In Spring 2006, my site was plagued with recurrent hardware problems causing serious downtime. At the time, the site was hosted on a dedicated server and I had no failover strategy whatsoever so when a hard disk failed on the server, you could expect a few days of downtime.
At the beginning of the summer, I got fed up and started investigating possible solutions to this problem and, after some experimentation, finally settled on DNS failover. Here are the results of the experimentation, originally posted on WebHosting Talk:
I run a site with about 1,000,000 unique visitors per month and recent server failures made me decide to get a failover server to minimize downtime. My goal wasn't to get 99.999% uptime but to be able to be back on track after a failure in a "reasonable" amount of time. After evaluating several solutions, I decided to go with DNS failover. Here's how the setup work:
1) mydomain.com points to main server with a very low TTL (time to live)
2) failover server replicates data from main server
3) when main server goes down, mydomain.com is changed to point to failover server
The drawback is the DNS propagation time since some DNS servers don't honor the TTL and there is some caching happening on the user's machine and browser. I looked for empirical data to gauge the extent of the problem but couldn't find any so I decided to setup my own experiment:
I start with mydomain.com pointing to the main server with a TTL of 1800 seconds (1/2 hour). I then change it to point to the failover server which simply port forwards to the main server. On the main server, I periodically compute the percentage of requests coming from the failover server which gives me the percentage of people for which the DNS change has propagated.
I made the DNS change at exactly 16:04 on 06/21/06 and here are the percentage of propagated users:
06/21/06 16:00 0 %
06/21/06 16:05 3 %
06/21/06 16:10 20 %
06/21/06 16:15 37 %
06/21/06 16:20 59 %
06/21/06 16:25 69 %
06/21/06 16:30 76 %
06/21/06 16:35 80 %
06/21/06 16:40 86 %
06/21/06 16:45 90 %
06/21/06 16:50 91 %
06/21/06 16:55 92 %
06/21/06 17:00 93 %
06/21/06 17:05 94 %
06/21/06 17:10 94 %
06/21/06 17:15 95 %
06/21/06 17:35 95 %
06/21/06 17:40 96 %
06/21/06 17:45 97 %
...
06/22/06 10:40 99 %
So even after 18 hours, there is still a certain percentage of users going to the old server so DNS failover is obviously not a 99.999% uptime solution. However, since more than 90% of the users are propagated in the first hour, the solution works well enough for me.
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This is not the first time that I have landed on this page.
But, this time there is something to share.
A pretty popular media site has setup *double* failover using the .
For branding reasons they determined that they should have a separate failover site rather than the free failover page that is included with the service.
As a bonus, if their production servers *and* failover servers go out of action, they will then failover to the default failover site maintained by the dns provider.
DNS Utils has some free utils to help with DNS problems.
http://www.dns-utils.com
Ok, having been here 5 times before :D
I read the article one more time, and it seems that it is a glass half-full and glass half-empty situation.
The change was made at 16:04, and the TTL was 30 minutes. Well, at 16:35, 31 minutes later, 80 percent had migrated over. In other words, within the allowed TTL, 80 percent had swung over. If the TTL had been 5 minutes, then 80 percent would have moved over by 16:10.
If you can keep 80 percent alive, or leave 100 percent dead, which is better?
Whoops!
Sorry, I was concentrating on the numbers so much that I missed your very last sentence.
Your post is being quoted at stackoverflow.com as a justification for *not* depending on dns failover as a means of improving availability. Obviously, this is not what you said in the last sentence of your original post.
I used http://www.postpart.com for failover and geo dns. It works for me.
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This may be irrelevant anymore but I just found this post because I was looking for a solution to a similar problem. My site is hosted by NameCheap, and when last week they were maintaining their servers and changing the HDD my site was on, the site was down for hours. And those problems lasted several days.
I am not really technical but love your idea of saving over 90% of all visitors when a server is down. Not sure I can use it though... Hope you continue writing posts for your blog! Darren
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For large scale sites DNS Failover really works great. I usually keep TTLs at 30 seconds and can show via Zabbix monitoring graphs full failover of all traffic between datacenters in less than 1 minute. It also works great for DNS load balancing where you need multiple sites hot and want to automate removing a site during outages. I'd be happy to share some zabbix graphs if you would like to see an example of a high volume site during a dns failover of traffic - anyone can email me at scott@dnshat.com if interested.
Thanks,
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