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The HA firewall through clustering service discussed above is rather costly and its configuration is complicated. It can be used by high-end customers. You can also set up an HA firewall with a network sprayer such as WebSphere Edge Components' Load Balancer, as shown in Figure 15-21. The Load Balancer is a load balancing product that divides up the workload generated by new connections among a group of back-end servers. This can be done either by changing the assignment between the host name and the IP address (DNS redirection), or by rerouting TCP and UDP traffic directly to the server with the lowest workload. Load Balancer also recognizes server failures and automatically keeps new requests from being dispatched to the failed server.
Figure 15-21 Load Balancer configured firewall
As mentioned before, the Load Balancer is a sophisticated load balancing tool. The clients target a cluster IP address configured on the LB server. The requests are then rerouted by the Load Balancer to the server with the lowest workload. The LB recalculates the workload of the servers either with information collected by the LB itself, such as the number of active connections and new connections, or with system information collected by the Metric Server running locally on the servers, such as CPU load or memory utilization.
Figure 15-22 Load Balancer configured firewall after failover
The Load Balancer server is a single point of failure in the system. To prevent this, Load Balancer gives us the option to configure a backup Load Balancer server that automatically takes over in case of a failure. The actual load information and client routing tables are shared between the two Load Balancer servers, so nearly all connections can be preserved in the case of a breakdown. Some external scripts are automatically executed during takeover, and they can be modified to provide the high availability of the firewall. This is similar to the scenario described for the LDAP servers with the Load Balancer, see Figure 15-16.
Detailed information about the Load Balancer and the WebSphere Edge Components can be found in Chapters 4 and 5 of the IBM WebSphere V6 Scalability and Performance Handbook, SG24-6392.
