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How Multiple Server Hosting impacts your website's uptime

By Godfrey Heron
Posted Tuesday, June 15, 2004

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Hosting of web sites has essentially become a commodity.
There is very little distinguishing one hosting company
from the next. Core plans and features are the same and
price is no longer a true determining feature. In fact,
choosing a host based on the cheapest price can be more
expensive in the long term with respect to reliability
issues and possible loss of sales as a result of website
downtime.

Selecting a host from the thousands of providers and
resellers can be a very daunting task, which may result
in a hit and miss approach. But although hosting may have
become a commodity, one distinguishing feature that you
must always look out for is reliability.

At the heart of any hosting company's reliability is
redundancy. This ensures that if a problem exists at one
point, there will be an alternative which ensures
continuity as seemlessly and transparently as possible.

Most hosts do employ redundant network connections. These
are the high speed pipes that route data from the server
to your web browser. But, redundant 'multiple web servers'
have been extremely rare and very expensive, requiring
costly routing equipment which has previously been used
only in mission critical applications of Fortune 500
companies.

However, a very neat but little known Domain Name
Server(DNS) feature called 'round robin' allows the
selection and provision of a particular IP address from a
'pool' of addresses when a DNS request arrives.

To understand what this has to do with server reliability
it's important to remember that the Domain Name Server
(DNS) database maps a host name to their IP address. So
instead of using a hard to remember series of numbers
(IP address) we just type in your web browser
(www.yourdomain.com), to get to your website.

Now, typically it takes at at least 2 to 3 days to
propagate or ‘spread the word’ of your DNS info throughout
the internet. That's why when you register or transfer a
domain name it isn't immediately available to the person
browsing the web.

This delay has stymied the security benefits of hosting
your site on multiple servers, as your site would be down
for a couple of days if something went awry with one
server. You would have to change your DNS to reflect your
second server and wait days before the change was picked
up in routers on the internet.

However, the round robin DNS strategy solves this
predicament, by mapping your domain name to more than
one IP address.

Select hosting companies now employ the DNS round robin
technique in conjunction with'failover monitoring'.

The DNS round robin failover monitoring process starts
by a web hosting company setting up your site on two or
more independent web servers (preferably with different
IP blocks assigned to them). Your domain name will
therefore have 2 or more IP Addresses assigned to it.

Then the failover monitor watches your web server(s)
by dispatching data to a URL you specify and looking for
particular text in the results. When the system detects
that one of your IP addresses is returning an error,
and the others aren't, it pulls that IP address out of the
list. The DNS then points your domain name to the working
IP address/s

If any of your IP's come back online they are restored
to the IP pool. This effectively and safely keeps your site
online – even if one of your web servers is down.

The average failure detection and recovery time with a
system like this can be as low as 15 minutes. This time
varies depending on the speed of your site and the nature
of the failure and also how long other ISP's cache (save)
your DNS information.

The time taken for other ISP's caching your information
can be manipulated in the failover monitor by lowering the
"time to live" (TTL) cache settings. These are the
settings that other ISP's will use to determine how long
to cache your DNS information.

Of course you must bear in mind the matter of how
frequently data is synchronized between your website's
servers. This will be the hosting company's
responsibility, and this may become complicated where
databases and user sessions are involved.

The very expensive hardware based failover monitoring
systems that point a virtual IP address to other ISP's,
while behind the scenes juggling a number of unique IP
addresses on different servers, is of course the most
'elegant' solution to multi server hosting.

That way, the whole issue of ISP's caching your
information does not come into play.

Therefore, for site's that need to have true 99.99995%
uptime, without huge outlays of money, the technology
is readily available and certain proprietory failure
monitoring systems are now relatively cheap to apply.


About the Author
Godfrey Heron is the Website Manager of the Irieisle
Multiple Domain Hosting Services company.Signup for your
free trial, and host multiple web sites on one account:
(http://www.irieisle-online.com)