How Multiple Server Hosting impacts your website's uptime
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
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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