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Peer-to-peer overlay networks are widely used in distributed systems. Based on whether a regular topology is maintained among peers, peer-to-peer networks can be divided into two categories: structured peer- to-peer networks in which peers are connected by a regular topology, and unstructured peer-to-peer networks in which the topology is arbitrary. Structured peer-to-peer networks usually can provide efficient and accurate services but need to spend a lot of efforts in maintaining the regular topology. On the other hand, unstructured peer-to-peer networks are extremely resilient to the frequent peer joining and leaving but this is usually achieved at the expense of efficiency. The objective of this work is to design a hybrid peer-to-peer system for distributed data sharing which combines the advantages of both types of peer-to-peer networks and minimizes their disadvantages. The proposed hybrid peer-to-peer system is composed of two parts: the first part is a structured core network which forms the backbone of the hybrid system; the second part is multiple unstructured peer- to-peer networks each of which is attached to a node in the core network. The core structured network can narrow down the data lookup within a certain unstructured network accurately, while the unstructured networks provide a low cost mechanism for peers to join or leave the system freely. A data lookup operation first checks the local unstructured network and then the structured network. This two-tier hierarchy can decouple the flexibility of the system from the efficiency of the system. Our simulation results demonstrate that the hybrid peer-to-peer system can utilize both the efficiency of structured peer-to-peer network and the flexibility of the unstructured peer-to-peer network and achieve a good balance between the two types of networks.