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Some kind of spillover - Is this possible?


Pat O

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Okay so we have a web application which is fronted by NetScaler.

The setup we would like to achieve is this:

3 Servers with on each a web server which serves the app.
3 Backup Server with on each a web server which serves the app.

We would like to make a LBVS that forward request to the 3 main server, but when one of the main server goes offline or unreachable, that Netscaller use one of the 3 backup server to replace it.

Or if you prefer, each time one of the main server is unavailable, that netscaler pick one backup server to replace the unavailable main server.

Is this possible?

I tried the Health spillover, but then it switch everything to the backup servers when the threshold is reached. Our app is not multiple server aware, so if you get switched to a new server you loose everything you did on the first and have to log back in again and restart what you were doing, so switching the two good server to the backup server when only one server has a problem, is more of a nightmare than having one down server...

In other words we have session affinity on this app.

Thanx to anyone who has an idea how to proceede

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In some way yes.

When configuring the spillover you have to use the BackupVirtual server field to tell it which VS to use when the threshold is reached.

But as I said in the previous entry, I can't use that because it switch ALL the server, not just the one that failed.
So in my case if I have 3 servers, all 3 servers are made unavailable and every user is switched to the backup servers and thus all my users have lost their work, they must log back in the app and redo their work.

That is unacceptable.

What I would like is when one server goes down, the Netscaler pick one server in the backup virtual server and use it to replace the unavailable server, without touching to the two working servers.

Think of it as a pool of server, where you have a pool of server that can be used when one of the main server goes down.

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You're not going to be able to do service level failover on a single appliance. The only good way to make that work is to have the "front end" Netscaler, which has an LB VIP with services that each point to a "back end" Netscaler, with one VIP for each pair you want to act as failover. On the back end Netscaler, you would have three VIPs that are directly accessible, and three VIPs that are not directly accessible. For each of the VIPs that is directly accessible, you would set its partner non-directly accessible VIP as the Backup Virtual Server. Thus, the front end Netscaler handles load balancing between the servers, and the back end Netscaler handles the failover portion. This is not possible on a single appliance or HA pair because you cannot point a service to another VIP on the same appliance.

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