Hi, Partly because I never like straightforward solutions, I am looking to implement a non-standard failover system that owes its origins to mixing RAID 5 with some beer. The idea is to have machines A, B and C, configured as follows: 1) Any given process is running on TWO machines at the same time. If a process or machine fails, then a new backup is started on the third machine. There is thus a rotation around the nodes. 2) Packets destined for any of the machines should be received by ALL of the machines. 3) If the primary process is on A, then replies from B and C should still be generated, but be transparently dropped. Likewise, if the primary process is on B or C. Let's say you have an Apache process running on A and B. B is shadowing A on everything. It's at the same point, has the same connections established, etc. Failover becomes merely "ungagging" B. How is this a LARTC problem? Uhhh... because this approach is seriously abusing the entire networking stack. We essentially have all three machines running with an identical MAC and IP address visible to the outside. The "distinct" address is purely internal. What this requires is a way of tricking all three machines into believing that they are the sole recipients. This keeps the stacks in a uniform state, which means we can fail-over the connections without having to either checkpoint or copy stateful information, both of which get ugly when you start talking about lots of information. This leads to the second network-related problem. If you have two identical machines starting from identical states, and processing identical streams, then they should end up in identical states - ie: crashed. This is easily fixed. If A is the machine you are starting all the processes on, and B is your "mirror", then C needs to take up the excess load. In other words, A+C is a cluster, and B+C is a second cluster. Processes migrated to C from A or B aren't mirrored. (This is akin to RAID 5's partial backup.) So, the three machines need to be seen from four distinct views: A) From outside, a single machine is visible. B) From the HA perspective, there is one primary machine, one mirror machine and one spare C) From the load-balance perspective, there are two overlapping clusters. D) From the LAN perspective, there are three distinct, uniquely-addressable machines. Using Linux' advanced networking, BPF and netfilter layers, connection mirroring for HA/load-balancing purposes should be straight-forward. Much more so than using the "wedge" concept proposed by MIT. Because you need send nothing more than flags between the machines, it should also be less expensive on the LAN and processor. So... how would you go about doing this? __________________________________ Do you Yahoo!? Yahoo! Mail SpamGuard - Read only the mail you want. http://antispam.yahoo.com/tools _______________________________________________ LARTC mailing list / LARTC@xxxxxxxxxxxxxxx http://mailman.ds9a.nl/mailman/listinfo/lartc HOWTO: http://lartc.org/
