Structured P2P Models
There are different types of structured P2P systems, but by far the most popular is Distributed Hash Table (DHT). DHT is a decentralized system, that stores key-value pairs, so that any peer connected into the network can effectively retrieve the value associated with a given key. By this protocol the files will be found (but not downloaded), even if peers that have the file would not be alive. It is the protocol that makes the job, and not any preferred node, so the system is fully decentralized and in this way similar to pure P2P systems.