Monitoring with Sentinel in Redis

Introduction

Sentinel helps watch over Redis servers to keep them running smoothly. It notices problems and can fix them automatically.

You want to make sure your Redis server is always available without manual checks.

You need automatic failover if the main Redis server stops working.

You want to monitor multiple Redis servers in a group.

You want alerts when Redis servers have issues.

You want to keep your Redis system stable and reliable.

Syntax

Redis

sentinel monitor <master-name> <ip> <port> <quorum>

master-name: A name you give to the main Redis server.

quorum: Number of Sentinels that must agree a master is down before action.

Examples

This tells Sentinel to watch a Redis server named 'mymaster' at IP 127.0.0.1 on port 6379. It needs 2 Sentinels to agree before failover.

Redis

sentinel monitor mymaster 127.0.0.1 6379 2

Sentinel watches 'redis-main' on IP 192.168.1.10 port 6380, requiring 3 Sentinels to agree on failure.

Redis

sentinel monitor redis-main 192.168.1.10 6380 3

Sample Program

This configuration sets up Sentinel to monitor a Redis master named 'mymaster' at localhost port 6379. It marks the master as down if no response in 5 seconds, sets failover timeout to 10 seconds, and allows one replica to sync in parallel during failover.

Redis

sentinel monitor mymaster 127.0.0.1 6379 2
sentinel down-after-milliseconds mymaster 5000
sentinel failover-timeout mymaster 10000
sentinel parallel-syncs mymaster 1

Important Notes

Sentinel needs at least 3 instances running for reliable failover decisions.

Make sure all Sentinels have the same configuration for the monitored masters.

Sentinel automatically promotes a replica to master if the current master fails.

Summary

Sentinel watches Redis servers to detect failures automatically.

It can promote replicas to master to keep Redis available.

Use 'sentinel monitor' to tell Sentinel which Redis server to watch.

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