CLUSTERS

Example of clustering six servers for mail and applications
If you have a larger organization and six servers available, you can create a larger cluster. In this configuration, be sure that you do not create more replicas than necessary because of the extra workload created by cluster replication. In a large cluster, you can distribute databases equally across servers or you can dedicate certain servers to particular tasks. For example, Servers 1 and 2 could be mail servers, and Servers 3, 4, 5, and 6 could be application servers with applications divided evenly among them. Critical databases would have replicas on multiple servers. In this configuration, you could even dedicate one of the servers as a backup server for critical databases only. In this case, the server would receive real-time cluster replication events so that the databases are always up-to-date, but users would not be able to access the databases on this server. It would be used strictly to store up-to-date copies of databases in case the primary copies of the databases become damaged or deleted.

In the following figure, there are six servers -- two servers used primarily for mail and four servers used for applications. Some databases, such as DB1, are more important than others, so they have more replicas. In this scenario, you can replicate all of the mail databases from one mail server to the other mail server, if the servers are powerful enough to handle the load if failover occurs. If the two mail servers are not powerful enough, you can distribute some mail replicas to one or more of the other servers in the cluster.

Six-server cluster for mail and applications