CLUSTERS

How the Internet Cluster Manager works
The Internet Cluster Manager (ICM) lets you use IBM® Lotus® Domino™ clusters to provide failover and workload balancing to HTTP clients (Internet browsers) when they access Domino Web servers. This makes your Web servers and databases highly available to clients. You can run the ICM on a Lotus Domino Enterprise serve or, a Lotus Domino Utility server. You install and configure Domino clusters as you normally would, and then you configure the ICM. The ICM supports the HTTP and HTTPS protocols.

The ICM acts as an intermediary between HTTP clients and the Domino Web servers in a cluster. When Domino Web servers are running in a cluster, they generate URLs that direct HTTP client requests to the ICM. The ICM maintains information about the availability of servers and databases in the cluster. When the ICM receives a client request, it redirects the client to the most available server that contains a replica of the requested database.

The ICM sends periodic probes to the Web servers in the cluster to determine their status and availability. When the ICM receives a client request, it looks at the information in the Cluster Database Directory to find a server that contains the requested database. The ICM determines the most available server that contains the requested database and then redirects the client to that server. This results in the client closing the session with the ICM and opening a new session with the selected server. The user may see this as a change in the host name in the URL. The user may also see the path to the database change in the URL because the database may have a different path on the target server.

If the page that a Web server displays to a client includes links to other databases, the Web server includes the host name of the ICM in the URLs to those databases in the following instances:


This ensures that users accessing those links go through the ICM.

Note In cases not mentioned above, you can use the Redirect URL command to create links to other servers.

The following figure shows an HTTP client asking the ICM to open a database, and the ICM redirecting the client to the best server that contains the requested database, Server 2. The client then connects directly to Server 2.

The ICM redirecting a client to a cluster server

The ICM can run on a server in the cluster or outside the cluster. When the ICM runs on a server in the cluster, it accesses the local copy of the Cluster Database Directory. When the ICM runs on a server outside the cluster, it selects a server in the cluster and accesses the Cluster Database Directory on that server. If the server that the ICM selects becomes unavailable, this connection fails over to another server in the cluster.

The ICM always uses its local copy of the Domino Directory. Therefore, the ICM must be in the same Domino domain as the cluster.

Performance considerations

In most cases, users will experience better performance when you use the ICM. The overhead of using the ICM is very small, but the benefit to performance from workload balancing can be significant. In cases where the workload was already balanced, there will not be a significant increase or decrease in performance.