Sentinel quorum concept in Redis - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to specify the quorum number in Redis Sentinel configuration.

Redis

sentinel monitor mymaster 127.0.0.1 6379 [1]

Drag options to blanks, or click blank then click option'

A0

B2

C1

D3

Attempts:

3 left

💡 Hint

Common Mistakes

Setting quorum to 0 disables failover detection.

Setting quorum to 1 may cause false failovers.

The quorum value defines how many Sentinels need to agree before a failover is initiated. Usually, 3 is a common quorum number.

2fill in blank

medium

Complete the command to check the quorum value for a monitored master in Redis Sentinel.

Redis

SENTINEL quorum [1]

Drag options to blanks, or click blank then click option'

Amymaster

Bquorum

Cmaster1

Dfailover

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'quorum' as the master name causes errors.

Using 'failover' is not a valid master name.

The SENTINEL quorum <mastername> command returns the quorum value for the specified master.

3fill in blank

hard

Fix the error in the Sentinel configuration line to correctly set quorum to 2.

Redis

sentinel monitor mymaster 127.0.0.1 6379 [1]

Drag options to blanks, or click blank then click option'

Atwo

B2

C02

D0x2

Attempts:

3 left

💡 Hint

Common Mistakes

Using words like 'two' instead of digits.

Using hexadecimal or leading zeros.

The quorum must be a numeric integer value without quotes or prefixes.

4fill in blank

hard

Fill both blanks to complete the Sentinel failover timeout and parallel sync settings.

Redis

sentinel failover-timeout mymaster [1]
sentinel parallel-syncs mymaster [2]

Drag options to blanks, or click blank then click option'

A180000

B1

C30000

D3

Attempts:

3 left

💡 Hint

Common Mistakes

Setting failover timeout too low causes premature failovers.

Setting parallel syncs too high may overload the master.

Failover timeout is usually set in milliseconds (e.g., 180000 ms = 3 minutes). Parallel syncs is the number of replicas synced at once, often 1.

5fill in blank

hard

Fill all three blanks to complete the Sentinel configuration for down-after-milliseconds, failover timeout, and quorum.

Redis

sentinel down-after-milliseconds mymaster [1]
sentinel failover-timeout mymaster [2]
sentinel monitor mymaster 127.0.0.1 6379 [3]

Drag options to blanks, or click blank then click option'

A10000

B180000

C2

D30000

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up timeout values.

Setting quorum too low or too high.

Down-after-milliseconds is the time to consider a master down (e.g., 10000 ms). Failover timeout is the max time for failover (e.g., 180000 ms). Quorum is the number of Sentinels needed to agree (e.g., 2).

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