Sunday, July 17, 2011

High Availability (failover) - Round-robin DNS

I'm coming back to this blog, but I'll be posting only short notes. I need to learn a lot these days, but there's no much time to document all that learning.

  • Round-robin DNS is a feature provided by implementations like BIND9 with which you can load-balance your services, provided that all the servers are in sync. Though many people argue that this mechanism is not intended for failover which is my main focus of attention.

  • The way it works is pretty straight forward:

  1. You define several records with the same name, but pointing to different IP addresses.

  2. The priority among those records gets permuted every now and then.

  3. Mixed with the proper specification of the TTL parameter you can distribute requests more or less equally.

The high availability issue is a very interesting one. This was my first approximation to that matter, and there are many more alternatives left to explore. One drawback of this option is that servers need to be in perfect sync and for the time being I'm using  an asynchronous solution for replication.

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