Sentinel configuration in Redis

Introduction

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

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

You need automatic switching to a backup Redis server without manual work.

You want to monitor Redis servers and get alerts if something goes wrong.

You want to manage multiple Redis servers easily in a group.

You want to avoid downtime in your app that uses Redis.

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 Sentinels must agree before switching to a backup.

Examples

This tells Sentinel to watch a Redis server called 'mymaster' at IP 127.0.0.1 on port 6379, and it needs 2 Sentinels to agree to switch if the master fails.

Redis

sentinel monitor mymaster 127.0.0.1 6379 2

This sets Sentinel to consider the master down if it doesn't respond for 5000 milliseconds (5 seconds).

Redis

sentinel down-after-milliseconds mymaster 5000

This sets the maximum time Sentinel will try to switch to a backup to 60000 milliseconds (1 minute).

Redis

sentinel failover-timeout mymaster 60000

This allows only 1 replica to sync with the new master at the same time during failover.

Redis

sentinel parallel-syncs mymaster 1

Sample Program

This configuration sets up Sentinel to watch a Redis master at 192.168.1.100:6379, requires 2 Sentinels to agree before failover, waits 10 seconds before marking the master 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

Make sure all Sentinel instances have the same configuration for the master name and quorum.

Sentinel configuration is usually placed in a sentinel.conf file and loaded when Sentinel starts.

Changing Sentinel settings requires restarting Sentinel or sending a CONFIG REWRITE command.

Summary

Sentinel configuration tells Redis how to watch and protect your main server.

Key settings include the master name, IP, port, quorum, and timing values.

Proper Sentinel setup helps keep your Redis service reliable and automatic.

Practice

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

easy

A. To provide a graphical user interface for Redis

B. To store data persistently on disk

C. To monitor Redis servers and perform automatic failover if the master fails

D. To increase the speed of Redis queries

Solution

1. Step 1: Understand Sentinel's role

   Redis Sentinel monitors Redis servers to detect failures.

2. Step 2: Identify failover capability

   Sentinel automatically switches to a backup server if the master fails.

3. Final Answer:

   To monitor Redis servers and perform automatic failover if the master fails -> Option C

4. Quick Check:

   Sentinel = automatic failover [OK]

Hint: Sentinel watches and switches masters automatically [OK]

Common Mistakes:

• Confusing Sentinel with data storage
• Thinking Sentinel speeds up queries
• Assuming Sentinel provides UI

2. Which of the following is the correct syntax to configure a Redis Sentinel to monitor a master named mymaster on IP 192.168.1.100 and port 6379?

easy

A. sentinel monitor mymaster 192.168.1.100 6379 2

B. monitor sentinel mymaster 192.168.1.100 6379 2

C. sentinel set mymaster 192.168.1.100 6379 2

D. sentinel watch mymaster 192.168.1.100 6379 2

Solution

1. Step 1: Recall Sentinel monitor command format

   The correct command starts with sentinel monitor followed by master name, IP, port, and quorum.

2. Step 2: Match the correct syntax

   sentinel monitor mymaster 192.168.1.100 6379 2 matches the correct order and keywords.

3. Final Answer:

   sentinel monitor mymaster 192.168.1.100 6379 2 -> Option A

4. Quick Check:

   sentinel monitor = correct syntax [OK]

Hint: Sentinel monitor command starts with 'sentinel monitor' [OK]

Common Mistakes:

• Swapping 'monitor' and 'sentinel'
• Using 'set' or 'watch' instead of 'monitor'
• Incorrect order of parameters

3. Given the following Sentinel configuration snippet:

sentinel monitor mymaster 10.0.0.1 6379 2
sentinel down-after-milliseconds mymaster 5000
sentinel failover-timeout mymaster 60000

What does the down-after-milliseconds setting specify?

medium

A. The time Sentinel waits before declaring the master as down

B. The time Redis waits before accepting writes

C. The timeout for client connections to Redis

D. The time Sentinel waits before starting a failover

Solution

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

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

2. Step 2: Differentiate from failover timeout

   Failover timeout is different; it controls failover duration, not detection.

3. Final Answer:

   The time Sentinel waits before declaring the master as down -> Option A

4. Quick Check:

   down-after-milliseconds = detection wait time [OK]

Hint: 'down-after-milliseconds' means detection delay [OK]

Common Mistakes:

• Confusing with failover timeout
• Thinking it affects client connections
• Assuming it controls Redis write delay

4. You have this Sentinel configuration line:

sentinel monitor mymaster 10.0.0.1 6379

What is wrong with this configuration?

medium

A. IP address format is incorrect

B. Master name should be 'master' not 'mymaster'

C. Port number should be 6380 for Sentinel

D. Missing the quorum number at the end

Solution

1. Step 1: Check required parameters for sentinel monitor

   The command requires master name, IP, port, and quorum number.

2. Step 2: Identify missing quorum

   The quorum number (minimum votes to failover) is missing in the line.

3. Final Answer:

   Missing the quorum number at the end -> Option D

4. Quick Check:

   Quorum number required in sentinel monitor [OK]

Hint: Sentinel monitor needs quorum number last [OK]

Common Mistakes:

• Ignoring quorum number
• Changing port unnecessarily
• Assuming master name must be 'master'

5. You want to configure Redis Sentinel to monitor a master and automatically failover if it is down. Which combination of settings is necessary to ensure Sentinel detects failure quickly and performs failover within 1 minute?

hard

A. Set down-after-milliseconds to 60000 only

B. Set down-after-milliseconds to 5000 and failover-timeout to 60000

C. Set failover-timeout to 5000 only

D. Set down-after-milliseconds to 60000 and failover-timeout to 5000

Solution

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

   This controls how fast Sentinel marks master as down; 5000 ms means 5 seconds.

2. Step 2: Understand 'failover-timeout'

   This controls how long failover can take; 60000 ms means 60 seconds (1 minute).

3. Step 3: Combine settings for quick detection and failover

   Setting down-after-milliseconds to 5000 and failover-timeout to 60000 ensures detection in 5 seconds and failover within 1 minute.

4. Final Answer:

   Set down-after-milliseconds to 5000 and failover-timeout to 60000 -> Option B

5. Quick Check:

   Fast detection + 1 min failover = Set down-after-milliseconds to 5000 and failover-timeout to 60000 [OK]

Hint: Fast detection needs low down-after; failover-timeout sets max failover time [OK]

Common Mistakes:

• Swapping values of down-after and failover-timeout
• Setting only one parameter
• Using too high down-after-milliseconds