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IIS Failover Senerio.

Posted on 2007-12-05
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I administer some web application that require extremely high availability.  My organization has two data centers, both have independent subnets/vlans.  I would like to mirror my DB server from data center #1 to a DB server in data center #2 and then do the same with my web servers.

i know i can use the built-in mirroring functionality of SQL Server, but would be the best thing to use for IIS?  Can i use NLB even though they are on two different networks?  If I can use NLB, then can it be configured to only allow connections to the webserver in data center #2 if the web server in data center #1 is unavailable or will it be forced to do true load balancing?

I do not really need to load balance since the web apps I server aren't high traffic apps, they just require extremely high availability.

Thank you for your input!
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Question by:jasco4617
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Ted Bouskill earned 250 total points
ID: 20415361
Microsoft's built in load balancing should be on the same subnet (network).  You could use a hardware NLB solution which might be able to span subnets.

Mirroring may not work the way you expect.  One copy is live, the other is not.
http://aspalliance.com/1388_Database_Mirroring_in_Microsoft_SQL_Server_2005.all
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by:digital_thoughts
ID: 20415394
I think as long as you can access the IIS Server in data center #2 from data center #1 and vice versa, then you should be able to use NLB without an issue.

Do the users access the web application via direct server url? i.e. http://servername/appname/ ? Or some other means? You might be able to use DNS to give you the failover you need, where it will attempt access on the primary server first, but if unavailable, it will try the second.
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by:jasco4617
ID: 20415395
i thought about doing something like, the only problem is that if i have the hardware NLB in datacenter #1 and data center #1 goes down, then the NLB cluster it creates will go down.  If i am wrong on how that would act, then let me know.
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by:jasco4617
ID: 20415430
Digital...

the way it is currently set is that there is a single IP address that has a hostname of say hello.world.com.  that IP address is added to the IP address list for my NIC and then the IIS website is assigned that IP.

I am by far not a networking guy and I was unaware that DNS could do fail over.

Thanks.
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by:Ted Bouskill
ID: 20417528
digital_thoughts: DNS cannot do the load balancing.  It will only support DNS round robin but that is not reliable because if a server dies, DNS round robin will continue to send requests to the dead server.  Also, NLB does not work properly in two sites.  All servers must be in the same subnet.  I know from experience.

In regard to the hardware NLB you actually use redundant hardware at one site.  If one goes down, the other takes over.

By the way, if you are really paranoid about losing a data center then you need 4 NLB devices or two NLB arrays.  One at each site.
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by:jasco4617
ID: 20422751
tedbill...

in response to your first post:  

What I had in mind if MS's NLB could work would be to have a the NLB set so that the web server in the 2nd data center would only be used by setting the Load Weight to 100 on the appropriate web server.  Then configure the web app in the 2nd data center to point to the mirrored instance of the database.  That way if data center #1 goes down the mirrored DB will become the principle server and the web server in data center #2 would start receiving traffic.  

However, if NLB will not work across subnets and i can't get my organization to create a vlan that spans the two locations then that idea is useless.

Also, the web and db are located on the same server so if one goes down both do.
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by:Ted Bouskill
ID: 20424407
Well I have managed to get NLB working across a subnets but it didn't work well (it was over a year ago so I can't remember why I decided not to use it)
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by:dtripp33
ID: 20508570
I have looked into an application that might help. Simple Failover http://www.simplefailover.com/ This will monitor your website(s) on many different levels and adjust where the DNS points to based on the results. It is highly customizable and fairly low cost.
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