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Redis Sentinel Failover | An Introduction

Did you know that Redis offers a high-availability solution called Redis Sentinel?

It offers several features like monitoring, automatic failover, notification, and setting up configuration.

• Monitoring:

  Sentinel continuously monitors the Redis instances, detecting failures and performing necessary actions.
• Notification:

  It notifies about the status of Redis instances. This enables proactive response to potential issues.
• Automatic Failover:

  Sentinel automates the failover process. This results in uninterrupted service by promoting a new master when the current master becomes unreachable.
• Configuration Setup:

  It helps with the configuration setup with parameters like quorum, timeout thresholds, and synchronization settings.

Quorum-Based Failover

A key concept in Redis Sentinel is quorum-based failover. Quorum is the minimum number of Sentinel processes required to agree on the failure of a Redis master before starting a failover. This ensures that failovers occur only when a majority of Sentinels reach a decision.

For example, if there are three Sentinel instances configured with a quorum of two, a failover will occur only when two out of three Sentinels agree that the master node is unreachable. This mechanism prevents split-brain scenarios and ensures the reliability of failover operations.

Configuration Parameters

Several configuration parameters in the `sentinel.conf` file decide the behavior of Redis Sentinel:

• sentinel monitor:

  Defines the master node to monitor, along with its IP address, port, and quorum number.
• down-after-milliseconds:

  Sets the threshold for categorizing nodes as not-reachable based on the response time to ping commands.
• parallel-syncs:

  It is the number of slaves to synchronize with the master simultaneously during failover.
• failover-timeout:

  It decides the duration for Sentinel to wait before attempting failover again after a failed attempt.

Methods for Failover

Redis Sentinel offers multiple methods for starting failover:

• Method 1: Manual Failover Command
  1. Connect to a Sentinel node using `redis-cli`.
  2. Then, run the failover command to force failover without requiring agreement from other Sentinels.
• Method 2: Sentinel Command Line Interface
  1. Connect to a Sentinel instance and get information about configured masters and replicas.
  2. Then, start the failover using the `SENTINEL failover` command, allowing the system to choose the correct replica for promotion.

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Conclusion

In brief, our Support Experts introduced us to Redis Sentinel and automated failover in Redis deployments.