ArcGIS Server 10.6 introduces a new optimized internal application server for managing services. This optimized app server significantly improves the performance of many administrative operations in ArcGIS Server compared to prior releases. Operations affected by this update include publishing new services, and starting and stopping existing services.
The optimized app server is used for all new sites, as well as for upgraded sites that contain a single cluster. For upgraded sites containing multiple clusters, the legacy application server architecture is used.
Clusters have been deprecated. The use of clusters for new deployments is strongly discouraged and existing deployments are advised to migrate away from use of clusters as part of ongoing maintenance and upgrades.
It is possible to manually change to the legacy application server architecture. See System Properties to learn how to do this using the REST API.