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.
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.
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 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. Additional real servers may be added to the network as extra capacity is required.
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