How Redis Sentinel Works: Overview and Example

Redis Sentinel is a system that monitors Redis servers to detect failures and automatically promotes a replica to master if the current master fails. It uses sentinel monitor to watch servers and sentinel failover to switch roles, ensuring high availability without manual intervention.

Syntax

Redis Sentinel uses configuration commands to monitor and manage Redis instances. The main commands are:

• sentinel monitor <master-name> <ip> <port> <quorum>: Sets up monitoring of a master Redis server.
• sentinel failover <master-name>: Triggers a manual failover to promote a replica.
• sentinel get-master-addr-by-name <master-name>: Gets the current master address.

redis

sentinel monitor mymaster 127.0.0.1 6379 2
sentinel failover mymaster
sentinel get-master-addr-by-name mymaster

Example

This example shows how to configure Redis Sentinel to monitor a master server and perform automatic failover.

bash

# sentinel.conf
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

# Start sentinel with:
redis-sentinel sentinel.conf

# Commands to check master address:
redis-cli -p 26379 sentinel get-master-addr-by-name mymaster

Output

127.0.0.1
6379

Common Pitfalls

Common mistakes when using Redis Sentinel include:

• Setting quorum too low, causing false failovers.
• Not configuring enough Sentinel instances for reliable consensus.
• Failing to update client applications to use Sentinel for master discovery.
• Ignoring network partitions that can cause split-brain scenarios.

Always run at least three Sentinel instances and configure clients to query Sentinel for the current master.

redis

## Wrong: Single sentinel with quorum 1
sentinel monitor mymaster 127.0.0.1 6379 1

## Right: Three sentinels with quorum 2
sentinel monitor mymaster 127.0.0.1 6379 2

Quick Reference

Command	Description
sentinel monitor	Start monitoring a master server
sentinel failover	Force a failover to a replica
sentinel get-master-addr-by-name	Get current master address
sentinel reset	Reset Sentinel state for a master
sentinel masters	List all monitored masters
sentinel slaves	List replicas of a master

Key Takeaways

Redis Sentinel monitors Redis masters and replicas to detect failures automatically.

It performs automatic failover by promoting a replica to master when needed.

Configure multiple Sentinel instances with a proper quorum for reliability.

Clients should query Sentinel to discover the current master address.

Avoid network partitions and ensure Sentinel instances can communicate.