Features Required:

File Distribution: Users should be able to share files with other peers in the network by distributing the file in small chunks.
Peer Discovery: Peers should be able to discover and connect to other peers in the network to exchange files.
File Indexing: The system should maintain an index of available files and their associated metadata to facilitate searching and downloading.
Chunk Management: Peers should be able to request and exchange specific chunks of files with other peers to assemble complete files.
Piece Selection Strategy: The system should implement a piece selection strategy, such as rarest first or random first, to efficiently download missing chunks of files.
Piece Verification: Peers should verify the integrity of downloaded file chunks using checksums or hash functions.
Concurrency and Parallelism: The system should support concurrent downloading and uploading of multiple files and chunks to improve performance.
Bandwidth Management: The system should implement mechanisms to optimize bandwidth usage and avoid congestion in the network.

Design Patterns Involved or Used:

Observer Pattern: The Observer pattern can be used to notify peers about changes or updates in the availability of files or chunks.
Factory Pattern: The Factory pattern can be used to create different types of objects, such as peers, file handlers, or network connections, based on user requests.
Strategy Pattern: The Strategy pattern can be used to implement different piece selection strategies, allowing peers to choose the most appropriate strategy for their needs.
Proxy Pattern: The Proxy pattern can be used to manage the communication between peers, providing a level of indirection and encapsulation for network operations.

Code: Classes Implementation Based on Patterns Mentioned Above

// Peer class
class Peer {
    private String peerId;
    private List<File> sharedFiles;
    private List<Chunk> downloadedChunks;
    // Other attributes and methods

    public Peer(String peerId) {
        this.peerId = peerId;
        this.sharedFiles = new ArrayList<>();
        this.downloadedChunks = new ArrayList<>();
    }

    public void shareFile(File file) {
        sharedFiles.add(file);
    }

    public void downloadChunk(Chunk chunk) {
        downloadedChunks.add(chunk);
    }

    // Other operations related to file sharing and downloading
}

// File class
class File {
    private String fileId;
    private List<Chunk> chunks;
    // Other attributes and methods

    public File(String fileId) {
        this.fileId = fileId;
        this.chunks = new ArrayList<>();
    }

    public void addChunk(Chunk chunk) {
        chunks.add(chunk);
    }

    // Getters and setters
}

// Chunk class
class Chunk {
    private String chunkId;
    private byte[] data;
    private String fileId;
    // Other attributes and methods

    public Chunk(String chunkId, byte[] data, String fileId) {
        this.chunkId = chunkId;
        this.data = data;
        this.fileId = fileId;
    }

    // Getters and setters
}

// PieceSelectionStrategy interface
interface PieceSelectionStrategy {
    Chunk selectPiece(List<Chunk> availableChunks);
}

// RarestFirstStrategy class
class RarestFirstStrategy implements PieceSelectionStrategy {
    @Override
    public Chunk selectPiece(List<Chunk> availableChunks) {
        // Implement rarest first piece selection strategy
        // Return the rarest chunk from the available chunks
        return null; // Placeholder for the selected chunk
    }
}

// RandomFirstStrategy class
class RandomFirstStrategy implements PieceSelectionStrategy {
    @Override
    public Chunk selectPiece(List<Chunk> availableChunks) {
        // Implement random first piece selection strategy
        // Return a random chunk from the available chunks
        return null; // Placeholder for the selected chunk
    }
}

// Main Class
public class BitTorrentLikeSystem {
    public static void main(String[] args) {
        // Create peers
        Peer peer1 = new Peer("peer1");
        Peer peer2 = new Peer("peer2");

        // Create files
        File file1 = new File("file1");
        File file2 = new File("file2");

        // Share files among peers
        peer1.shareFile(file1);
        peer2.shareFile(file2);

        // Download chunks using different piece selection strategies
        PieceSelectionStrategy rarestFirstStrategy = new RarestFirstStrategy();
        PieceSelectionStrategy randomFirstStrategy = new RandomFirstStrategy();

        Chunk rarestChunk = rarestFirstStrategy.selectPiece(peer2.getSharedFiles().get(0).getChunks());
        peer1.downloadChunk(rarestChunk);

        Chunk randomChunk = randomFirstStrategy.selectPiece(peer1.getSharedFiles().get(0).getChunks());
        peer2.downloadChunk(randomChunk);
    }
}

In this code example, the Peer class represents a peer participating in the file sharing system, the File class represents a file that can be shared, and the Chunk class represents a chunk of a file. The PieceSelectionStrategy interface and its implementations (RarestFirstStrategy and RandomFirstStrategy) represent different strategies for selecting file chunks.

The code demonstrates the usage of classes based on the mentioned patterns, such as the Observer pattern for notifying peers about the availability of file chunks, the Factory pattern for creating peers, file objects, or network connections, the Strategy pattern for selecting file chunks using different strategies, and the Proxy pattern for managing communication between peers.

Please note that this is a simplified example, and a complete implementation of a peer-to-peer file sharing system like BitTorrent involves more complex components, such as peer discovery mechanisms, network protocols, distributed hash tables (DHTs), and optimizations for data transfer and bandwidth management.

Design (LLD) a peer-to-peer file sharing system like BitTorrent - Machine Coding