Sentinel quorum concept in Redis - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the Sentinel quorum in Redis Sentinel?

The Sentinel quorum is the minimum number of Sentinel instances that must agree a master is down before a failover is triggered.

beginner

Why does Redis Sentinel require a quorum before starting a failover?

To avoid false failovers caused by network glitches or temporary issues, ensuring that enough Sentinels agree the master is really down.

intermediate

How do you configure the quorum in Redis Sentinel?

You set the quorum number in the Sentinel configuration file using the 'sentinel monitor' command, for example: sentinel monitor mymaster 127.0.0.1 6379 2 (where 2 is the quorum).

beginner

What happens if the quorum is not reached in Redis Sentinel?

If the quorum is not reached, Sentinel will not start a failover, and the master will continue to be considered up.

intermediate

How does Sentinel quorum relate to high availability?

Quorum helps ensure that failover decisions are reliable, which is key for maintaining high availability and avoiding split-brain scenarios.

What does the Sentinel quorum represent in Redis Sentinel?

A The minimum number of Sentinels that must agree the master is down

B The number of slaves connected to the master

C The number of clients connected to Redis

D The maximum number of failovers allowed

The quorum is the minimum number of Sentinel instances that must agree the master is down before failover.

If the quorum is set to 3, how many Sentinels must agree the master is down to trigger failover?

A 1

B 2

C 3

D 4

The quorum number defines how many Sentinels must agree; if set to 3, at least 3 must agree.

What is the risk of setting the Sentinel quorum too low?

A Failover may never happen

B False failovers due to temporary network issues

C Slower failover process

D More memory usage

A low quorum can cause failover to trigger incorrectly during brief network glitches.

Where do you configure the Sentinel quorum value?

A In the Sentinel configuration file

B In the Redis server config file

C In the client application

D In the operating system settings

The quorum is set in the Sentinel configuration file using the 'sentinel monitor' command.

What happens if the quorum is not reached when a master is suspected down?

A Sentinel promotes a slave anyway

B Sentinel immediately triggers failover

C Sentinel shuts down

D Sentinel ignores the master status and continues monitoring

Without quorum agreement, Sentinel does not trigger failover and keeps monitoring.

Explain the role of quorum in Redis Sentinel failover process.

Think about how Sentinels decide together before acting.

Describe how to set and why to choose a proper quorum value in Redis Sentinel.

Consider the trade-off between quick failover and avoiding mistakes.

Practice

1.

What does the quorum setting in Redis Sentinel control?

easy

A. The maximum number of clients connected to Redis

B. The number of replicas a master must have

C. The timeout duration for client connections

D. The number of Sentinels that must agree a master is down before failover

Solution

1. Step 1: Understand Sentinel quorum purpose

   The quorum defines how many Sentinels must agree that the master is down before starting failover.

2. Step 2: Eliminate unrelated options

   Options about clients, timeouts, or replicas do not relate to quorum.

3. Final Answer:

   The number of Sentinels that must agree a master is down before failover -> Option D

4. Quick Check:

   Sentinel quorum = agreement count [OK]

Hint: Quorum means how many Sentinels agree on failure [OK]

Common Mistakes:

• Confusing quorum with client limits
• Thinking quorum is a timeout setting
• Mixing quorum with replica count

2.

Which of the following is the correct way to set the quorum to 3 in a Redis Sentinel configuration file?

sentinel monitor mymaster 127.0.0.1 6379 3

easy

A. sentinel set quorum 3

B. sentinel quorum mymaster 3

C. sentinel monitor mymaster 127.0.0.1 6379 3

D. sentinel quorum 3 mymaster

Solution

1. Step 1: Recall correct sentinel monitor syntax

   The sentinel monitor command includes the master name, IP, port, and quorum number at the end.

2. Step 2: Match options to syntax

   Only sentinel monitor mymaster 127.0.0.1 6379 3 matches the full correct syntax with quorum as the last argument.

3. Final Answer:

   sentinel monitor mymaster 127.0.0.1 6379 3 -> Option C

4. Quick Check:

   Monitor command ends with quorum number [OK]

Hint: Quorum is last number in sentinel monitor command [OK]

Common Mistakes:

• Using 'sentinel quorum' command which does not exist
• Placing quorum before IP or port
• Confusing 'set' command with monitor

3.

Given 5 Sentinels monitoring a master with quorum set to 3, what happens if only 2 Sentinels report the master as down?

medium

A. Failover starts immediately

B. Failover does not start because quorum is not reached

C. All Sentinels restart

D. The master is removed from monitoring

Solution

1. Step 1: Understand quorum requirement

   Quorum is 3, meaning at least 3 Sentinels must agree the master is down to start failover.

2. Step 2: Analyze 2 Sentinels reporting down

   Since only 2 Sentinels report down, quorum is not met, so failover will not start.

3. Final Answer:

   Failover does not start because quorum is not reached -> Option B

4. Quick Check:

   Quorum unmet = no failover [OK]

Hint: Failover needs quorum Sentinels agreeing [OK]

Common Mistakes:

• Assuming failover starts with any Sentinel down report
• Thinking Sentinels restart on partial failure
• Believing master is removed automatically

4.

You configured 4 Sentinels with quorum set to 3, but failover never happens even when the master is down. What is the most likely cause?

medium

A. Quorum is set higher than the number of Sentinels available

B. Sentinel IP addresses are incorrect

C. The master is not actually down

D. The Redis version does not support Sentinel

Solution

1. Step 1: Check quorum vs Sentinels count

   Quorum is 3 but only 4 Sentinels exist; if some Sentinels are unreachable, quorum may not be met.

2. Step 2: Identify cause of failover not triggering

   If quorum is effectively higher than reachable Sentinels, failover won't start.

3. Final Answer:

   Quorum is set higher than the number of Sentinels available -> Option A

4. Quick Check:

   Quorum > available Sentinels blocks failover [OK]

Hint: Quorum must be ≤ number of active Sentinels [OK]

Common Mistakes:

• Ignoring Sentinel network issues
• Assuming master is always down
• Blaming Redis version without checking

5.

You have 7 Sentinels monitoring a Redis master. To ensure high availability, you want failover to start only if a majority agrees the master is down. What is the best quorum value to set?

hard

A. 4

B. 3

C. 5

D. 7

Solution

1. Step 1: Calculate majority of 7 Sentinels

   Majority means more than half, so (7 / 2) = 3.5, rounded up to 4.

2. Step 2: Choose quorum equal to majority

   Setting quorum to 4 ensures failover starts only if most Sentinels agree.

3. Final Answer:

   4 -> Option A

4. Quick Check:

   Majority quorum = 4 for 7 Sentinels [OK]

Hint: Quorum = majority of Sentinels (half + 1) [OK]

Common Mistakes:

• Setting quorum less than half
• Setting quorum equal to total Sentinels
• Not rounding up majority