Automatic failover in Redis - Step-by-Step Execution

Concept Flow - Automatic failover

Primary Redis Server

↓

Health Check by Sentinel

↓

Continue

↓

Replica Becomes New Primary

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Clients Redirected to New Primary

The Sentinel monitors the primary Redis server. If it detects failure, it promotes a replica to primary and redirects clients automatically.

Execution Sample

Redis

SENTINEL MONITOR mymaster 127.0.0.1 6379 2
SENTINEL FAILOVER mymaster

Sentinel monitors the primary Redis server and triggers failover to a replica if needed.

Execution Table

Step	Action	Condition	Result	Notes
1	Sentinel checks primary server	Primary responds	No failover	Primary is healthy
2	Sentinel checks primary server	Primary does not respond	Failover triggered	Primary is down
3	Sentinel selects replica	Replica is reachable	Replica promoted	Replica becomes new primary
4	Clients redirected	Clients connect to new primary	Operations continue	Failover complete
5	Sentinel continues monitoring	New primary responds	System stable	Monitoring resumes

💡 Failover completes when a replica is promoted and clients connect to it successfully.

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
Primary Status	Alive	Down	Down	Down
Replica Status	Replica	Replica	Promoted to Primary	Primary
Client Connection	Primary	Primary (failed)	Switching	New Primary

Key Moments - 3 Insights

Why does Sentinel promote a replica instead of restarting the primary?

Sentinel promotes a replica because the primary is unresponsive (see execution_table step 2), ensuring minimal downtime by switching to a healthy server.

How do clients know to connect to the new primary?

Clients are redirected after failover (execution_table step 4) because Sentinel updates the configuration or clients query Sentinel to find the current primary.

What happens if no replicas are available during failover?

Failover cannot complete without a reachable replica (execution_table step 3), so the system remains down until a replica is available.

Visual Quiz - 3 Questions

Look at the execution_table, at which step does Sentinel detect the primary is down?

A Step 2

B Step 3

C Step 1

D Step 4

💡 Hint

Check the 'Condition' and 'Result' columns in step 2 for primary server status.

According to variable_tracker, what is the status of the replica after step 3?

A Down

B Promoted to Primary

C Still a replica

D Disconnected

💡 Hint

Look at the 'Replica Status' row after step 3 in variable_tracker.

If the primary server never fails, what will the failover process do?

A Promote a replica anyway

B Clients will be redirected

C Failover will not trigger

D Sentinel stops monitoring

💡 Hint

Refer to execution_table step 1 where primary responds and no failover occurs.

Concept Snapshot

Automatic failover in Redis Sentinel:
- Sentinel monitors primary Redis server health.
- If primary fails, Sentinel promotes a replica to primary.
- Clients are redirected to the new primary automatically.
- Ensures high availability with minimal downtime.
- Requires at least one healthy replica for failover.

Full Transcript

Automatic failover in Redis works by Sentinel continuously checking if the primary Redis server is alive. If Sentinel detects the primary is down, it promotes one of the replicas to become the new primary. Then, clients are redirected to connect to this new primary so operations can continue without interruption. This process ensures the system stays available even if the main server fails. Sentinel keeps monitoring the new primary to maintain system stability.