[Building Sakai] about sakai's architecture

[Small Cat]

I have been read file " sakai_structure " in
sakai-src-2.5.4\reference\docs\architecture
, and I have some problems I don't understand. I hope you can explain it to
me ^^
- in sakai, we have one structure, or many structures ( each tool is one
structure / each site is one structure ? ) ???
- what node is root node of an sakai application
- each node is an data entity as announcement,gradebook... right or wrong ?

and where I can download some sample code ( without tasklisttool in
Programmer's cafe ) for sakai 2.5.4 (and  I want a sample code which can
show me " sakai structure " in the above document )
Thank you & have a good day !!!
( I'm sorry if my English is not good >"< )
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