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1. Background

In distributed system, data is replicated across multiple servers for fault tolerance and high availability.

Once system decides to maintain multiple copies of data, another problem arises: how to make sure that all replicas are consistent??

2. Dive Deep

2.1 Definition

• Quorum
  • Minimum number of servers on which a distributed operation needs to be performed successfully before declaring the operation’s overall success

2.2 How it works?

• Suppose a database is replicated on 5 machines, then quorum refers to the minimum number of machines that perform the same action for a given transaction in order to decide the final operation for that transaction
• So in a set of 5, three machines form the majority quorum, quorum enforces the consistency requirement needed for distributed operations
• Quorum Number
  • N / 2 + 1
• Quorum is achieved when nodes follow the below protocol R + W > N
  • R minimum read nodes
  • W minimum write nodes
  • N nodes in the quorum group

2.3 Where is it used?

• Chubby
  • Use paxos for leader election, which use quorum to ensure strong consistency
• Cassandra
  • Ensure data consistency, each write request can be configured to be successful only if the data has been written to at least a quorum of replica nodes
• Dynamodb
  • Writes to a sloppy quorum of other nodes in the system
  • All read/ write operations are performed on the first N healthy nodes from the preference list, which may not always be the first N nodes encountered walking the consistent hashing ring

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