This topology provides a highly available and load balanced service. A minimum of 4 nodes is required for this topology. Real-Servers can be added to the network as additional capacity is required. A sample configuration is provided for both a A single virtual service and a A network of virual services.
At any given time one Linux-Director is active. The active linux-director accepts traffic for a virtual IP address and load balances traffic using LVS. The virtual IP address is typically advertised to end-users though DNS. The two linux-directors monitor each other using Heartbeat and in the event of the active linux-director failing the hot-standby assumes the virtual IP address and access to 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.
The active linux-director acts as the gateway router for the network and would typically be fitted with two Network Interface Cards (NICs). Optionally, the linux-directors could have packet filtering enabled to control the traffic which is able to enter and leave the server network. When configuring LVS to forward packets using NAT, RFC 1918 [offsite] private address space may be used to reduce the number of externally routable IP addresses required.
When a linux-director receives a connection from a end-user it makes a decision on which real-server 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 real servers by periodically requesting a known page and checking that the response contains an expected string. If a real-server fails then the server is taken out of the pool of real-servers and will be reinserted once it comes back on line.
The Real-Servers may run a variety of services including The Apache HTTP Server [offsite]. Additional real-servers may be added to the network as extra capacity is required.
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