inexpensive computers in unrelated administrative domains . LOOKING UP in P 2 P

43 Systems T he main challenge in P2P computing is to design and implement 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 computation and storage resources on computers across the Internet; and • The decentralized and distributed 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, without any centralized servers or hierarchy? This problem is at the heart of any P2P system. It is not addressed well by most popular systems currently in use, and it provides a good example of how the challenges of designing P2P systems can be addressed. The recent algorithms developed by several research groups for the lookup problem present a simple and general interface, a distributed hash table (DHT). Data items are inserted in a DHT and found by specifying a unique key Designing and implementing a robust distribution system composed of inexpensive computers in unrelated administrative domains.