Distributed hash table

DHT en

A distributed hash table (DHT) is a class of a decentralized distributed system that provides a lookup service similar to a hash table: (key, value) pairs are stored in the DHT, and any participating node can efficiently retrieve the value associated with a given key. Responsibility for maintaining the mapping from keys to values is distributed among the nodes, in such a way that a change in the set of participants causes a minimal amount of disruption.

Overview

DHTs form an infrastructure that can be used to build more complex services, such as distributed file systems, peer-to-peer file sharing systems, content delivery networks, domain name systems, and blockchains. They are designed to scale to large numbers of nodes and to handle continual node arrivals, departures, and failures.

Key Concepts

• Key-Value Pair: The fundamental unit of data in a DHT, consisting of a unique key and its associated value.
• Node: A participant in the DHT network that stores a portion of the key-value pairs.
• Routing: The process of finding the node responsible for a given key.
• Overlay Network: The virtual network formed by the nodes in a DHT, which is used to route messages.

Operation

DHTs use a consistent hashing algorithm to assign keys to nodes. Each node is responsible for a range of keys, and the DHT ensures that each key is stored on the appropriate node. When a node joins or leaves the network, the DHT redistributes the keys to maintain balance.

Insertion

To insert a key-value pair, a node computes the hash of the key to determine the responsible node and then stores the pair on that node.

Lookup

To retrieve a value, a node computes the hash of the key and routes a query to the responsible node, which returns the value.

Applications

DHTs are used in various applications, including:

• BitTorrent: A peer-to-peer file sharing protocol that uses DHTs for decentralized tracking.
• Kademlia: A DHT used in several peer-to-peer networks.
• Chord: A protocol and algorithm for a DHT.
• Pastry: A scalable, distributed object location and routing substrate for wide-area applications.

Advantages

• Scalability: DHTs can handle a large number of nodes and data.
• Fault Tolerance: DHTs can continue to function even when nodes fail or leave the network.
• Decentralization: There is no single point of failure or control.

Challenges

• Security: Ensuring data integrity and preventing malicious nodes from disrupting the network.
• Load Balancing: Evenly distributing keys among nodes to prevent any single node from becoming a bottleneck.
• Latency: Minimizing the time it takes to route messages and retrieve data.

See Also

• Distributed computing
• Peer-to-peer
• Hash table
• Blockchain
• Content delivery network
• Domain name system