Reference

Grokking the Advanced System Design Interview - Quorum
分布式系统理论之Quorum机制

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Background

In Distributed Systems, data is replicated across multiple servers for fault tolerance and high availability.
How to make sure that all replicas are consistent?

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Definition

Quorum

• A quorum is the minimum number of servers on which a distributed operation needs to be performed successfully before declaring the operation’s overall success.

Example:

• A database is replicated on 5 machines.
• A quorum refers to the minimum number of machines that perform the same action (commit or abort) for a given transaction in order to decide the final operation for that transaction.

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Solution

What value should we choose for a quorum?
* More than half of the number of nodes in the cluster: (N/2 + 1)

Quorum is achieved when nodes follow the below protocol: R + W > N, where:

• N = nodes in the quorum group
• W = minimum write nodes
• R = minimum read nodes
• If a distributed system follows R + W > N rule, then every read will see at least one copy of the latest value written.

The following two things should be kept in mind before deciding read/write quorum:

• R=1 and W=N ⇒ full replication (write-all, read-one): undesirable when servers can be unavailable because writes are not guaranteed to complete.
• Best performance (throughput/availability) when 1 < r < w < n, because reads are more frequent than writes in most applications.

标签：Quorum,read,minimum,nodes,operation,quorum
来源： https://blog.csdn.net/heyunxiang/article/details/120742520