Sentinel quorum concept in Redis - Step-by-Step Execution

Concept Flow - Sentinel quorum concept

Sentinel nodes monitor master

↓

Detect master failure?

↓

Continue monitoring

↓

Votes >= quorum?

↓

Wait and retry

Sentinel nodes watch the master. If failure is detected, they vote. If votes reach quorum, failover starts.

Execution Sample

Redis

SENTINEL MONITOR master 127.0.0.1 6379 2
SENTINEL SET master quorum 2
-- Master fails
Sentinels vote
Votes reach quorum
Failover starts

Sentinels monitor a master with quorum 2; when master fails, votes are counted and failover starts if quorum reached.

Execution Table

Step	Event	Sentinel Votes	Quorum	Action
1	Master is healthy	0	2	Continue monitoring
2	Master failure detected by Sentinel 1	1	2	Wait for more votes
3	Master failure detected by Sentinel 2	2	2	Wait for more votes
4	Votes reach quorum	2	2	Start failover process
5	Failover completed	2	2	New master elected

💡 Failover starts after quorum (2 votes) is reached.

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
Sentinel Votes	0	1	2	2
Quorum	2	2	2	2
Action	Continue monitoring	Wait for more votes	Wait for more votes	Start failover process

Key Moments - 2 Insights

Why does Sentinel wait after only one vote?

Because quorum is 2, one vote is not enough to start failover (see execution_table step 2).

What happens if quorum is not reached?

Sentinels keep monitoring and retry voting until quorum is reached or master recovers (implied by step 2 and 3).

Visual Quiz - 3 Questions

Look at the execution_table, what is the Sentinel Votes count at step 3?

A 2

B 0

C 1

D 3

💡 Hint

Check the 'Sentinel Votes' column at step 3 in execution_table.

At which step does the failover process start?

A Step 3

B Step 4

C Step 2

D Step 5

💡 Hint

Look at the 'Action' column for when 'Start failover process' happens.

If quorum was set to 3, what would happen at step 3?

A Failover starts

B Votes reach quorum

C Wait for more votes

D Failover completes

💡 Hint

Compare quorum value in variable_tracker and votes at step 3.

Concept Snapshot

Sentinel quorum is the minimum votes needed to start failover.
Sentinels monitor master and vote on failure.
Failover starts only if votes >= quorum.
Quorum prevents false failovers.
Set quorum based on number of Sentinels.

Full Transcript

Redis Sentinel nodes continuously monitor the master server. When a failure is detected by any Sentinel, it votes that the master is down. These votes are counted, and only if the number of votes reaches or exceeds the configured quorum does Sentinel start the failover process to elect a new master. If the votes are below quorum, Sentinels wait and continue monitoring. This quorum mechanism helps avoid false failovers caused by temporary network issues or isolated Sentinel failures. In the example, with quorum set to 2, failover starts only after two Sentinels agree the master is down.