Distributed Systems - Quorum vs. Raft vs 2PC

Quorum vs. Raft: The Hierarchy of Distributed Systems In distributed systems, we often confuse "how we store data" with "how we govern it." To build reliable systems, we must distinguish between a mathematical property (Quorum), an orchestration protocol (Raft), and a logical contract (ACID). 1. The Functional Split The primary difference lies in which "Plane" the logic occupies: Quorum (The Data Plane Rule): This is a mathematical requirement for Durability. It defines how many nodes must acknowledge a piece of data to ensure it isn't lost ($W + R > N$). It is "blind"—it doesn't care who sends the data, only that it is stored safely. Raft (The Full System Protocol): Raft provides Ordering. It tightly couples the Control Plane (Who is the leader?) with the Data Plane (Replicating the log). It ensures that every node sees the exact same sequence of events. 2. The "Fencing" Problem These approaches differ most during a network failure: Raft (Built-in Protection): Because Raft handles