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Friday, April 30, 2010

Release It! - Chapter 11.2 Virtual IP Addresses

Cluster servers, such as HP ServiceGuard or Veritas Cluster Server, give non-clustered applications the appearance of being clustered. They work by detecting if the instance is running or not and starting one up if needed.  They use Virtual IP Addresses to redirect traffic as needed.  A Virtual IP Addess is an IP address that can be moved from one NIC to another.  Some lower layer TCP/IP manipulations have to be done in order make it work. In general, if your application calls any other service through a virtual IP, it must be prepared for the possibility that the next TCP packet isn’t going to the same interface as the last packet. This can cause IOExceptions in strange places.  Ruthless testing is a good idea in this case.  See if you can configure your testing environment to use Virtual IPs and repoint them to see how your system reacts.

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