Paxos (computer Science)

Paxos (computer Science)

Paxos is a family of protocols for solving consensus in a network of unreliable processors. Consensus is the process of agreeing on one result among a group of participants. This problem becomes difficult when the participants or their communication medium may experience failures.

Consensus protocols are the basis for the state machine approach to distributed computing, as suggested by Leslie Lamport and surveyed by Fred Schneider. The state machine approach is a technique for converting an algorithm into a fault-tolerant, distributed implementation. Ad-hoc techniques may leave important cases of failures unresolved. The principled approach proposed by Lamport et al. ensures all cases are handled safely.

The Paxos protocol was first published in 1989 and named after a fictional legislative consensus system used on the Paxos island in Greece. It was later published as a journal article in 1998.

The Paxos family of protocols includes a spectrum of trade-offs between the number of processors, number of message delays before learning the agreed value, the activity level of individual participants, number of messages sent, and types of failures. Although no deterministic fault-tolerant consensus protocol can guarantee progress in an asynchronous network (a result proved in a paper by Fischer, Lynch and Paterson), Paxos guarantees safety (freedom from inconsistency), and the conditions that could prevent it from making progress are difficult to provoke.

Paxos is normally used in situations requiring durability (for example, to replicate a file or a database), in which the amount of durable state could be large. The protocol attempts to make progress even during periods when some bounded number of replicas are unresponsive. However, a reconfiguration mechanism is available, and can be used to drop a permanently failed replica, or to add new replicas to the group.