Intermediate LevelSystem Design & Architecture
What are Quorum Reads and Writes ($R + W > N$) in distributed databases?
❓ Interview Prompt: Explain Quorum Consistency in Cassandra/DynamoDB, Replication Factor ($N$), Read Quorum ($R$), Write Quorum ($W$), and Strong Consistency formulas.
In distributed masterless databases (Cassandra), consistency is tunable using Quorum Reads and Writes:
Quorum Formula:
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$N$: Replication Factor (Total node copies).
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$W$: Write Quorum (Number of nodes that must acknowledge a write).
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$R$: Read Quorum (Number of nodes responding to a read).
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Strong Consistency Condition: If $R + W > N$, the read and write quorums overlap on at least 1 node, guaranteeing the client always reads the latest written value!
Code Implementation Example
sql
-- Executing Quorum Read in Cassandra CQL
CONSISTENCY QUORUM;
SELECT * FROM user_profiles WHERE id = 'user_100';💡Setting QUORUM consistency level in Cassandra.
🎯 Interview Key Takeaway
Configure R + W > N in distributed databases to achieve Strong Consistency across quorum node overlaps.
#System Design#Quorum#Cassandra#Consistency#Intermediate
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