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Quorum Consensus

  • Overview
  • Design scope

Overview

Quorum consensus can guarantee consistency for both read and write operations. Some definitions first:

  • N: Number of replicas.
  • W: A write quorum of size W. For a write operation to be considered as successful, write operation must be acknowledged from W replicas.
  • R: A read quorum of size R. For a read operation to be considered as successful, read operation must wait for responses from at least R replicas.

A coordinator acts as a proxy between the client and the nodes.

$W = 1$ does not mean data is written on one server. For instance, with the configuration in previous example, data is replicated at s0, s1, and s2. $W = 1$ means that the coordinator must receive at least one acknowledgement before the write operation is considered successful.

The configuration of W, R, and N, is a typical tradeoff between latency and consistency. If $W = 1$ or $R = 1$, an operation is returned quickly because a coordinator only needs to wait for a response from any of the replicas. If $W > 1$ or $R > 1$, the system offers better consistency; however, the query will be slower because the coordinator must wait for the response from the slowest replica.

Design scope

How to configure N, W, and R to fit our use case? Here are some of the possible setups:

  • If $R = 1$, and $W = N$, the system is optimized for a fast read.

  • If $W = 1$ and $R = N$, the system is optimized for fast write.

  • If $W + R > N$, strong consistency is guaranteed (Usually $N = 3$, $W = R = 2$) because there must be at least one overlapping node that has the latest data to ensure consistency.

  • If $W + R <= N$, strong consistency is not guaranteed.