Two-Phase Commit, Paxos, and Raft: Core Concepts and Workflows

Two‑Phase Commit (2PC) ensures ACID properties across multiple nodes by using a single coordinator and multiple participants. It consists of a prepare phase where the coordinator asks participants whether they can commit, followed by a commit phase where the coordinator instructs participants to either commit or roll back based on the collected responses.

Potential issues include participant failures, mitigated by setting a timeout after which the transaction is considered failed, and coordinator failures, addressed by replicating the operation log to a standby coordinator that can take over.

Paxos

Paxos solves the consensus problem in a message‑passing distributed system where processes may be slow, killed, restarted, or messages may be delayed, lost, or duplicated. It defines three roles: Proposer (suggests a value), Acceptor (votes on proposals), and Learner (learns the chosen value).

Each proposal is a pair [n, v] where n is a unique, monotonically increasing sequence number and v is the proposed value. Acceptors record the highest‑numbered proposal they have seen and reject any proposal with a lower number.

When a proposer receives responses from a majority of acceptors, it can send an accept request. Learners observe that a majority of acceptors have accepted a proposal and thus learn the chosen value.

Raft

Raft simplifies consensus by introducing a leader elected among followers and candidates. Followers receive periodic heartbeat messages from the leader; if a follower does not receive a heartbeat within a random election timeout (typically 150‑300 ms), it becomes a candidate and starts an election.

The candidate requests votes from all other nodes. If it receives votes from a majority, it becomes the leader.

The leader then handles client modifications by appending them to its log, replicating the entries to followers, waiting for a majority of followers to acknowledge, and finally committing the entries so that all nodes agree on the state.