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.
Load-balancing and GIS server clusters
In a multiple-machine site, load balancing is handled among all GIS server machines in a single cluster using ports 4004 and above. If your site uses a single cluster, all GIS servers in the site will load-balance requests. If your site uses more than one cluster, load-balancing is handled among all GIS servers within each cluster.
For example, an incoming request is assigned to an available GIS server in the cluster, even if the request was directed to a specific machine within or outside of the cluster. That GIS server then does the work of drawing the map, finding the address coordinate, running the geoprocessing tool, and so on, and returns the result to the client. If a machine is offline or the service that was requested is running in a different cluster, the request is forwarded to the cluster containing the service. The GIS server machines in that cluster load-balance and process the request accordingly.
In large sites with a single cluster, you can configure the site to remove the load balancing functionality between GIS servers. This reduces network traffic between machines in the site, reduces load on your network, and helps improve monitoring of GIS servers in your site. For full instructions, see Remove load balancing between GIS server machines.
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.