Load Balancer is just like a network card that used in the switch.
OR
Sometimes the same physical device serves as any combination of firewall, proxy server, router, and/or load balancer. Sometimes the load balancer is an actual hardware device with the software embedded (sometimes referred to as a content switch), while other times the load balancer is just software installed on a machine of your choosing.
Load balancers often have three types of logic for distributing incoming HTTP requests. They are:
1.Round-robin: With this method, the load balancer simply takes each incoming request and sends it to the next Web server. For instance, if Figure 1 represented our actual environment, the first request to reach the load balancer would be directed to Web server 1, the second to server 2, the third to server 3, the fourth to server 4, and the fifth back to server 1.
2.Sticky sessions (cookie-based): The round-robin form of load balancing completely ignores the concept of user sessions, so that during a single session one user could be passing requests through several different Web servers. For an e-commerce application, this won't work. For these types of applications, on sites that use sessions, the load balancer needs to be able to identify a particular user and keep that user pointed to the same Web server throughout the entire session.
Even this doesn't ensure a balanced load on each server. If somehow all of the users assigned to Web server 1 spent hours using the application and all the other users just spent minutes, Web server 1 could get overloaded while the other servers went underutilized. That's why many load balancers have dozens of load-balancing options and algorithms. They may balance by total traffic volume, they may monitor the resource utilization on the Web server to decide which server is least utilized at the moment the next request is received, or they may exercise any number of other possibilities
3.Sticky sessions (IP-based): incoming traffic is “stuck” to a single web server based on the IP address of the requesting client. If a server goes down, the sessions that were tried to it are distributed to other servers.
Some Load balancers are:Nginx reverse proxy load balancer,HAProxy
Good links are:
http://community.neustar.biz/community/wpm/blog/2009/02/02/tips-for-testing-with-load-balancers
http://www.ibm.com/developerworks/rational/library/4784.html/
OR
Sometimes the same physical device serves as any combination of firewall, proxy server, router, and/or load balancer. Sometimes the load balancer is an actual hardware device with the software embedded (sometimes referred to as a content switch), while other times the load balancer is just software installed on a machine of your choosing.
Load balancers often have three types of logic for distributing incoming HTTP requests. They are:
1.Round-robin: With this method, the load balancer simply takes each incoming request and sends it to the next Web server. For instance, if Figure 1 represented our actual environment, the first request to reach the load balancer would be directed to Web server 1, the second to server 2, the third to server 3, the fourth to server 4, and the fifth back to server 1.
2.Sticky sessions (cookie-based): The round-robin form of load balancing completely ignores the concept of user sessions, so that during a single session one user could be passing requests through several different Web servers. For an e-commerce application, this won't work. For these types of applications, on sites that use sessions, the load balancer needs to be able to identify a particular user and keep that user pointed to the same Web server throughout the entire session.
Even this doesn't ensure a balanced load on each server. If somehow all of the users assigned to Web server 1 spent hours using the application and all the other users just spent minutes, Web server 1 could get overloaded while the other servers went underutilized. That's why many load balancers have dozens of load-balancing options and algorithms. They may balance by total traffic volume, they may monitor the resource utilization on the Web server to decide which server is least utilized at the moment the next request is received, or they may exercise any number of other possibilities
3.Sticky sessions (IP-based): incoming traffic is “stuck” to a single web server based on the IP address of the requesting client. If a server goes down, the sessions that were tried to it are distributed to other servers.
Some Load balancers are:Nginx reverse proxy load balancer,HAProxy
Good links are:
http://community.neustar.biz/community/wpm/blog/2009/02/02/tips-for-testing-with-load-balancers
http://www.ibm.com/developerworks/rational/library/4784.html/
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