Sentinel quorum concept in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding Sentinel Quorum

In Redis Sentinel, what does the quorum value represent?

A The number of replicas that must be synchronized with the master

B The minimum number of Sentinels that must agree a master is down before failover starts

C The maximum number of clients that can connect to the master

D The total number of Redis nodes in the cluster

💡 Hint

Think about how Sentinels decide to promote a new master.

The quorum is the count of Sentinels that must agree a master is unreachable before failover begins. This prevents false failovers.

❓ query_result

intermediate

Sentinel Quorum Check Result

Given a Redis Sentinel setup with 5 Sentinels and quorum set to 3, what will happen if only 2 Sentinels report the master as down?

A Sentinel will restart the master node

B Failover will start immediately

C Failover will not start because quorum is not reached

D All Sentinels will disconnect from the master

💡 Hint

Quorum means minimum agreement needed.

Failover requires at least quorum number of Sentinels to agree the master is down. With only 2 out of 5, quorum 3 is not met, so no failover.

📝 Syntax

advanced

Correct Sentinel Configuration for Quorum

Which Redis Sentinel configuration line correctly sets the quorum to 4 for a master named 'mymaster'?

A quorum sentinel mymaster 4

B sentinel mymaster quorum 4

C sentinel mymaster 4 quorum

D sentinel quorum mymaster 4

💡 Hint

The correct order is sentinel quorum, master name, then value.

The correct syntax is sentinel quorum <master-name> <number>. Option D follows this format.

❓ optimization

advanced

Optimizing Sentinel Quorum for Reliability

You have 7 Sentinels monitoring a Redis master. To balance between fast failover and avoiding false positives, which quorum value is best?

A 4

B 1

C 7

D 2

💡 Hint

Quorum should be a majority but not all Sentinels.

Setting quorum to 4 (a majority of 7) ensures enough Sentinels agree before failover, reducing false positives but allowing timely failover.

🔧 Debug

expert

Diagnosing Failover Failure Due to Quorum

In a Redis Sentinel cluster with 3 Sentinels and quorum set to 2, failover never triggers even though the master is down. What is the most likely cause?

A One Sentinel is not reachable, so only 2 Sentinels remain and quorum is not met

B Quorum is set too low, causing failover to be disabled

C The master is not actually down, so Sentinels do not agree

D Sentinel quorum must be equal to the total number of Sentinels

💡 Hint

Check Sentinel connectivity and quorum requirements.

If one Sentinel is unreachable, only 2 Sentinels remain active. With quorum 2, both must agree. If one is down or disconnected, quorum cannot be reached, so failover won't trigger.