Quorum
The minimum number of nodes that must agree for a read or write to succeed. With N replicas, W+R > N ensures overlap between write and read sets for consistency.
What is Quorum?
The minimum number of nodes that must agree for a read or write to succeed. With N replicas, W+R > N ensures overlap between write and read sets for consistency.
Quorum is a advanced concept that sits in the Distributed Systems Core area of system design. Engineers reach for it whenever they need to reason about real-world trade-offs in that space — not just for textbook correctness, but because real production systems at companies like Netflix, Amazon, and Google make these decisions every day.
If you want to go deeper than this definition — with diagrams, code, and a quiz to lock it in — work through the "Quorum" lesson linked below. It walks through the why, the mechanism, the trade-offs, and how the giants actually use it in production.
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See also
Related glossary terms you might want to look up next.
Replication
Keeping copies of the same data on multiple servers. Improves read performance and provides fault tolerance if one server goes down.
Consensus
The process of getting multiple nodes in a distributed system to agree on a single value. The foundation of distributed databases and coordination services.
Leaderless Replication
A replication approach where any node can accept reads and writes. Uses quorum reads/writes for consistency. Cassandra and DynamoDB use this model.