How DNS Round Robin Failover Monitoring Improves Uptime
Due to this lack of differentiation, potential users look for other small features that distinguish hosting companies from one another. One such feature is redundancy which ensures the website owner that if an issue were to arise, there’s an alternative to continue to function as normal.
Most web hosting providers offer redundant network connections which route data from their server to each website across a high speed line. It’s extremely rare and can be costly to find redundant multiple web servers. The expense is high due to the cost of the routing equipment.
What is DNS Round Robin Failover Monitoring?
A few multi-server hosting companies utilize the “round robin” DNS (domain name server) strategy which maps a domain name to multiple IP addresses. Accompanying the round robin DNS strategy is “failover monitoring” which observes a web server or servers to pick out which IP address is functional.
DNS databases map a host name (website) to that specific IP (Internet Protocol) address. This is an easier way to access websites as opposed to remembering various IP addresses in order to display specific web pages.
When a site is first registered, it takes on average two to three days to proliferate the information around the Internet. For this reason the site is usually not available to an individual browsing the Internet for a set period of time.
Unfortunately, this delay blocks the security advantages of having a site on multiple servers. A similar occurrence arises when a server goes down causing websites to no longer be functional. Therefore the DNS would have to change to the second server which would take a few more days before it was spread throughout the Internet.
Luckily, the round robin DNS strategy disentangles this dilemma by mapping the domain name to multiple IP addresses. Since the probability of one of those IP addresses being functional is high, there’s a likely chance that the website will stay running.
How does it work?
The whole process begins with the web hosting provider. They set up the site on two separate web servers with different IP’s assigned to each of them. This means that domain will have two or more dedicated IP’s.
Then the round robin DNS feature automatically selects an IP address from a pool when the DNS request is initially delivered. As this is happening, the failover monitor is observing the server by sending a small amount of data to the URL looking specifically for text in the results. If one IP address returns an error, it pulls that IP address out of the pool. The DNS then connects the site to one of the IP addresses not returning an error.
As other IP addresses become available, they are placed back into the IP list for future use. This procedure is a safe and efficient method of keeping websites functional even if one web server is down.
With this method, the amount of time to detect the error and recover can be as low as 15 minutes depending on the speed of the website; length of time for ISP’s to cache the DNS information and the nature of the failure.
When other ISP’s cache the information on a website, they determine the amount of caching time. A nice feature of the failover monitor is the ability to lower the “time to live” (TTL) cache settings. This decreases the amount of time an ISP takes to recognize the information on a website.
Shortcomings of this Method
Although the DNS round robin failover monitoring method is efficient and reliable, there are a few downfalls. First, the hardware is extremely expensive. This is considered the most elegant solution to multi server hosting. Also, there are other failure monitoring technologies available that are cheaper than the hardware required for this method.
Also, this method does not offer support for server affinity which is a load balancing system’s ability to manage a user’s requests. Since DNS round robin caches the servers IP address, when the cache expires, it must make an additional request to the DNS server for that IP address. If the request doesn’t return the same IP address, session information is lost.In the end, it’s better to search for web hosting providers that offer multi-server support in the event that one server fails. The DNS round robin failover monitoring procedure is an excellent process for improving server uptime and allowing websites to be functional the majority of the time.
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