System Design Fundamentals

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Why Quorum?

The Problem That Quorum Solves

Why we store data on multiple servers

In a distributed system, we usually keep copies of the same data on multiple servers. These copies are called replicas.

We do this for two reasons:

  1. Fault tolerance. If one server crashes, the data still exists on other servers.
  2. High availability. Users can read from whichever server is closest or least busy.

So far, so good.

But replication creates a new problem

Once you have multiple copies of the data, you have to keep them in sync

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Vinay Prashanth

Vinay Prashanth

· 3 years ago

Could you please explain further the idea/logic behind "1 < r < w < n" by providing any example?

birolandhiscat

birolandhiscat

· 3 years ago

For 3 nodes A,B,C quorum group has size 2. However if we do 1 write (A) and 2 reads (B and C) we are not reading up-to-date data, despite 1 + 2 > 2.

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