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Geo-DNS can be used to balance traffic across data-centers. I found this (old) article where it is described why Geo-DNS implies availability issues if one of the IPs (data centers) goes down.

Could someone confirm or deny such issues? Alternatives? Is a plain DNS with multiple A records better?

Thanks, Valentino

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This is more or less becoming a duplicate of the discussion in… ? – Jesper Mortensen Oct 2 '09 at 11:47
Well, the answer to this question is useful to answer the other question. IMHO, this question has a value in itself. – vmiazzo Oct 19 '09 at 13:46
What happened is, people answering "the other question" have already answer to this: Geo-DNS creates availability issues. Anyway, no one answered directly this question. What is the standard way to handle such situation on SF? Should I answer the question? – vmiazzo Oct 19 '09 at 13:49

Multiple A records is definitely not the answer. If one of your hosts dies, the DNS will still happily dish out the dead IP address.

GSLB (Global Server Load Balancing) or equivalent, will do health checks on all your servers, and will dish out a single IP for the server that is "closest" to the client, or multiple addresses but always excluding dead hosts, and will provide the preferred order based on "closeness". Fundamentally, you need something that performs a health check of all your hosts, and stops DNS from giving out the IP address of any dead hosts.

Anything that relies on the individual client's interpretation of what to do with multiple A records is going to cause issues, as each client (Operating System, browser, ISP DNS caching, etc.) is different.

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AFAIK the only way for a DNS to suggest a data center instead of another is to reply with just the IP (or IPs) associated to that data center. If the data center becomes unreachable then all those IP are also unreachables. This means that, even if smart HTML browsers are able to instantly try another A record , all the attempts will fail until the local cache entry expires and a new DNS lookup is done, fetching the new working IPs (I assume DNS automatically suggests to a new data center when one fail). So, "smart DNS" cannot assure instant fail-over. – vmiazzo Sep 30 '09 at 14:35

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