Backend & APIs · Advanced
Build a BitTorrent Client
Write a BitTorrent client that parses a .torrent file, contacts the tracker, and downloads a real file in pieces over the peer wire protocol.
You build a working BitTorrent client that decodes the bencoded metainfo in a .torrent file, computes the info hash, and queries the tracker to get a list of peers. You then open TCP connections to those peers, complete the BitTorrent handshake, exchange the peer wire protocol's messages, and request the file one block at a time, verifying each piece against its SHA-1 hash before assembling the final file. It is worth building because it combines binary protocol parsing, concurrent network I/O, and data integrity into one end-to-end system that actually downloads real data. It teaches the realities of designing against a wire protocol and managing many simultaneous connections, which is exactly the skill set backend and distributed-systems roles look for.
What you build
- Parses bencoded .torrent metainfo and computes the info hash
- Requests peers from an HTTP tracker with the correct query
- Performs the BitTorrent handshake over TCP
- Speaks the peer wire protocol: bitfield, interested, unchoke, request, piece
- Downloads pieces as blocks and reassembles them in order
- Verifies each completed piece against its SHA-1 hash
- Downloads from multiple peers concurrently
What it teaches
- Bencode parsing and binary serialization
- Designing against a real wire protocol
- TCP socket programming and handshakes
- Concurrent network I/O with goroutines or async
- Data integrity via SHA-1 piece verification
- Peer-to-peer and distributed download mechanics
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Take it further
- Support multi-file torrents and directory reassembly.
- Add peer pipelining and a piece-selection strategy like rarest-first.
- Implement seeding so your client can upload to others.
