Hosts in the cluster may be described as the master host, master candidates, management hosts, compute hosts, the web server host, or the DB host.
By default, EGO sets the number of CPUs on hosts by the number of physical CPUs. If you have a dual-core processor, you may wish to change the default behavior and set the number of CPUs by cores on hosts.
There is only one master host at a time. However, if master candidates are specified and the master host ever fails, another host automatically takes over the master host role, allowing work to continue. This process is called failover. When the master host recovers, the role switches back again.
Hosts that can act as the master are called master candidates. This includes the original master host and all hosts that can take over the role in a failover scenario. All master candidates must be management hosts.
During master host failover, the system is unavailable for a few minutes while hosts are waiting to be contacted by the new master.
The master candidate list defines which hosts are master candidates. By default, the list includes just one host, the master host, and there is no failover. If you configure additional candidates to enable failover, the master host is first in the list. If the master host becomes unavailable, the next host becomes the master. If that host is also unavailable, the next host is considered to become the master, and so on down the list. A short list with two or three hosts is sufficient for practical purposes.
For failover to work properly, the master candidates must share a file system and the shared directory must always be available.
Management hosts belong to the ManagementHosts resource group. These hosts are not expected to execute workload units for users. Management hosts are expected to run services such as the web server and web services gateway. The master host and all master candidates must be management hosts.
Management hosts share configuration files, so a shared file system is needed among all management hosts.
A management host is configured when you run egoconfig mghost on the host. The tag mg is assigned to the management host, to differentiate it from a compute host.
Compute hosts are distributed to cluster consumers to execute workload units. By default, compute hosts belong to the ComputeHosts resource group.
The ComputeHosts group excludes hosts with the mg tag, which is assigned to management hosts when you run egoconfig mghost. If you create your own resource groups to replace ComputeHosts, make sure they also exclude hosts with the mg tag.
By default, the number of slots on a compute host is equal to the number of CPUs.
The DB host is the host that runs the database used for Reporting.
For a demo 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, the Reporting feature 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.