Peer-to-Peer (P2P) overlay network aims to be a feasible platform for building federated but autonomous digital libraries. However, due to a plethora number of P2P infrastructures and corresponding functionalities, it is often not easy to choose appropriate candidates for specific applications. This paper is devoted for this issue by comparing some typcial P2P systems widely used in digtal library or databbase communities and extending an open discussion on how to determine proper infrastructures according to specific system requirements.