ArcGIS for Server vs. ArcGIS spatial data server

Question

I just found that there is a product called ArcGIS spatial data server.
And reading about it, found it is very similar to ArcGIS for server (just subset of it).
So, what is the difference between the two?
is it just a subset of the full server, so maybe it is cheaper?

MarcoBoeringa · Answer

A comprehensive overview of current limitations / requirements of Spatial Data Server is also in this Help topic:

A quick tour of authoring feature services for ArcGIS Spatial Data Server

nicogis · Answer

Extra info: details here http://resources.arcgis.com/en/help/main/10.1/01sq/01sq00000005000000.htm
ArcGIS Spatial Data Server is provided as a separate installation with all editions of ArcGIS for Server Enterprise
and
You can choose to allow editing through your feature service when you publish it provided the data source is a database and you have an ArcGIS for Server Standard or ArcGIS for Server Advanced license. Feature services are read-only if published under an ArcGIS for Server Basic license. Data that participates in a geodatabase cannot be edited through an ArcGIS Spatial Data Server feature service at any ArcGIS for Server license level

MarcoBoeringa · Answer

I just found that there is a product called ArcGIS spatial data server.
And reading about it, found it is very similar to ArcGIS for server (just subset of it).
So, what is the difference between the two?
is it just a subset of the full server, so maybe it is cheaper?

If ArcGIS Server and ArcGIS Spatial Data Server (SDS) actually share any programming code is something only ESRI can tell, so your question of "whether it is a subset of the full server" is unanswerable... ESRI presents them as two separate products.

From a functionality point of view, SDS does not support any of the advanced geodatabase functionality like topologies, network datasets, parcel fabrics etc. So if your goal is to build a geodatabase incorporating any of this, and create a webapplication having some of this advanced functionality, than SDS is NOT the way to go, you will need ArcGIS for Server.

In addition, Spatial Database Server was primarily designed to access and serve editable Feature Service layers from non-Geodatabase repository containing Spatial databases with native spatial types, although you can access and serve Geodatabase layers in a read-only mode - and as Simple Features only - too.

Please note it also is primarily targeted at serving web Feature Services, not Image Services, which it can't even create according to the documentation I saw. IDK, but performance is likely not to be on par with fast cached Image Services of the same data.