Sentinel architecture in Redis

Introduction

Sentinel architecture helps keep your Redis database safe and working well by watching over it and fixing problems automatically.

You want your Redis database to keep working even if one server stops.

You need automatic switching to a backup Redis server when the main one fails.

You want to be alerted if your Redis servers have problems.

You want to manage multiple Redis servers easily without manual checks.

Syntax

Redis

sentinel monitor <master-name> <ip> <port> <quorum>
sentinel down-after-milliseconds <master-name> <milliseconds>
sentinel failover-timeout <master-name> <milliseconds>
sentinel parallel-syncs <master-name> <number>

master-name is a name you give to your main Redis server.

quorum is how many Sentinel instances must agree a server is down before action.

Examples

This tells Sentinel to watch a Redis server called 'mymaster' at IP 127.0.0.1 and port 6379. It needs 2 Sentinels to agree before marking it down.

Redis

sentinel monitor mymaster 127.0.0.1 6379 2

Sentinel will consider 'mymaster' down if it does not respond for 5000 milliseconds (5 seconds).

Redis

sentinel down-after-milliseconds mymaster 5000

Sentinel will try to switch to a backup server within 60000 milliseconds (1 minute) if 'mymaster' fails.

Redis

sentinel failover-timeout mymaster 60000

Sample Program

This configuration sets up Sentinel to watch a Redis master server at IP 192.168.1.100 on port 6379. It requires 2 Sentinels to agree before marking the master down. It waits 10 seconds before deciding the master is down, allows 3 minutes for failover, and syncs one replica at a time.

Redis

sentinel monitor mymaster 192.168.1.100 6379 2
sentinel down-after-milliseconds mymaster 10000
sentinel failover-timeout mymaster 180000
sentinel parallel-syncs mymaster 1

Important Notes

Sentinel runs as a separate process alongside Redis servers.

Multiple Sentinel instances improve reliability by agreeing on server status.

Failover means switching to a backup Redis server automatically to keep your app running.

Summary

Sentinel architecture watches Redis servers to detect failures.

It automatically switches to backups to keep data available.

It helps manage Redis servers easily and reliably.

Practice

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

easy

A. To compress Redis data for faster transmission

B. To store large amounts of data persistently

C. To monitor Redis servers and automatically handle failover

D. To provide a graphical user interface for Redis

Solution

1. Step 1: Understand Sentinel's role

   Redis Sentinel monitors Redis servers to detect failures.

2. Step 2: Identify Sentinel's key feature

   It automatically switches to a backup server if the main one fails, ensuring availability.

3. Final Answer:

   To monitor Redis servers and automatically handle failover -> Option C

4. Quick Check:

   Sentinel = automatic failover [OK]

Hint: Sentinel watches servers and switches on failure [OK]

Common Mistakes:

• Confusing Sentinel with data storage
• Thinking Sentinel provides UI
• Assuming Sentinel compresses data

2. Which of the following is the correct way to start a Redis Sentinel instance using the command line?

easy

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

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

C. redis-cli sentinel start /path/to/sentinel.conf

D. redis-start sentinel /path/to/sentinel.conf

Solution

1. Step 1: Recall Redis Sentinel startup command

   The correct command to start Sentinel is redis-sentinel followed by the config file.

2. Step 2: Check each option

   redis-sentinel /path/to/sentinel.conf matches the correct syntax; others are invalid commands or flags.

3. Final Answer:

   redis-sentinel /path/to/sentinel.conf -> Option A

4. Quick Check:

   Sentinel starts with redis-sentinel command [OK]

Hint: Sentinel uses redis-sentinel command, not redis-server [OK]

Common Mistakes:

• Using redis-server with --sentinel flag
• Trying to start Sentinel via redis-cli
• Using non-existent redis-start command

3. Given a Redis Sentinel setup monitoring a master named mymaster, what will the command redis-cli -p 26379 SENTINEL get-master-addr-by-name mymaster return?

medium

A. The configuration file path of Sentinel

B. The IP and port of the current master server

C. The list of all slaves under the master

D. An error because the command is invalid

Solution

1. Step 1: Understand the command purpose

   The command queries Sentinel on port 26379 for the address of the master named 'mymaster'.

2. Step 2: Identify expected output

   It returns the IP and port of the current master server Sentinel monitors.

3. Final Answer:

   The IP and port of the current master server -> Option B

4. Quick Check:

   get-master-addr-by-name returns master address [OK]

Hint: get-master-addr-by-name returns master IP and port [OK]

Common Mistakes:

• Confusing master address with slave list
• Expecting config file path as output
• Assuming command is invalid

4. You configured Redis Sentinel but it never triggers failover when the master goes down. Which of the following is the most likely cause?

medium

A. Sentinel quorum is not reached due to insufficient Sentinel instances

B. The master server has persistence disabled

C. The Redis version is too new for Sentinel

D. The client application does not use SSL

Solution

1. Step 1: Understand failover conditions

   Sentinel requires a quorum of Sentinel instances to agree before failover.

2. Step 2: Analyze the cause of no failover

   If quorum is not reached, Sentinel will not promote a slave to master.

3. Final Answer:

   Sentinel quorum is not reached due to insufficient Sentinel instances -> Option A

4. Quick Check:

   Failover needs quorum agreement [OK]

Hint: Failover needs quorum; check Sentinel count [OK]

Common Mistakes:

• Blaming persistence for failover
• Assuming Redis version blocks Sentinel
• Thinking SSL affects failover

5. In a Redis Sentinel cluster with 5 Sentinel nodes monitoring 3 Redis masters, how many Sentinel nodes must agree to trigger a failover for a master if the quorum is set to 3?

hard

A. At least 2 Sentinel nodes must agree

B. Only 1 Sentinel node needs to agree

C. All 5 Sentinel nodes must agree

D. At least 3 Sentinel nodes must agree

Solution

1. Step 1: Understand quorum setting

   The quorum setting defines the minimum number of Sentinels that must agree to start failover.

2. Step 2: Apply quorum to the cluster

   With quorum set to 3, at least 3 Sentinel nodes must agree to trigger failover.

3. Final Answer:

   At least 3 Sentinel nodes must agree -> Option D

4. Quick Check:

   Quorum = minimum agreeing Sentinels [OK]

Hint: Quorum number = minimum Sentinels needed for failover [OK]

Common Mistakes:

• Thinking only 1 Sentinel is enough
• Assuming all Sentinels must agree
• Confusing quorum with total Sentinel count