Monitoring with Sentinel in Redis - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is Redis Sentinel?

Redis Sentinel is a system that helps monitor Redis servers, automatically switch to a backup if the main server fails, and notify you about server status.

beginner

How does Sentinel detect a Redis server failure?

Sentinel regularly checks Redis servers by sending ping messages. If a server does not respond after a set time, Sentinel marks it as down.

intermediate

What is a 'failover' in Redis Sentinel?

Failover is the process where Sentinel promotes a backup Redis server to become the new main server when the current main server fails.

beginner

Which configuration file is used to set up Redis Sentinel?

Redis Sentinel uses a configuration file usually named 'sentinel.conf' where you define monitored Redis servers and Sentinel settings.

beginner

What command starts a Redis Sentinel instance with a configuration file?

The command is: redis-sentinel /path/to/sentinel.conf

What role does Redis Sentinel NOT perform?

A Store application data

B Automatically failover to a replica

C Monitor Redis servers

D Send notifications about server status

Redis Sentinel monitors and manages Redis servers but does not store application data itself.

How does Sentinel decide a Redis master is down?

A When it misses several ping responses

B When it receives a shutdown command

C When the disk is full

D When a client disconnects

Sentinel marks a master as down after missing multiple ping responses.

What happens during a Sentinel failover?

A Data is deleted

B All Redis servers are restarted

C A new Redis master is elected from replicas

D Sentinel shuts down

Failover means promoting a replica to master to keep Redis available.

Which file do you edit to configure Redis Sentinel?

A sentinel-settings.conf

B redis.conf

C redis-sentinel.conf

D sentinel.conf

The sentinel.conf file contains Sentinel configuration.

Which command starts Redis Sentinel with a config file?

A redis-server sentinel.conf

B redis-sentinel /path/to/sentinel.conf

C redis-cli sentinel start

D sentinel start

Use 'redis-sentinel' followed by the config file path to start Sentinel.

Explain how Redis Sentinel monitors and manages Redis servers to ensure high availability.

Think about how Sentinel keeps Redis running smoothly even if a server fails.

Describe the steps to configure and start Redis Sentinel for monitoring a Redis master server.

Focus on configuration and command to launch Sentinel.

Practice

1. What is the main purpose of Redis Sentinel in monitoring?

easy

A. To provide a graphical user interface for Redis

B. To automatically detect Redis server failures and promote replicas

C. To increase Redis memory capacity

D. To store data persistently on disk

Solution

1. Step 1: Understand Sentinel's role

   Sentinel monitors Redis servers to detect failures automatically.

2. Step 2: Identify Sentinel's key feature

   It can promote replicas to master to keep Redis available during failures.

3. Final Answer:

   To automatically detect Redis server failures and promote replicas -> Option B

4. Quick Check:

   Sentinel monitors and promotes replicas [OK]

Hint: Sentinel watches and promotes Redis servers automatically [OK]

Common Mistakes:

• Confusing Sentinel with data storage
• Thinking Sentinel increases memory
• Assuming Sentinel provides GUI

2. Which command correctly tells Sentinel to monitor a Redis master named mymaster at IP 192.168.1.10 on port 6379 with quorum 2?

easy

A. sentinel monitor mymaster 192.168.1.10 6379

B. sentinel watch mymaster 192.168.1.10 6379 2

C. sentinel add mymaster 192.168.1.10 6379 2

D. sentinel monitor mymaster 192.168.1.10 6379 2

Solution

1. Step 1: Recall Sentinel monitor syntax

   The correct syntax is: sentinel monitor <name> <ip> <port> <quorum>.

2. Step 2: Match the command to syntax

   sentinel monitor mymaster 192.168.1.10 6379 2 matches the syntax exactly with all required parameters.

3. Final Answer:

   sentinel monitor mymaster 192.168.1.10 6379 2 -> Option D

4. Quick Check:

   Correct sentinel monitor syntax [OK]

Hint: Use 'sentinel monitor' with name, IP, port, quorum [OK]

Common Mistakes:

• Using 'sentinel watch' instead of 'monitor'
• Omitting quorum number
• Using 'sentinel add' which is invalid

3. Given the following Sentinel configuration snippet:

sentinel monitor mymaster 10.0.0.5 6379 2
sentinel down-after-milliseconds mymaster 5000
sentinel failover-timeout mymaster 10000

What does the down-after-milliseconds setting control?

medium

A. Time Redis waits before shutting down

B. Time replicas wait before syncing

C. Time Sentinel waits before marking master as down

D. Time Sentinel waits before starting failover

Solution

1. Step 1: Understand 'down-after-milliseconds'

   This setting defines how long Sentinel waits to consider the master as unreachable.

2. Step 2: Match setting to behavior

   It controls the time Sentinel waits before marking the master as down.

3. Final Answer:

   Time Sentinel waits before marking master as down -> Option C

4. Quick Check:

   down-after-milliseconds = time to mark down [OK]

Hint: down-after-milliseconds sets master down detection time [OK]

Common Mistakes:

• Confusing with Redis shutdown time
• Thinking it controls replica sync delay
• Assuming it delays failover start

4. You configured Sentinel with:

sentinel monitor mymaster 10.0.0.5 6379 2

But Sentinel never triggers failover even when the master is down. What is a likely cause?

medium

A. Quorum is set too high for available Sentinels

B. Master IP address is incorrect

C. Sentinel service is not running

D. Redis master port is blocked by firewall

Solution

1. Step 1: Understand quorum role

   Quorum is the number of Sentinels that must agree master is down to trigger failover.

2. Step 2: Analyze failover condition

   If quorum is higher than available Sentinels, failover never triggers.

3. Final Answer:

   Quorum is set too high for available Sentinels -> Option A

4. Quick Check:

   Failover needs quorum Sentinels agreeing [OK]

Hint: Check quorum matches number of running Sentinels [OK]

Common Mistakes:

• Ignoring quorum size
• Assuming IP or port issues cause failover block
• Not verifying Sentinel service status

5. You have 3 Sentinel instances monitoring a Redis master with quorum 2. One Sentinel detects the master is down and starts failover, but the other two Sentinels disagree. What will happen?

hard

A. Failover will not proceed until at least 2 Sentinels agree

B. Failover proceeds immediately with one Sentinel's decision

C. Sentinel with disagreement will restart Redis master

D. All Sentinels shut down to avoid split-brain

Solution

1. Step 1: Understand quorum in failover

   Failover requires quorum number of Sentinels to agree master is down.

2. Step 2: Analyze scenario with 3 Sentinels and quorum 2

   Only one Sentinel detects down, so quorum not reached; failover waits.

3. Final Answer:

   Failover will not proceed until at least 2 Sentinels agree -> Option A

4. Quick Check:

   Failover needs quorum agreement [OK]

Hint: Failover needs quorum agreement, not just one Sentinel [OK]

Common Mistakes:

• Thinking one Sentinel can trigger failover alone
• Assuming Sentinels restart Redis master
• Believing Sentinels shut down on disagreement