Peer-to-Peer Architecture

Client-Server vs Peer-to-Peer

In the conventional client-server model, clients request resources and servers provide them. Servers are always-on, centrally managed, and hold the canonical data. This model is simple, easy to secure, and easy to reason about — but it creates bottlenecks: if the server fails or is overloaded, all clients are affected.

In Peer-to-Peer (P2P) architecture, every node acts as both a client and a server. Nodes share resources (bandwidth, storage, compute) directly with each other without a central coordinator. This eliminates single points of failure and allows the system's capacity to grow automatically as nodes join — each new peer brings resources as well as consuming them.

P2P Topology Types

Distributed Hash Tables (DHTs)

A Distributed Hash Table (DHT) is the key data structure enabling structured P2P. It provides a key-value lookup service distributed across all nodes. Each node is responsible for a range of keys. Lookups for a key route through O(log N) hops, where N is the number of nodes. Kademlia is the most widely deployed DHT — it powers BitTorrent's peer discovery and IPFS.

// Kademlia DHT lookup (simplified)
// Each node has a 160-bit ID. XOR distance determines routing.

function lookup(targetKey):
  candidates = kBucket.getClosestNodes(targetKey, k=20)

  loop:
    query each candidate: "who do you know closest to targetKey?"
    newCandidates = merge results
    if no closer nodes found:
      return closest known nodes to targetKey
    candidates = newCandidates

// Each hop brings you closer to the target node.
// O(log N) hops in a network of N nodes.

Gossip Protocols

Gossip protocols (also called epidemic protocols) are a communication mechanism used in P2P and distributed systems for propagating information. Each node periodically selects a random set of neighbors and shares its state. Like a rumor spreading through a crowd, information propagates exponentially fast without any central coordinator. Cassandra uses gossip for cluster membership and failure detection; AWS DynamoDB uses it for ring membership.

BitTorrent: P2P File Distribution

BitTorrent is the most successful P2P file distribution protocol. A file is split into chunks; peers download chunks from multiple sources simultaneously and upload chunks they have to others. As a file becomes more popular, more peers have chunks to share — the system's bandwidth increases with demand (unlike client-server where more demand degrades service).

Blockchain as P2P Architecture

Blockchain is built on a P2P foundation. Every node in a Bitcoin or Ethereum network holds a full copy of the ledger. Transactions are gossiped across the peer network; consensus (Proof of Work, Proof of Stake) determines which transactions are accepted without a central authority. The architectural properties — decentralization, censorship resistance, Byzantine fault tolerance — flow directly from the P2P substrate.

Trade-offs of P2P