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1. Load Balancing And Yahoo!

2. A high-volume site like Yahoo! knows that the actual
quality of service any web server provides to end users
basically depends on network-transfer speed and server
response time. Network-transfer speed refers to the
Internet-link bandwidth while server-response time
depends upon resources including fast CPU, lots of RAM
and good I/O performance. Once these resources are
exhausted and the web-server is encountering heavy
traffic, a problem would surely arise.

3. Load Balancing

4. A problematic situation pertaining to difficulty in
handling high volumes of incoming traffic can be solved
either through installing more RAM on existing machines
or replacing the CPU with a faster one. The use of faster
or dedicated SCSI controllers and disks with shorter
access time can also be done. Software can be tuned so
that the operating system parameters and web server
software can be adjusted to achieve better performance.

5. An alternative approach is to improve performance by
increasing the number of web servers. This approach
would attempt to distribute traffic unto a cluster of
back-end web servers that need not be large-scale
machines. Web server scalability is achieved when more
servers are added to distribute the load among the
group of servers or server cluster.

6. This is what load balancing is all about. It involves the
fine tuning of a computer system, network or disk
subsystem in order to more evenly distribute the data
and/or processing across available resources. Load
balancing is distributing, processing and
communications activity evenly across a computer
network so that no single device is overwhelmed. Busy
websites usually use two or more web servers in a load
balancing scheme so that when one server gets
overwhelmed with requests, traffic is forwarded to
another server with more capacity.

7. There are two probable reasons why a company could
want to load balance traffic across firewalls. One is for
purely technical reasons and the other is centered on
winning business. The technical aspect should be quickly
addressed as soon as funds and environment allow.

8. When there is only one web server responding to all
incoming HTTP requests for a website, it may not be
able to perform accordingly especially if the website has
gained popularity. Loading of web pages will be very
slow and some users would have to wait for their
requests to be processed. It can come to a point where
upgrading the server hardware is no longer cost
effective due to the increased traffic and connections to
a website.

9. Yahoo! was granted a patent from a filing done in 1999
regarding coordinating information between multiple
servers that share information as well as servers that
may cache some of the information. Load balancing
devices are becoming very common in supporting high-traffic
websites. These devices evolve as websites grow
in terms of size, complexity and traffic flow.

10. The presence of multiple web servers in a server group
requires that HTTP traffic be evenly distributed among
the servers. These servers should appear as a single web
server to the web client. The load balancer simply
intercepts each request and redirects it to an available
server in the server cluster.

11. Methods of Load Balancing

12. Load balancing can be achieved in a number of ways.
Choice would depend on the individual requirement,
available features, complexity of implementation and
the cost. The user company would have to determine its
circumstances to determine which option would work
best.

13. The Round Robin DNS Load Balancing is one of the early
adapted load balancing techniques. The built-in round
robin feature of BIND of a DNS server facilitates cycling
through the IP addresses corresponding to a group of
servers in a cluster. It is a fairly simple and inexpensive
method which is very easy to implement. However, its
downside is that the DNS server does not have any
knowledge of server availability thus may continually
point to an unavailable server. It has the ability to
differentiate by IP address but not by server port. There
is also the possibility that the IP address is cached by
other name servers which would result to request not
being sent to the load balancing DNS

14. server.

15. In Hardware Load Balancing, hardware load balances
route TCP/IP packets to various servers in a cluster. This
method is said to provide a powerful topology with high
availability. It uses circuit level network gateway to
route traffic. Its one downside is the higher cost incurred
as compared to other methods.

16. The most commonly used method is Software Load
Balancing. Load balancers often come as an integrated
component of expensive web server and application
server software packages. This method is more
configurable based on requirements and can incorporate
intelligent routing base on multiple input parameters.
An additional hardware needs to be provided to isolate
the load balancers.

17. Algorithm of Server Load Balancing

18. When HTTP requests are assigned to any server picked
randomly among the group of servers, this is called
random allocation. It is possible that one server may be
assigned more requests than the others, but generally
each server gets its share of the load. It can be very easy
to implement but the risk of overloading one while
under-utilizing another is big.

19. The IP sprayer assigns the requests to a list of the
servers on a rotating basis when the round-robin
allocation is used. The first request goes to a randomly
picked server in a group so that the entire first request
need not go to the same server especially if more than
one IP sprayer is involved. The circular order is followed
in redirecting the traffic for subsequent requests. The
server which has been assigned a request moves to the
end of the list to ensure that all servers are equally
assigned. The allocation is much orderly than random
but it may not be enough based on processing overhead
required and when there are differences in server
specification in a server group.

20. The shortcoming of the round-robin allocation has been
eliminated by the weighted round-robin version. In this
case, a server that is capable of handling twice as much
load as the other can get a weight of two. This means
that the IP sprayer will assign two requests to the
powerful server as against one request assigned to the
weaker one. This takes care of the capacity of the
servers in the group. However, it does not consider the
advanced load balancing requirements like processing
time for individual request. An efficient load balancer
should be capable of intelligent monitoring that would
help it direct requests to the server that is more capable
of handling them.

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