A quorum (majority voting) refers to the minimum number of votes (or participants) required in a distributed system, database, or consensus algorithm to make a decision valid.

It ensures that even if some nodes fail or give conflicting answers, the system still reaches a consistent and reliable decision.

---

📌 In Distributed Systems (like databases, consensus, leader election):

• A quorum is the minimum number of nodes that must agree for an operation (read or write) to be considered successful.
• Typically, it’s defined as more than half of the total nodes (⌊N/2⌋ + 1).

• Example: If you have 5 nodes in a cluster:

  • Quorum = 3 (majority).
  • Any decision (like electing a leader or committing a write) needs at least 3 nodes agreeing.

---

📌 Why Majority Voting (Quorum) is Important:

1. Consistency – Prevents two conflicting decisions from being accepted at the same time.
2. Fault Tolerance – System continues to work even if some nodes fail.
3. Avoids Split-Brain – Ensures only one “side” of the cluster makes valid decisions.

---

📌 Example in Databases:

• Write Quorum (W): Minimum nodes that must confirm a write.
• Read Quorum (R): Minimum nodes that must confirm a read.

• Rule: R + W > N ensures strong consistency. Example: In Cassandra with 5 replicas (N=5):

  • W = 3, R = 3 → guarantees overlap → consistent reads/writes.

---

👉 In short: Quorum (majority voting) = Minimum number of participants required to agree, usually a simple majority, to make a decision valid in distributed systems.

---