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This might be a pretty basic question, but my small company is new to HA infrastructure and we are having discussions about what to expect.

We have a KEMP HA load balancer in our network. According to the Kemp web site, Kemp supports "stateful failover to ensure uninterrupted support of users sessions during the transition of traffic to the hot standby unit." (their quote)

In my mind, that means if I have 2 web servers (IIS, if it's important), that if my user is connected through the Kemp to Server 1 and then Server 1 drops offline, then the user will seemlessly be routed through Server 2 - and the user will not notice (no log in, etc.)

Is my understanding correct?

TIA! Rich

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This is typically how HA works. However, you will want to make sure that it is configured correctly. Otherwise, if the sessions are not replicated over to the second server then it will cause the users to automatically be logged out. (this isn't a true logout, but rather, they're hitting a different server which they are not yet authenticated) You will want to check and make sure that your web application supports the passing of session information between the servers. Also be aware of any kind of file storage as if they are being independently hosted on the first machine, any kind of downloads when the failover happens will result in a 404 on the failover server since the file technically doesn't exist on the second server.

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Thank you, this is helpful. The app is ASP.Net and is configured to use SQL Server Sessions, so the sessions should be shared. Our file sharing is done with UNCs, so that should not be an issue either. Still, when we shut down the "active" web server, there are some times when HTTP Posts are lost - for example, AJAX calls fail after the fail over. (I tried to vote this up, but I don't have enough points.) – richb01 Oct 4 '13 at 9:25
I tried to vote you up but, apparently I don't have enough "reputation" to say "Good job!" lol – richb01 Oct 25 '13 at 10:55

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