The following document describes the inner workings of an assignment I completed for my university during my third year. The goal of the current assignment is the construction of a peer-to-peer network monitored by a central tracker server.
The implementation of this project requires only Java 11.
| Entity | Brief description |
|---|---|
| Tracker | The tracker is a single object monitoring the various communications between the peer entities. The tracker also holds information about the location of all files and fragments inside the network. |
| Peer | A peer constitutes a small part of the network which is capable of storing files and file fragments as well as downloading files from other peers. |
The tracker consists of the Tracker class and the MessageHandler class. The Tracker class stores the information about every peer that ever registered, every peer that is currently active and the files each peer has. A peer's successful and unsuccessful attempts to seed a file are also documented at the tracker. A MessageHandler instance is created by the Tracker every time a new connection reaches the tracker at port 55217. The MessageHandler class is a thread that manages a single connection up until it terminates.
The tracker is initiated using the following terminal command: java Tracker.
Note: The SavedPeer, LoggedInPeer and SavedFile classes are storage containers for peer-related data.
The Peer entity is made from the following classes: Peer, PeerStub, PeerTimer and PeerInputHandler. The Peer class is the main part of the peer entity. A single PeerStub and one PeerTimer exist inside a Peer class instance.
The PeerStub instance acts as a container for the server socket which receives connections from other peers or the tracker. After a successful connection, the peer stub creates a PeerInputHandler thread to look after the connection. The PeerInputHandler can perform a the following operations:
- Respond to requests that wish to determine the peer's status(check-active-requests).
- Send a small file(less than 1 MB in size) to another peer.
- Seed a piece belonging to a file which the peer possesses in their entirety.
A peer's available files(including the downloaded ones) are stored in the shared_dir/<peer_name>/ directory, where <peer_name> refers to the name used to register at the tracker.
A PeerFactory is used to create many peers using the following command:
java PeerFactory <peer_number>, where <peer_number> refers to the number of peers to be created.
Currently, the project is setup so that all peers attempt to download all files that exist in the network. Files bigger than 1 MB are partitioned into 1 MB pieces. Each peer gets informed by the tracker about the pieces he does not possess and then requests a piece at random by one of it's seeders. A seeder is a peer that fully possesses a specific file. When the seeder receivers a requests, it initiates its PeerTimer.
Every time the PeerTimer ticks, a random requests gets executed. That means, that the seeder sends a piece it possesses to another peer which has requested that piece. If a peer successfully downloads all the pieces belonging to a file, it assembles the pieces into a single file and becomes one of its seeder.
