Why Sentinel provides high availability in Redis - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to check the Sentinel status command.

Redis

redis-cli [1]

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

ASENTINEL slaves mymaster

BSENTINEL masters

CSENTINEL get-master-addr-by-name mymaster

DPING

Attempts:

3 left

💡 Hint

Common Mistakes

Using PING instead of SENTINEL command

Using slaves command when asked for status

The SENTINEL masters command lists all monitored masters, showing Sentinel's view of the cluster status.

2fill in blank

medium

Complete the code to failover a master using Sentinel.

Redis

redis-cli [1] mymaster

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

ASENTINEL failover

BSENTINEL slaves

CSENTINEL reset

DSENTINEL monitor

Attempts:

3 left

💡 Hint

Common Mistakes

Using monitor instead of failover

Using reset which clears state

The SENTINEL failover mymaster command forces Sentinel to start a failover for the master named 'mymaster'.

3fill in blank

hard

Fix the error in the command to get the master address.

Redis

redis-cli SENTINEL [1] mymaster

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

Aget-master-addr

Bget-master-name-by-addr

Cget-slave-addr-by-name

Dget-master-addr-by-name

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong command names

Confusing slave and master commands

The correct command is get-master-addr-by-name to get the IP and port of the master.

4fill in blank

hard

Fill both blanks to create a Sentinel configuration command.

Redis

SENTINEL [1] mymaster [2] 6379

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

Amonitor

Bport

C127.0.0.1

Dfailover

Attempts:

3 left

💡 Hint

Common Mistakes

Using failover instead of monitor

Using port instead of IP address

The command SENTINEL monitor mymaster 127.0.0.1 6379 tells Sentinel to monitor the master at IP 127.0.0.1 and port 6379.

5fill in blank

hard

Fill both blanks to create a command that resets Sentinel state for a master.

Redis

SENTINEL [1] [2]

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

Areset

Bmymaster

Dfailover

Attempts:

3 left

💡 Hint

Common Mistakes

Using failover instead of reset

The command SENTINEL reset mymaster resets Sentinel's state for the master named 'mymaster'.

Practice

1. What is the main reason Redis Sentinel provides high availability?

easy

A. It compresses data to save storage space.

B. It increases the speed of Redis queries.

C. It encrypts Redis data for security.

D. It monitors Redis servers and promotes replicas automatically on failure.

Solution

1. Step 1: Understand Sentinel's role in monitoring

   Sentinel continuously watches Redis servers to detect if the master fails.

2. Step 2: Recognize automatic failover

   When the master fails, Sentinel promotes a replica to master automatically to keep service running.

3. Final Answer:

   It monitors Redis servers and promotes replicas automatically on failure. -> Option D

4. Quick Check:

   Sentinel = Automatic failover [OK]

Hint: Sentinel watches and switches masters automatically [OK]

Common Mistakes:

• Confusing Sentinel with data compression tools
• Thinking Sentinel speeds up queries
• Assuming Sentinel only encrypts data

2. Which of the following is the correct way to configure Redis Sentinel to monitor a master named mymaster on port 6379?

easy

A. sentinel monitor mymaster 127.0.0.1 6379 2

B. sentinel watch mymaster 127.0.0.1 6379 2

C. sentinel track mymaster 127.0.0.1 6379 2

D. sentinel check mymaster 127.0.0.1 6379 2

Solution

1. Step 1: Recall Sentinel configuration command

   The correct command to start monitoring a master is sentinel monitor.

2. Step 2: Verify command syntax

   The syntax is sentinel monitor <name> <ip> <port> <quorum>, matching sentinel monitor mymaster 127.0.0.1 6379 2.

3. Final Answer:

   sentinel monitor mymaster 127.0.0.1 6379 2 -> Option A

4. Quick Check:

   Monitor command = sentinel monitor [OK]

Hint: Use 'sentinel monitor' to watch a master server [OK]

Common Mistakes:

• Using 'watch' instead of 'monitor'
• Confusing 'track' or 'check' commands
• Wrong command syntax order

3. Given a Redis Sentinel setup monitoring a master and two replicas, what will happen if the master server stops responding?

medium

A. Sentinel will promote one replica to master automatically.

B. Sentinel will do nothing and wait for manual restart.

C. Sentinel will shut down all Redis servers.

D. Sentinel will delete all data to prevent corruption.

Solution

1. Step 1: Understand Sentinel's failure detection

   Sentinel detects when the master is down by monitoring its health.

2. Step 2: Recognize Sentinel's failover action

   Upon detecting failure, Sentinel promotes a replica to master automatically to maintain availability.

3. Final Answer:

   Sentinel will promote one replica to master automatically. -> Option A

4. Quick Check:

   Failover = Replica promoted [OK]

Hint: Master down? Sentinel promotes replica automatically [OK]

Common Mistakes:

• Thinking Sentinel waits for manual restart
• Assuming Sentinel shuts down servers
• Believing Sentinel deletes data

4. You configured Redis Sentinel but it never promotes a replica after the master fails. What is the most likely cause?

medium

A. The replicas have more memory than the master.

B. The Redis version is too new for Sentinel to work.

C. Sentinel quorum is not reached due to insufficient Sentinel nodes.

D. Sentinel does not support automatic failover.

Solution

1. Step 1: Understand Sentinel quorum requirement

   Sentinel requires a quorum (minimum number of Sentinels agreeing) to trigger failover.

2. Step 2: Identify cause of no failover

   If quorum is not reached, Sentinel will not promote replicas automatically.

3. Final Answer:

   Sentinel quorum is not reached due to insufficient Sentinel nodes. -> Option C

4. Quick Check:

   Failover needs quorum [OK]

Hint: Check Sentinel quorum count if failover doesn't happen [OK]

Common Mistakes:

• Blaming replica memory size
• Assuming Redis version blocks Sentinel
• Thinking Sentinel lacks failover feature

5. In a Redis Sentinel cluster with 5 Sentinel nodes monitoring 3 replicas and 1 master, what happens if 2 Sentinels lose network connectivity but the master fails?

hard

A. All replicas are shut down to avoid split-brain.

B. Failover proceeds because quorum of Sentinels still agree on master failure.

C. Sentinel automatically adds new nodes to replace disconnected ones.

D. Failover is blocked because any Sentinel failure stops the process.

Solution

1. Step 1: Calculate quorum in a 5-node Sentinel cluster

   Quorum is usually a majority; with 5 nodes, quorum is 3.

2. Step 2: Assess failover with 2 disconnected Sentinels

   3 Sentinels remain connected, so quorum is met and failover proceeds.

3. Step 3: Understand Sentinel behavior on partial network loss

   Sentinel does not block failover if quorum is reached despite some nodes being disconnected.

4. Final Answer:

   Failover proceeds because quorum of Sentinels still agree on master failure. -> Option B

5. Quick Check:

   Quorum met = failover proceeds [OK]

Hint: Failover needs majority Sentinels, not all connected [OK]

Common Mistakes:

• Thinking any Sentinel loss blocks failover
• Assuming Sentinel auto-adds nodes
• Believing replicas shut down on master failure