 1998).
In general, it is often the case that weblications are used more intensively and frequently than traditional web pages. Users want the services that weblications have to offer (Norman, 2000). Therefore, they are willing to invest more time in learning the functionality of a weblication for the payoff of increased productivity.
Guidelines
1. Use Full screen to focus users’ attention, hide all tools, menus and bars make the browser less confusing.
2. Minimize the use of windows, so won’t get lost behind other windows.
3. Emphasize the contents and makes the window less obtrusive.
4. Use consistent layout enhances usability.
5. Use rollovers, appropriate application make screen clear for reading. If overuse, may result in a flickering effect.
6. Use ALT-overs when the immediacy of a rollover is not needed.
7. Avoid double clicks. Users already get use to use single click for web interactions.
8. Use the conventions of link selection in web, since users already get use to it.
9. Use inherent functionality of visual elements, users already expect this from their interface.
10. Use both saturated and unsaturated link colors, let user easy to determine which link/service is already executed.
11. Use underlined fonts as hot-spots
12. Use pull-down menus, radio buttons, and checkboxes as utilized online.
13. Use plug-ins and frames as tools for weblication design. More functionality and richer form of interaction or information display.
14. Use motion cues and animation as a feedback mechanism.
15. Use common functionality between a weblication and web browsers. Save efforts.
16. Exploit the similarities in the basic functionality of all weblications. Promote standardization and eliminates problems from implementation of non-standardize interaction.
17. Utilize the resource potential of the web in the design of a weblication.
18. Manage time and workflow within a weblication. The resulting product will be more task-oriented, guide users through the actions necessary for task completion.
19. Consider the aesthetic integrity of the interface, Aesthetic presentation can give a weblication a personality, provide users enjoyment or familiarity, and a sense of trust and professionalism
http://www.lukew.com/resources/articles/web_applications.html
While developing web applications we also need to follow standards to provide quality applications for all end users.
This document provides guidelines for effective design and implementation for web application including web services. "weblications"—a term first coined by Bruce Tognazzini (Norman, 2000)—are defined as serious software being delivered as a service over the Web (Nail, 1998).
In general, it is often the case that weblications are used more intensively and frequently than traditional web pages. Users want the services that weblications have to offer (Norman, 2000). Therefore, they are willing to invest more time in learning the functionality of a weblication for the payoff of increased productivity.
Guidelines
1. Use Full screen to focus users’ attention, hide all tools, menus and bars make the browser less confusing.
2. Minimize the use of windows, so won’t get lost behind other windows.
3. Emphasize the contents and makes the window less obtrusive.
4. Use consistent layout enhances usability.
5. Use rollovers, appropriate application make screen clear for reading. If overuse, may result in a flickering effect.
6. Use ALT-overs when the immediacy of a rollover is not needed.
7. Avoid double clicks. Users already get use to use single click for web interactions.
8. Use the conventions of link selection in web, since users already get use to it.
9. Use inherent functionality of visual elements, users already expect this from their interface.
10. Use both saturated and unsaturated link colors, let user easy to determine which link/service is already executed.
11. Use underlined fonts as hot-spots
12. Use pull-down menus, radio buttons, and checkboxes as utilized online.
13. Use plug-ins and frames as tools for weblication design. More functionality and richer form of interaction or information display.
14. Use motion cues and animation as a feedback mechanism.
15. Use common functionality between a weblication and web browsers. Save efforts.
16. Exploit the similarities in the basic functionality of all weblications. Promote standardization and eliminates problems from implementation of non-standardize interaction.
17. Utilize the resource potential of the web in the design of a weblication.
18. Manage time and workflow within a weblication. The resulting product will be more task-oriented, guide users through the actions necessary for task completion.
19. Consider the aesthetic integrity of the interface, Aesthetic presentation can give a weblication a personality, provide users enjoyment or familiarity, and a sense of trust and professionalism
Sunday, January 20, 2008
Lecture 1 summary - Browser summary W3School
Based on the static from w3school.com by 2007 Microsoft IE family got 55.9% share, where Firefox got 36.3% share, the share for Microsoft IE is droping where the pupolarity of Firefox is increasing for the passing 3 years.
Friday, January 18, 2008
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