  Sharing musical files via the Internet was the essential motivation of early
P2P systems. Despite of the great success of the P2P file sharing systems,
these systems support only "simple" queries. The focus in such systems is how
to carry out an efficient query routing in order to find the nodes storing a
desired file. Recently, several research works have been made to extend P2P
systems to be able to share data having a fine granularity (i.e. atomic
attribute) and to process queries written with a highly expressive language
(i.e. SQL). These works have led to the emergence of P2P data sharing systems
that represent a new generation of P2P systems and, on the other hand, a next
stage in a long period of the database research area. ? The characteristics of
P2P systems (e.g. large-scale, node autonomy and instability) make impractical
to have a global catalog that represents often an essential component in
traditional database systems. Usually, such a catalog stores information about
data, schemas and data sources. Query routing and processing are two problems
affected by the absence of a global catalog. Locating relevant data sources and
generating a close to optimal execution plan become more difficult. In this
paper, we concentrate our study on proposed solutions for the both problems.
Furthermore, selected case studies of main P2P data sharing systems are
analyzed and compared.
