"Round Robin" DNS for redundancy as well as load balancing
If you are unfamiliar with the concept of Round Robin DNS configurations, please go to the previous page in our tutorial: <<Round Robin DNS configuration
Round Robin: Normally used as a load balancing system, round robin can also be applied to provide a redundant web server configuration to provide protection in case of a web server going down.
Redndancy Schemes: If you have websites or internet services that are critical or you cannot afford outage if a server fails for any reason, there are a number of different redundancy schemes available. By having
the services available on two geographically and topologically seperated servers ensures a very high probablility to one of them being available at any one time and many solutions address the issue of sending all traffic to one of
them should the other one fail. Many of these require a third management server which again introduces a single point of failure and, of course, requires the costs and administration of a third server.
Using Round Robin DNS configuration for redundancy: A round robin configuration employs two or more name servers registered to the domain which point to different servers. The name servers are served in turn and, thus,
the servers providing the service will share the internet traffic accordingly. A more detailed description of this technique is described here. To extend this ability to provide redundancy, the DNS servers for the domain
should be on the same physical machine as the services that they are pointing to. In this way, should one of the servers fail, the DNS will fail with it and the requesting machine will ask for the name server again. The next request
from the name server will give another DNS server which is on a physically different machine and this DNS will also point to itself. This ensures that the DNS request is only served by a machine that can also provide the service. Let's
look at an example of one of our high traffic sites using this configuration:
www.metric-conversions.org.
The name servers for this domain name are as follows:
ns1.metric-conversions.org. [87.106.57.223] [DE]
metric-conversions.org. NS ns1.metric-conversions.org.
metric-conversions.org. NS ns2.metric-conversions.org.
metric-conversions.org. A 87.106.57.223
ns1.metric-conversions.org. A 87.106.57.223
ns2.metric-conversions.org. A 72.55.156.214
www.metric-conversions.org. CNAME metric-conversions.org.
ns2.metric-conversions.org. [72.55.156.214] [CA]
metric-conversions.org. NS ns1.metric-conversions.org.
metric-conversions.org. NS ns2.metric-conversions.org.
metric-conversions.org. A 72.55.156.214
ns1.metric-conversions.org. A 87.106.57.223
ns2.metric-conversions.org. A 72.55.156.214
www.metric-conversions.org. CNAME metric-conversions.org.
Normally, the two servers share the traffic equally providing load balancing. Should the first server go down, one of the following two scenarios will take place when accessing metric-conversions.org
Scenario A
1) Request for name server, second name server ns2.metric-conversions.org is served
2) www.metric-conversions.org translates to metric-conversions.org which translates to 72.55.156.214
3) Request successfully made to working server.
Scenario B
1) Request for name server, first name server ns1.metric-conversions.org is served
2) ns1.metric-conversions.org fails. Re-request name server for metric-conversions.org, second name server ns2.metric-conversions.org is served
3) www.metric-conversions.org translates to metric-conversions.org which translates to 72.55.156.214
4) Request successfully made to working server.
Of course, an ISP which has cached the DNS previously will still try to access the failed server but by changing the TTL (time to live) setting for the DNS record to a few minutes the outage due to this case will be kept to a minimum. More
elaborate and complex solutions may have advantages over this method but for many applications, this will provide a very simple adequate solution to implementing redundancy with just two servers.
We offer a service to implement this configuration on our servers if you would prefer not to do this yourself: business web site hosting services >>
We offer a search engine optimization service and we have a website hosting solution. Please contact our Isle of Wight office for more information.
