Redis Sentinel is a system designed to help manage Redis instances, providing high availability and monitoring capabilities. It ensures that your Redis deployment remains available and operational even in the event of failures.

• Monitoring: Sentinel constantly checks if your master and replica instances are working as expected.
• Notification: Sentinel can notify administrators or other systems when a Redis instance goes down.
• Automatic Failover: If a master instance fails, Sentinel can promote one of the replicas to be the new master.
• Configuration Provider: Sentinel acts as a source of authority for clients to discover the current master instance.

• A running Redis master instance.
• One or more Redis replica instances.

1. Install Redis Sentinel: Redis Sentinel is included with the Redis installation. Ensure Redis is installed on your system.

2. Configure Sentinel: Create a configuration file for Sentinel. Below is an example configuration:

   # sentinel.conf
   port 26379
   dir /tmp
   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
   

   • port 26379: The port on which Sentinel will run.
   • dir /tmp: Directory for Sentinel to store its state.
   • sentinel monitor mymaster 127.0.0.1 6379 2: Monitor a master instance named mymaster running on 127.0.0.1:6379 with a quorum of 2.
   • sentinel down-after-milliseconds mymaster 5000: Consider the master down if it is unresponsive for 5000 milliseconds.
   • sentinel failover-timeout mymaster 10000: Timeout for the failover process.
   • sentinel parallel-syncs mymaster 1: Number of replicas that can be re-synchronized in parallel during a failover.

3. Start Sentinel: Run the following command to start Sentinel with the configuration file:

   redis-sentinel /path/to/sentinel.conf
   

Let's assume you have a Redis master running on 127.0.0.1:6379 and two replicas on 127.0.0.1:6380 and 127.0.0.1:6381.

1. Configure the Master and Replicas: Ensure the replicas are properly configured to replicate from the master.

   # On replica 1 (port 6380)
   redis-cli -p 6380
   > SLAVEOF 127.0.0.1 6379
   
   # On replica 2 (port 6381)
   redis-cli -p 6381
   > SLAVEOF 127.0.0.1 6379
   

2. Create Sentinel Configuration: Create a sentinel.conf file with the following content:

   port 26379
   dir /tmp
   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
   

3. Start Sentinel: Run Sentinel using the configuration file.

   redis-sentinel /path/to/sentinel.conf
   

Sentinel continuously monitors the master and replica instances. You can check the status using the following command:

This command will provide information about the monitored master instances.

If the master instance fails, Sentinel will automatically promote one of the replicas to be the new master. You can observe the failover process in the Sentinel logs.

1. Simulate a Master Failure: Stop the master instance.

   redis-cli -p 6379 SHUTDOWN
   

2. Observe Failover: Check the Sentinel logs to see the failover process. You can also use the following command to verify the new master:

   redis-cli -p 26379 SENTINEL masters
   

• Quorum Configuration: Ensure the quorum value in the Sentinel configuration is correctly set. It should be the majority of Sentinels that need to agree on the failover.
• Network Configuration: Make sure all Sentinel instances can communicate with each other and with the Redis instances.
• Resource Allocation: Ensure that the system running Sentinel has enough resources (CPU, memory) to handle the monitoring and failover processes.

Redis Sentinel is a powerful tool for managing Redis instances, providing high availability and automatic failover capabilities. By setting up and configuring Sentinel, you can ensure that your Redis deployment remains resilient and operational even in the face of failures. In the next section, we will explore Redis Cluster Mode, which provides a way to scale Redis horizontally.