Fileserver defense

Fileserver.md

@@ -1,8 +1,8 @@
 # Fileserver
 
-The fileserver is a simple frontend to a disk storage space that contains 
+The fileserver is a simple frontend to a disk storage space that contains auxiliary files for assignments, archives with job configuration and files submitted by users and evaluation results. These files are the only ones required for backend to run, so dedicated fileserver gives the possibility of testing backend separately. Also, one fileserver instance could be shared among multiple API instances (with the same broker), so common files does not need to be duplicated in each API instance.
-auxiliary files for assignments, files submitted by users and evaluation 
+
-results. 
+One exception is that important files with character of database entry (but not stored in database due to size) are stored directly in filesystem of API server. But this fact does not devaluate benefit of separate fileserver. From security point of view, fileserver should be completely isolated from public internet to keep the data safe while API server must be public from its nature.
 
 For a description of the communication protocol used by the frontend 
 and workers, see the [Communication](#communication) chapter.