Nextra's K2 and large-scale CL apps

Hi everyone,

I read Espen Vestre's paper on Nextra's core ISP system with great  
interest,
(http://www.franz.com/services/conferences_seminars/jlugm00/conference/ 
Talk05_Vestre.pdf)
and it inspired me to start learning Common Lisp (I already have  
experience with Java, Python, Tcl, and other languages).

It doesn't surprise me that CL is suited for applications like this.  
However, I have some questions about Nextra's system. I hope that  
someone on this list can address them to the extent that they are  
methods or patterns commonly used by CL coders.

  - What is the purpose of the "Sherpa" layer (layer 2)? Is there an  
open source equivalent? Why doesn't the CL code access the database  
directly?

  - How do CL applications like this usually handle the  
object-relational mapping? Do you write tedious code that creates a  
CLOS instance for every row in a database, and equivalent code to  
update the database when the CLOS object changes, or is there some  
library to make this easier?

  - Many CL success stories mention being able to update the system  
"live" as a huge advantage. How exactly is this implemented? Is there  
some way to telnet directly into a running server, or does the  
programmer have to implement some kind of toplevel and way of  
communicating with it?

  - The article mentions Java and Perl libraries for S-expression based  
TCP protocols. Are there any open source libraries for non-Lisp  
languages that handle such protocols?

Thanks,
Guan