Webopedia defines server farm clustering as follows:
“A server farm is a group of networked servers that are
housed in one location. A server farm streamlines
internal processes by distributing the workload between
the individual components of the farm and expedites
computing processes by harnessing the power of multiple
servers.
The farms rely on load-balancing software that
accomplishes such tasks as tracking demand for
processing power from different machines, prioritizing
the tasks and scheduling and rescheduling them
depending on priority and demand that users put on
the network. When one server in the farm fails,
another can step in as a backup.”
It is important to note, that typically, web servers,
which are load-balanced in such a manner, display one
external IP address to the public Internet, while
using internal network IP’s to communicate between
the clustered servers and load balancer.
Now this is indeed fantastic! Not only do you receive
web site peak demand scalability with web server
clusters, but you also have the built-in
“high uptime availability” component which is so
important.
However this is only half of the picture.
There are very important cautionary notes to keep in
mind.
Where web hosting is concerned, availability depends
on two things:
1.Hardware reliability (RAID drives, server
clustering etc) within the Data Center;
2.High Bandwidth Internet Connectivity to the
Data Center / Network Operating Center (NOC).
Now, with all your well thought out server clustering
solutions, what would be the result, if, (as had
recently occurred in a very high profile web company),
a fire in the Network vicinity had caused the entire
Data Center to shut down power for hours. Or, a
bandwidth provider to the NOC had router problems.
All your websites would be showing the dreaded “Page
Cannot be Displayed” page.
The ideal solution therefore would be to employ
clustering solutions with servers in entirely
different Data Centers with different bandwidth
providers. Redundant Data Centers eliminate the NOC
itself being a single point of failure. This
scenario becomes interesting at this point, because
the difficulty of addressing the potential problems
now increase exponentially.
We now have to deal with DNS caching, the concept of
failover, and how static and dynamic web
applications respond to failure events.
Failover and Load balancing are frequently used
interchangeably, however they are in fact quite
different.
·Load Balancing refers to physically sharing
servers capacity, so that one server is not
overloaded and swamped with requests.
·Failover however, is the process that manually
or automatically switches a failed server or
bandwidth provider to a standby server or
network if the primary system fails or is
temporarily shut down for servicing.
As such, failover software is an important function of
mission-critical systems that rely on constant
accessibility.
One of the inherent difficulties with failover for Web
Hosting companies operating on different networks
is the limitations imposed by the DNS caching system.
As DNS records are passed from the original DNS
servers (i.e., ns1/ns2.your-domain.com), they are
cached or stored at several different ISP’s along the way.
Which is why it takes a while for a newly
registered domain name to resolve to its IP address.