Sentinel configuration in Redis - Step-by-Step Execution

Concept Flow - Sentinel configuration

Start Sentinel

↓

Load Config File

↓

Connect to Master

↓

Monitor Master & Slaves

↓

Detect Failover Conditions

↓

If Master Down?

No→Continue Monitoring

Yes↓

Elect New Master

↓

Reconfigure Slaves

↓

Notify Clients

↓

Continue Monitoring

Sentinel starts, loads config, monitors master and slaves, detects failures, triggers failover, and continues monitoring.

Execution Sample

Redis

sentinel monitor mymaster 127.0.0.1 6379 2
sentinel down-after-milliseconds mymaster 5000
sentinel failover-timeout mymaster 60000
sentinel parallel-syncs mymaster 1

Configures Sentinel to monitor a master Redis instance and sets failover parameters.

Execution Table

Step	Action	Evaluation	Result
1	Start Sentinel process	Sentinel reads config	Sentinel ready to monitor
2	Load 'sentinel monitor' config	Identify master 'mymaster' at 127.0.0.1:6379	Master registered for monitoring
3	Load 'down-after-milliseconds'	Set timeout to 5000 ms	Failure detection timer set
4	Load 'failover-timeout'	Set failover timeout to 60000 ms	Failover timing configured
5	Load 'parallel-syncs'	Set max parallel syncs to 1	Sync limit set
6	Connect to master and slaves	Connections established	Monitoring begins
7	Check master health	Master responds	Continue monitoring
8	Simulate master down	No response within 5000 ms	Mark master as down
9	Start failover election	Sentinels vote	New master elected
10	Reconfigure slaves	Slaves point to new master	Replication updated
11	Notify clients	Clients informed of new master	Clients reconnect
12	Continue monitoring	Sentinel resumes checks	System stable
13	Exit	No further actions	Sentinel runs continuously

💡 Sentinel runs continuously; exit here means end of this trace.

Variable Tracker

Variable	Start	After Step 2	After Step 8	After Step 9	Final
master_status	unknown	up	down	new_master_elected	new_master_elected
failover_timer	not set	not set	started	running	running
slave_config	default	default	default	reconfigured	reconfigured

Key Moments - 3 Insights

Why does Sentinel wait 5000 ms before marking the master as down?

Sentinel uses the 'down-after-milliseconds' setting (step 3 and 8) to avoid false alarms from temporary network issues.

How does Sentinel decide who becomes the new master?

Sentinel nodes vote during failover election (step 9) to choose the best candidate, ensuring a coordinated failover.

What happens to the slaves after a new master is elected?

Slaves are reconfigured to replicate from the new master (step 10), so data stays consistent.

Visual Quiz - 3 Questions

Look at the execution table, at which step does Sentinel mark the master as down?

A Step 8

B Step 9

C Step 7

D Step 10

💡 Hint

Check the 'Evaluation' column for 'No response within 5000 ms' in step 8.

According to variable_tracker, what is the 'master_status' after step 9?

A up

B down

C new_master_elected

D unknown

💡 Hint

Look at the 'master_status' row under 'After Step 9' column.

If 'parallel-syncs' was set to 2 instead of 1, which step would be affected?

A Step 3

B Step 5

C Step 9

D Step 11

💡 Hint

Refer to step 5 where 'parallel-syncs' is loaded and set.

Concept Snapshot

Sentinel configuration commands set monitoring and failover parameters.
Key commands: sentinel monitor, down-after-milliseconds, failover-timeout, parallel-syncs.
Sentinel monitors master and slaves, detects failures, triggers failover.
Failover includes electing new master, reconfiguring slaves, notifying clients.
Sentinel runs continuously to maintain Redis high availability.

Full Transcript

Sentinel configuration involves setting up monitoring for a Redis master instance and defining failover behavior. The process starts by loading configuration commands such as 'sentinel monitor' to specify the master, 'down-after-milliseconds' to set failure detection timeout, 'failover-timeout' for failover duration, and 'parallel-syncs' to limit slave synchronization. Sentinel connects to the master and slaves, continuously checking their health. If the master does not respond within the timeout, Sentinel marks it as down and initiates a failover election among Sentinel nodes. Once a new master is elected, slaves are reconfigured to replicate from it, and clients are notified to reconnect. This cycle repeats to ensure Redis availability.

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