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Thread: redundancy?

  1. #1
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    redundancy?

    I'm not sure if this is possible, but what I'm looking to do is to have a complete, live back up of my hosting site on a different server in case the first server goes down. Is there a way to do this without having to make changes separately to each site, so that when one site is updated, so is the other, and, (and I know this is the kicker) to have the domain point to the first site for every day use and ONLY to point to the backup one in case of server being down?

    I don't know if I explained this correctly, so let me know if you need clarification.

    Thanks!
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  2. #2
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    Well I know exactly what you mean, a 'mirrored site' that only comes into view if the 'normal server' is down.

    But I don't know the answer in full,
    I'm sure someone will explain for the 'technicaly challenged' like me.
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  3. #3
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    That's exactly what I'm looking for.
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  4. #4
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    If your site has only static content, you can publish to your main server and then use a tool like rsync to synchronize the changes to the backup server. Things are trickier if you have a dynamic site that is driven by data in a database - then you have to make sure that data written to the database is synchronized as well which is more involved. It can be done in MySQL with it's replication support, but it's more complicated to get setup.

    As for the failover, if you can have a few minutes of downtime the easiest thing to do is to setup your DNS with short TTLs (say 5 minutes) and manually point your DNS records to the backup server when there is an outage. You'll want to make sure you're using a DNS provider that's not in the same datacenter as your primary site, or else you'll likely lose both if anything major happens to cause your main server to go down.

    Also, you'll need to setup monitoring of your server, so you know when it needs to be failed over. There are a number of third-party services that can do this for you.

    Each piece of this can be made more complex in order to cut down on the downtime you suffer, but with a greater investment in time and money - it really comes down to a tradeoff of how expensive downtime is to your site.

  5. #5
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    The only problem with using DNS failover is the caching. Most clients have a DNS cache which isn't cleared for several hours, so if they visit your site before the outage their DNS won't show the failover IP for them until their cache clears.

    Therefore, to constant visitors that type of failover won't work, if the outage occurs before the user tries to connect to the site, then it will work.


  6. #6
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    Thanks! This is exactly the info I needed.
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  7. #7
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    Not with a super-low (i.e. 5 minutes) TTL.

    Quote Originally Posted by devonblzx View Post
    The only problem with using DNS failover is the caching. Most clients have a DNS cache which isn't cleared for several hours, so if they visit your site before the outage their DNS won't show the failover IP for them until their cache clears.

    Therefore, to constant visitors that type of failover won't work, if the outage occurs before the user tries to connect to the site, then it will work.
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  8. #8
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    Quote Originally Posted by mripguru View Post
    Not with a super-low (i.e. 5 minutes) TTL.
    That may work great in theory for that purpose but some ISPs will not respect such a low TTL (unfortunately).

    A nice solution, if you don't mind round-robin and can keep your content synchronized, would be to use a TTL that isn't too low or too high (i.e. 900, 1800, 3600, etc.) but keep two or more A records by default. When one of the servers specified fails, remove its A record automatically. Most browsers will recognize the fact that there are multiple IP addresses and gracefully toggle between them in the case of an outage, with the only impact being a delay in initially displaying the site (which isn't quite as bad as a full outage)--also, by removing the failed server from the set of A records, you mitigate that concern.

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