Why Sentinel provides high availability in Redis - Visual Breakdown

Concept Flow - Why Sentinel provides high availability

Start: Redis Master and Slaves Running

↓

Sentinel Monitors Master

↓

Detects Master Failure?

No→Continue Monitoring

Yes↓

Sentinel Elects New Master from Slaves

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Update Clients to Use New Master

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Resume Normal Operations with New Master

Sentinel watches the Redis master. If it fails, Sentinel picks a new master from slaves and tells clients to use it, keeping the system available.

Execution Sample

Redis

1. Sentinel monitors master
2. Detects failure
3. Elects new master
4. Updates clients
5. System continues

This sequence shows how Sentinel detects failure and switches to a new master to keep Redis available.

Execution Table

Step	Sentinel Action	Condition	Result	System State
1	Monitor master	Master is healthy	No fail detected	Master active, slaves syncing
2	Monitor master	Master not responding	Fail detected	Master down
3	Start failover	Fail confirmed by quorum	Elect new master	One slave promoted
4	Update clients	New master elected	Clients redirected	Clients connected to new master
5	Resume monitoring	New master active	Normal operation	High availability restored

💡 Sentinel stops failover after new master is elected and clients are updated

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
Master Status	Healthy	Down	Down	Down	New Master Active
Failover State	None	Fail Detected	Failover In Progress	Failover Done	None
Client Connection	To Master	To Master (Failing)	Switching	To New Master	To New Master

Key Moments - 2 Insights

Why does Sentinel wait for quorum before electing a new master?

Sentinel waits for quorum to avoid false failover; this ensures multiple Sentinels agree the master is down (see execution_table step 3).

What happens if clients are not updated after failover?

Clients would keep trying the old master and fail; updating clients (step 4) is crucial to maintain availability.

Visual Quiz - 3 Questions

Look at the execution_table at which step does Sentinel detect the master failure?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Check the 'Condition' and 'Result' columns in execution_table row for Step 2

According to variable_tracker, what is the client connection state after step 4?

A Connected to new master

B Switching connection

C Connected to old master

D Disconnected

💡 Hint

Look at 'Client Connection' row under 'After Step 4' in variable_tracker

If Sentinel did not update clients after failover, what would happen?

A Clients connect to new master automatically

B Clients fail to connect and lose availability

C Clients continue normal operation

D Sentinel restarts the master

💡 Hint

Refer to key_moments explanation about client updates after failover

Concept Snapshot

Sentinel monitors Redis master continuously.
If master fails, Sentinel confirms failure with quorum.
Sentinel elects a slave as new master.
Clients are updated to use new master.
This automatic failover keeps Redis highly available.

Full Transcript

Redis Sentinel provides high availability by monitoring the master server. It continuously checks if the master is working. When Sentinel detects the master is down, it waits for agreement from other Sentinels (quorum) to avoid mistakes. Then, Sentinel chooses one of the slaves to become the new master. After that, Sentinel updates all clients to connect to this new master. This process ensures Redis keeps working without manual intervention, maintaining high availability.