A Virtual Server (VS) can utilize a single instance of a Spatial Server (single threaded) or multiple instances of a Spatial Server (multi threaded), depending on how the properties for that VS are defined. By default ArcMap Server is single threaded, and all other types of Virtual Servers (Image, Feature, Query, Geocode, Extract, Metadata) are multithreaded.
Recycling the Spatial Server is recommended if you are hosting ArcMap Services. By default, only Spatial Servers associated with the ArcMap Server are recycled. The properties of single-threaded servers and recyclable servers are defined in the appserver.properties directory located at <ArcIMS Install>\Appserver on Windows and <ArcIMS Install>/Middlware/Application_Server/ on UNIX.
If a VS is single threaded, only one instance of a Spatial Server can be utilized by that VS. In addition, that Spatial Server cannot be used by any other VS.
If a VS is multithreaded, it can consume multiple instances of a Spatial Server and any other multithreaded VS can also utilize that Spatial Server.
If the property of a VS is viewed when recycling is in progress, the displayed list of servers and the association of those Spatial Servers to the VS may be incorrect. A single-threaded server (that is already associated with a single threaded VS) that is still under restoration may show up as not being associated with any virtual server. Also, a recyclable multithreaded server may become available for addition to a single threaded VS.
It is possible to add this single threaded Spatial Server to other single threaded or multi threaded VS, violating the single instance per Spatial Server rule. Similarly it is also possible to add a multithreaded recyclable server to a VS that contains single threaded servers, violating the single threaded properties of a VS.
During recycling, until a Spatial Server is completely restored, the association of that Spatial Server to a Virtual Server is not defined.