This topology combines the function of Linux-Directors and Real-Servers into the same hosts, so that high availability and load balancing can be achived with just two nodes. A sample configuration is available.
At any given time one of the hosts is the active Linux-Director, is active, while the other is a hot stand-by. The active linux director accepts traffic for a virtual IP address. This is the IP address that is advertised to end-users though DNS. Connections are load balanced to both the localhost and the other node using LVS. The two nodes monitor each other using Heartbeat and in the event of the active linux-director failing the hot stand-by assumes the virtual address and the service is maintained. Connections are synchronised between the active and hot-standby linux-directors, so that when a failover occurs existing connections are able to continue, as long as the real-server that terminates them is still available.
When a linux-director receives a connection from an end-user it makes a decision on which Real-Server (itself or the other node) to forward the connection to. All packets for the life of this connection will be forwarded to the same real-server so the integrity of the connection between the end-user and the real-server is maintained.
ldirectord monitors the health of the the services on both nodes by periodically requesting a known page and checking that the response contains an expected string. If a service fails on a server, then the server is taken out of the pool of real-servers and will be reinserted once it comes back on line.
The return path for connections is directly onto the local area network, or gateway router. As direct routing is used in this configuration, the return packets do not need to pass back through the linux-director.
The servers may run a variety of services including The Apache HTTP Server [offsite]. Additional real-servers (that do not also act as linux-directors) may be added to the network as increased capacity is required. However, there are other topologies that scale better to higher bandwidth requirements.
Additional real-servers may be added to scale the virtual service horizontally, however these cannot act as linux-directors.
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Last Updated: Sat Mar 4 16:33:57 2006 +0900
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