The cluster components

The Cluster Manager

A Cluster Manager runs on each server in a cluster and tracks the state of all the other servers in the cluster. It keeps a list of which servers in the cluster are currently available and maintains information about the workload on each server.

When you add a server to a cluster, Domino® automatically starts the Cluster Manager on that server. As long as the server is part of a cluster, the Cluster Manager starts each time you start the server.

Each Cluster Manager monitors the cluster by exchanging messages, called probes, with the other servers in the cluster. Through these probes, the Cluster Manager determines the workload and availability of the other cluster servers. When it is necessary to redirect a user request to a different replica, the Cluster Manager looks in the Cluster Database Directory to determine which cluster servers contain a replica of the requested database. The Cluster Manager then informs the client which servers contain a replica and the availability of those servers. This lets the client redirect the request to the most available server that contains a replica.

The tasks of the Cluster Manager include:

• Determining which servers belong to the cluster. It does this by periodically monitoring the Domino® Directory for changes to the ClusterName field in the server document and the cluster membership list.
• Monitoring server availability and workload in the cluster.
• Informing other Cluster Managers of changes in server availability.
• Informing clients about available replicas and availability of cluster servers so the clients can redirect database requests based on the availability of cluster servers (failover).
• Balancing server workloads in the cluster based on the availability of cluster servers.
• Logging failover and workload balance events in the server log file.

When it starts, the Cluster Manager checks the Domino® Directory to determine which servers belong to the cluster. It maintains this information in memory in the server's Cluster Name Cache. The Cluster Manager uses this information to exchange probes with other Cluster Managers. The Cluster Manager also uses the Cluster Name Cache to store the availability information it receives from these probes. This information helps the Cluster Manager perform the functions previously listed, such as failover and workload balancing.

To view the information in the Cluster Name Cache, type show cluster at the server console.

The Cluster Database Directory

A replica of the Cluster Database Directory (cldbdir.nsf) resides on every server in a cluster. The Cluster Database Directory contains a document about each database and replica in the cluster. This document contains such information as the database name, server name, path, replica ID, and other replication and access information. The cluster components use this information to perform their functions, such as determining failover paths, controlling access to databases, and determining which events to replicate and where to replicate them to.

The Cluster Database Directory Manager

The Cluster Database Directory Manager on each server creates the Cluster Database Directory and keeps it up-to-date with the most current database information. When you first add a server to a cluster, the Cluster Database Directory Manager creates the Cluster Database Directory on that server. When you add a database to a clustered server, the Cluster Database Directory Manager creates a document in the Cluster Database Directory that contains information about the new database. When you delete a database from a clustered server, the Cluster Database Directory Manager deletes this document from the Cluster Database Directory. The Cluster Database Directory Manager also tracks the status of each database, such as databases marked Out of service (unavailable for user access) or Pending delete from the cluster.

When there is a change to the Cluster Database Directory, the Cluster Replicator immediately replicates that change to the Cluster Database Directory on each server in the cluster. This ensures that each cluster member has up-to-date information about the databases in the cluster.

The Cluster Administrator

The Cluster Administrator performs many of the housekeeping tasks associated with a cluster. For example, when you add a server to a cluster, the Cluster Administrator starts the Cluster Database Directory Manager and the Cluster Replicator. The Cluster Administrator also starts the Administration Process, if it is not already running. When you remove a server from a cluster, the Cluster Administrator stops the Cluster Database Directory Manager and the Cluster Replicator. It also deletes the Cluster Database Directory on that server and cleans up records of the server in the other servers' Cluster Database Directories.

The Cluster Replicator

The Cluster Replicator constantly synchronizes data among replicas in a cluster. Whenever a change occurs to a database in the cluster, the Cluster Replicator quickly pushes the change to the other replicas in the cluster. This ensures that each time users access a database, they see the most up-to-date version. The Cluster Replicator also replicates changes to private folders that are stored in a database. Each server in a cluster runs one Cluster Replicator by default, although you can run more Cluster Replicators if there is a lot of activity in the cluster.

The Cluster Replicator looks in the Cluster Database Directory to determine which databases have replicas on other cluster members. The Cluster Replicator stores this information in memory and uses it to replicate changes to other servers. Periodically (every 15 seconds by default), the Cluster Replicator checks for changes in the Cluster Database Directory. When the Cluster Replicator detects a change in the Cluster Database Directory -- for example, an added or deleted database or a database that now has Cluster Replication disabled -- it updates the information it has stored in memory.

The Cluster Replicator pushes changes to servers in the cluster only. The standard replicator task (REPLICA) replicates changes to and from servers outside the cluster.