About GIS server clusters
GIS server machines configured to participate in an ArcGIS Server site can be organized into groups, called clusters. Each cluster can be configured to run a dedicated subset of services. For example, you could create one cluster to run all your map services and another cluster of servers (perhaps with higher processing power) to run your geoprocessing services.
All the GIS server machines in a cluster expose an identical set of services. The machines communicate with each other and attempt to assign each request to the machine that is most likely to be available. For example, in the diagram above, map service requests are routed to the GIS server participating in cluster A, while geoprocessing service requests are routed to the GIS servers participating in cluster B. Additionally, you can add or remove GIS server machines from your cluster to respond to changing patterns in demand.
About the default cluster
An ArcGIS Server site must have at least one cluster. When you first create a site, a cluster called default is automatically generated for you. This cluster is configured to house the GIS server machine you used to create the site and run the preconfigured services installed with ArcGIS Server.
It is strongly recommended that you do not delete the default cluster. If you delete the default cluster, any services assigned to it, including the pre-configured services that are critical to publishing and caching, will be stopped. If you require the services in the cluster to remain running, you'll need to edit the services to use another cluster before deleting the default. For instructions on how to change the cluster hosting a service, see Editing a GIS server cluster in Manager.
You can use ArcGIS Server Manager or ArcGIS for Desktop to create, edit, and delete GIS server clusters. For full instructions, see the links in the sections below.