Cluster overview

As you plan your installation, understand your hosts and decide what kind of cluster you want to create.

An overview of a typical cluster, showing management hosts, compute hosts, an external database, and a file server on a shared file system

A typical cluster contains an external database, a file server on a shared file system, and hosts. Hosts can be divided into two groups: management hosts and compute hosts. Management hosts provide specialized services to the cluster, while compute hosts run user workload.

Management hosts

Management hosts provide management services within the cluster. The primary host and all primary-candidate hosts must be management hosts. Other management hosts include the following components:
  • The web server hosting the cluster management console.
  • The web servers hosting the RESTful APIs.
  • The host running the data loaders and data purger for the reporting framework.
The primary host
The primary host is the first host installed in the cluster. The primary host controls the rest of the hosts in the cluster and is the interface to the clients of the cluster. The resource manager (vemkd) for the cluster resides on this host.
The primary candidates
There is only one primary host at a time. If the primary host fails, another host automatically takes over the primary host role. Hosts that can act as the primary are called primary candidates.
Web servers
  • The web server hosting the cluster management console. Only one management host is elected as the cluster management console web server; it does not, however, need to be a dedicated host. Any management host can be the cluster management console web server (decided when the cluster starts up).
  • Two management hosts which are the web servers hosting the RESTful APIs:
    • The ascd web server, which hosts the RESTful APIs for instance group management.
    • The REST web server, which hosts the RESTful APIs for resource management and package deployment.
Database host
The database host runs the database used for reporting in your cluster.
  • For a demonstration cluster, the Derby database runs as a system service (derbydb) on a management host that you specify during installation. There is no failover, so if this host fails, reporting does not work properly.
  • For a production cluster, you must use a supported commercial database, and the host does not need to be part of the cluster.

Compute hosts

Compute hosts provide computing resources to consumers. A cluster may contain any number of compute hosts, but must have at least one compute host.