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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.
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