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Server clustering boosts reliability, eases failover
System downtime is no longer just an internal matter, thanks to the advent of business
on the Web. Clustering is one option for keeping servers running smoothly.
By Edward Hurley, TechTarget
In this age of e-commerce and Web-enabled applications, companies are turning toward clusters of interconnected servers for the scalability and reliability needed to compete.
Server clusters are a broad category of systems that can range from computers sharing
storage to groups of servers that can "failover" or redistribute the workload of one to
another with the help of special software.
One of the major advantages to clustering is the ability to increase computing power by
adding another server or "node" to the cluster. This ability came in handy for the online
pharmacy Drugstore.com. Compaq had designed a system that would accommodate up to 3
million hits, the projected maximum for the site. However, 10 million people hit the site
shortly after launch. To keep the site online, Compaq rotated in more servers, said Mark
Silverberg, Compaq's technical marketing manager for the company's high-performance
server organization.
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Reliability is another advantage that arises from clustering, because some clusters
include management software that reallocates the workload of a server that fails. This
can help to minimize downtime, which can be especially costly for companies who guarantee
their customers system availability. Such is the case with Acxiom Corp., which helps
companies analyze their customer data. "We are truly required to be a 24x7 operation as
we guarantee 99.99% availability to our customers," said Tim Donar, the senior system
architect of the Little Rock, Ark.-based company.
Moreover, clustering software has progressed to a point where each cluster can be viewed
and supervised as a single system, Donar said. Acxiom used to limit its clusters to eight
nodes because each had to be managed separately. For example, adding user account
information used to require entering the information on each server in the cluster. Now
it's just a matter of inputting information once, Donar said.
Perhaps the simplest example of a server cluster is two servers that are connected so
that one mirrors the other and would step in during a failure. This complete redundancy
is expensive but is a consideration for businesses that can't afford to have a system
failure, said Brian Richardson, program director of open computing and server strategies
for Stamford, Conn.-based Meta Group Inc.
"You need to do an assessment of downtime. A cluster is a lot like insurance. You don't
want to spend more on it than the property you are protecting is worth," said
Richardson.
Yet system downtime is no longer just an internal matter. "What was a simple outage has
become headline news," said Dan Kusnetzky, vice president of System Software at
International Data Corp.
In order for failover to occur, each node in the cluster must be in constant contact with
the others, usually by way of an electronic pulse or heartbeat. When a node stops
emitting the heartbeat, the other nodes realize it has failed, and failover kicks in. The
work is then divvied up among the other clusters or taken on by just one, depending on
pre-determined instructions.
Clusters can be configured to failover in a variety of ways. For example, a node can
failover to another in a different location in the event of a disaster. Some
configurations have an extra node in the cluster that is usually idle. In cases of
failover, the idle node takes over and the cluster capacity isn't compromised. Such
functionality also comes into play during routine maintenance. For example, one node can
be taken offline and its work shifted to another server for upgrades or maintenance.
Virtually every server operating system can support clustering to some extent. Generally,
the choice of Unix or Windows, Linux or mainframe operating systems will be guided by the
applications architecture, said Richardson.
Windows NT servers can failover if one goes down, but clusters are limited to only four
nodes. By contrast, a cluster of IBM mainframes can completely share system resources and
appear for all intents and purposes as one system. Such clusters, however, can cost
millions of dollars.
Brisbane, Calif.-based TurboLinux offers clustering software that allows servers of
differing operating systems to be in a cluster. TurboLinux Cluster Server takes advantage
of the Linux operating system, an open source Unix cousin. Thanks to its shared ancestry
with Unix, Linux is very stable. It is also cost effective because traditional licensing
fees don't need to be paid to a software vendor, said Ly-Huong Pham, the company's chief
operating officer.
MORE ON THIS TOPIC:
SearchEnterpriseServers.com
has information on clustering for various types of servers.
SearchSystemsManagement.com's Best Web Links collection includes a section
on Configuration Management.
