Monitoring with Sentinel in Redis - Step-by-Step Execution

Concept Flow - Monitoring with Sentinel

Start Sentinel

↓

Sentinel monitors Redis instances

↓

Detects master failure?

No→Continue monitoring

Yes↓

Sentinel votes for failover

↓

Elect new master

↓

Update clients with new master info

↓

Resume monitoring

Sentinel continuously watches Redis servers, detects failures, elects a new master, and informs clients to keep the system running smoothly.

Execution Sample

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

This config sets up Sentinel to monitor a Redis master at 127.0.0.1:6379 with quorum 2 and failover timing.

Execution Table

Step	Action	Condition/Check	Result	System State Change
1	Sentinel starts monitoring master	Master reachable?	Yes	Monitoring active, no failover
2	Master stops responding	Master reachable?	No	Sentinel marks master as down
3	Sentinel nodes vote	Quorum reached?	Yes	Failover process initiated
4	Elect new master	Election complete?	Yes	New master assigned
5	Update clients	Clients notified?	Yes	Clients connect to new master
6	Resume monitoring	System stable?	Yes	Monitoring continues with new master
7	Master recovers	Master reachable?	Yes	Old master becomes replica
8	Exit	Failover complete and stable?	True	Sentinel continues monitoring

💡 Failover completes successfully and system stabilizes with new master

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 5	Final
master_status	up	down	down	down	down	replica
failover_initiated	false	false	true	true	true	false
current_master	original_master	original_master	original_master	new_master	new_master	new_master
clients_updated	false	false	false	false	true	true

Key Moments - 3 Insights

Why does Sentinel wait for votes before starting failover?

Sentinel waits for quorum votes (see Step 3) to avoid false failovers caused by temporary network issues.

What happens to the old master after failover?

After failover, the old master becomes a replica (see variable 'master_status' final value) to avoid split-brain.

How do clients know about the new master?

Sentinel updates clients after election (Step 5), so they connect to the new master automatically.

Visual Quiz - 3 Questions

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

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Check the 'Result' and 'System State Change' columns in Step 2

According to the variable tracker, what is the value of 'failover_initiated' after Step 3?

A false

B true

C undefined

D null

💡 Hint

Look at the 'failover_initiated' row under 'After Step 3' column

If the quorum was not reached at Step 3, what would happen next?

A Sentinel would continue monitoring without failover

B Failover would start anyway

C Clients would be immediately updated

D Old master would become replica

💡 Hint

Refer to the 'Condition/Check' and 'Result' columns at Step 3

Concept Snapshot

Sentinel monitors Redis masters continuously.
If master fails, Sentinel nodes vote (quorum) to confirm.
On quorum, failover elects a new master.
Clients are updated to connect to new master.
Old master becomes replica after recovery.
This ensures high availability automatically.

Full Transcript

Sentinel is a Redis tool that watches over Redis servers to keep them running smoothly. It checks if the master server is working. If the master stops responding, Sentinel nodes vote to confirm the failure. When enough nodes agree, Sentinel starts a failover process to pick a new master. Then, it tells all clients to use the new master. The old master becomes a replica to avoid conflicts. This process helps Redis stay available without manual intervention.