xapian document

Hi, all
     I am a recuit to xapian, I found it is a little diffcult to understand
the database and alogrithm in xapian.
     Is there any documnets describe xapian in detail ?
     I mean describe database, index, query and data structures in details,
     for example, I am confused with the following concept:
     1. How posting.DB record,DB postlist,DB files orginized, what kind of
information in these files?
     2. How data files compressed, which compression alogrithm used in
xapian ?

    Thanks && regards

    Jiang