Abstract
Structured peer-to-peer (P2P) overlay networks like Distributed Hash Tables (DHTs) map data items to the network based on a consistent hashing function. Such mapping for data distribution has an inherent load balance problem. Thus, a load balancing mechanism is an indispensable part of a structured P2P overlay network for high performance. The rapid development of P2P systems has posed challenges in load balancing due to their features characterized by large scale, heterogeneity, dynamism, and proximity. An efficient load balancing method should flexible and resilient enough to deal with these characteristics. This chapter will first introduce the P2P systems and the load balancing in P2P systems. It then introduces the current technologies for load balancing in P2P systems, and provides a case study of a dynamism-resilient and proximity-aware load balancing mechanism. Finally, it indicates the future and emerging trends of load balancing, and concludes the chapter.
Top2. Background
Over the past years, the immerse popularity of the Internet has produced a significant stimulus to P2P file sharing systems. A recent study of large scale characterization of traffic (Saroiu, 2002) shows that more than 75% of Internet traffic is generated by P2P applications. Load balancing is an inherent problem in DHTs based on consistent hashing functions. Karger et al. proved that the consistent hashing function in Chord (Karger, 1997) leads to a bound of O(log n) imbalance of keys between the nodes. Load imbalance adversely affects system performance by overloading some nodes, while prevents a P2P overlay from taking full advantage of all resources. One main goal of P2P overlays is to harness all available resources such as CPU, storage, and bandwidth in the P2P network so that users can efficiently and effectively access files. Therefore, load balancing is crucial to achieving high performance of a P2P overlay. It helps to avoid overloading nodes and make full use of all available resources in the P2P overlay.
Load balancing in DHT networks remains challenging because of their two unique features:
Key Terms in this Chapter
Peer-to-peer Network: A peer-to-peer network is a logical network on top of physical networks in which peers are organized without any centralized coordination.
Load Balancing Method: A method that controls the load in each node no more than the node’s capacity.
Heterogeneity: The instinct properties of participating peers, including computing ability, differ a lot and deserve serious consideration for the construction of a real efficient wide-deployed application.
Peer: A peer (or node) is an abstract notion of participating entities. It can be a computer process, a computer, an electronic device, or a group of them.
Dynamism/Churn: A great number of nodes join, leave and fail continually and rapidly, leading to unpredicted network size.
Structured Peer-to-Peer Network/Distributed Hash Table: A peer-to-peer network that maps keys to the nodes based on a consistent hashing function.
Proximity: Mismatch between logical proximity abstraction derived from DHTs and physical proximity information in reality, which is a big obstacle for the deployment and performance optimization issues for P2P applications.