January 14, 2012

1.1.4 Load Balancers

A load balancer is a network device that distributes the flow of network traffic between multiple network devices. The goal is to minimize network congestion and bottlenecks. Load balancers can be used to balance traffic to routers, web servers or other network devices either through round-robin techniques or more intelligent methods, e.g. taking into account the number of current connections or response time. It maximizes throughput and ensures the system has the capacity to handle incoming requests and ensure better allocation of resources

A load balancer can be implemented as a software or hardware. Under the most common implementation, the load balancer splits the traffic intended for a website into individual requests that are then rotated to redundant servers as they become available (if a server that should be available is busy or down, it is taken out of the rotation).

Load balancing allows the service to continue even in the face of server down time due to server failure or server maintenance. If you are load balancing across several servers and one of the servers fails, your service will still be available to your users, as the traffic will be diverted to the other servers in your server farm.
Load balancers are generally grouped into two categories: Layer 4 and Layer 7. Layer 4 load balancers act upon data found in network and transport layer protocols (IP, TCP, FTP, UDP). Layer 7 load balancers distribute requests based upon data found in application layer protocols such as HTTP.

Some industry standard algorithms are:
Round robin
Weighted round robin
Least connections
Least response time

Layer 7 load balancers can further distribute requests based on application specific data such as HTTP headers, cookies, or data within the application message itself, such as the value of a specific parameter.

References:
http://www.f5.com/glossary/load-balancer.html
http://www.wisegeek.com/what-is-load-balancing.htm
CompTIA Security+ Study Guide: Exam SY0-301, Fifth Edition by Emmett Dulaney
Mike Meyers' CompTIA Security+ Certification Passport, Second Edition by T. J. Samuelle

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