dht-paper - DATA T P2P systems are attractive for several...

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COMMUNICATIONS OF THE ACM February 2003/Vol. 46, No. 2 43 Systems T he main challenge in P2P computing is to design and imple- ment a robust and scalable distributed system composed of inexpensive, individually unreliable computers in unrelated administrative domains. The participants in a typical P2P system might include computers at homes, schools, and businesses, and can grow to several million concurrent participants. P2P systems are attractive for several reasons: • The barriers to starting and growing such systems are low, since they usually don’t require any special administrative or financial arrangements, unlike centralized facilities; • P2P systems offer a way to aggregate and make use of the tremendous com- putation and storage resources on computers across the Internet; and • The decentralized and distrib- uted nature of P2P systems gives them the potential to be robust to faults or intentional attacks, making them ideal for long-term storage as well as for lengthy computations. P2P computing raises many interesting research problems in distributed systems. In this article we will look at one of them, the lookup problem . How do you find any given data item in a large P2P system in a scalable manner, with- out any centralized servers or hierarchy? This problem is at the heart of any P2P system. It is not addressed well by most popular sys- tems currently in use, and it provides a good example of how the challenges of designing P2P systems can be addressed. The recent algorithms devel- oped by several research groups for the lookup problem present a sim- ple and general interface, a distrib- uted hash table (DHT). Data items are inserted in a DHT and found by specifying a unique key D ATA ± By Hari Balakrishnan, M. Frans Kaashoek, David Karger, Robert Morris, and Ion Stoica Designing and implementing a robust distribution system composed of inexpensive computers in unrelated administrative domains. L OOKING U P in P2P
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44 February 2003/Vol. 46, No. 2 COMMUNICATIONS OF THE ACM for that data. To implement a DHT, the underlying algorithm must be able to determine which node is responsible for storing the data associated with any given key. To solve this problem, each node main- tains information (the IP address) of a small number of other nodes (“neighbors”) in the system, forming an overlay network and routing messages in the overlay to store and retrieve keys. One might believe from recent news items that P2P systems are mainly used for illegal music-swap- ping and little else, but this would be a rather hasty conclusion. The DHT abstraction appears to pro- vide a general-purpose interface for location-inde- pendent naming upon which a variety of applications can be built. Furthermore, distributed applications that make use of such an infrastructure inherit robustness, ease-of-operation, and scaling properties. A significant amount of research effort is now being devoted to investigating these ideas (Proj-
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dht-paper - DATA T P2P systems are attractive for several...

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