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A mechanism of providing request distribution and failover for incoming HTTP requests is required. Without this mechanism, the Web server becomes a single point of failure, and scalability is limited to the size of the hardware used to host the Web server. A likely solution is to employ an IP sprayer, such as the Dispatcher component of the WebSphere Edge Components' Load Balancer.
Figure 15-2 shows a basic configuration that implements the Load Balancer as the network sprayer (actually the Dispatcher component of Load Balancer is distributing the workload). The Load Balancer is configured in a cluster with a backup Load Balancer, which maintains a heartbeat with the primary server. If the primary system fails, the backup system takes over the Virtual IP Address and processes requests from the HTTP clients.
Each Web server in the topology is configured with at least one physical IP address and a loopback alias configured with a shared Virtual IP Address, also called the cluster address. HTTP clients make HTTP requests to this Virtual IP Address. All requests are routed to the Load Balancer, which in turn sprays them among the members of the Web server cluster. The Web server cluster consists of identical Web servers running on different physical machines (or in different LPARs). In the event of a failure of one of the Web servers, the Dispatcher discontinues directing work to the failed server.
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When i5/OS HTTP servers are part of a Load Balancer Web server cluster, then the following is an example of the command that run on each of the i5/OS HTTP server systems to create a Virtual IP Address that can then be started: ADDTCPIFC INTNETADR('10.10.10.100') LIND(*VIRTUALIP)
SUBNETMASK('255.255.255.0')
In this command, 10.10.10.100 is the cluster address as defined in the Dispatcher configuration. Also, ensure that at least two system network interfaces - the cluster address and the interface defined within the Load Balancer that corresponds to the address of that specific HTTP server - are bound to each of the i5/OS HTTP server instances. Of course, choosing to bind all system interfaces to the i5/OS HTTP server instances is another option if that might be appropriate for your environment. |
Use the Manager component of Dispatcher to determine the status of Web servers. It provides weight values of each balanced Web server. Running the Manager component is optional, but it is necessary for dynamic weighting of the Web servers and also for identifying failed servers.
