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Sentinel Promotion

Introduction​

In the preceding section, we delved into the Replication technique, which is a powerful approach employing the REPLICAOF command for reliable data transfer. However, while this technique offers notable benefits, a minor drawback persists: the need for a downtime window, albeit a potentially brief one. This necessitates reconfiguration and restarts of your server applications.

In this section, we ascend to the next level of sophistication by introducing an advanced technique: harnessing the capabilities of Redis Sentinel. This technique promises to transcend the constraints of downtime, enabling you to execute a migration with practically uninterrupted service continuity.

Redis Sentinel​

Redis Sentinel stands as a distributed system designed to monitor and manage Redis instances, mainly geared towards achieving high availability and automatic failovers. By orchestrating the deployment of multiple Redis instances, Sentinel ensures that your application can thrive amidst node failures, maintaining system robustness. In essence, a Sentinel instance is just a Redis instance running in a special mode. The ability to automatically handle failovers can be used to perform migrations as well.

Migration Steps​

At a high level, here are the steps involved:

  • Start a new Dragonfly instance and configure it as a replica of the source (primary) Redis instance.
  • Replicate data from the source (primary) Redis instance to the new Dragonfly instance.
  • Allow replication to reach a steady state and monitor using the INFO replication command.
  • Let Sentinel promote the Dragonfly instance to become the new primary.

As you can see, these steps are similar to the Replication technique. The key difference here is to utilize Sentinel for the automatic and reliable transition, promoting the Dragonfly instance as the new primary.

In the following example, we will assume that:

  • The source Redis instance runs with the hostname redis-source, the IP address 200.0.0.1, and the port 6379.
  • The new Dragonfly instance runs with the hostname dragonfly, the IP address 200.0.0.2, and the port 6380.
  • The Sentinel instance runs with the hostname sentinel, the IP address 200.0.0.3, and the port 5000.

1. Source Instance​

Assume the original source Redis instance is running, and you can check its replication information:

redis-source:6379$> INFO replication
# Replication
role:master
connected_slaves:0

Assume the source Redis instance is already managed by a Sentinel instance or a Sentinel cluster. Otherwise, reconfiguration of your server applications is still needed which implies a potential downtime window. However, if your applications are running on a container orchestrator such as Kubernetes, the rolling update mechanism can help minimize the downtime.

2. Configure Sentinel​

If your source Redis instance is not managed by Sentinel yet, follow the steps below. Otherwise, continue to Configure Replication.

To start a Redis instance in Sentinel mode, you can use a minimal sentinel.conf file like the following:

# sentinel.conf
port 5000
sentinel monitor master-instance 200.0.0.1 6379 1
sentinel down-after-milliseconds master-instance 5000
sentinel failover-timeout master-instance 60000

The generalized form for the sentinel monitor configuration flag is as follows. The example above is very minimal for illustration purpose only. In a production environment, it is crucial to pick reasonable values for quorum and other configurable values. Read more about Sentinel configuration here.

sentinel monitor <master-name> <ip> <redis-port> <quorum>

Run a Redis instance in the Sentinel mode:

$> redis-server sentinel.conf --sentinel

Once the Sentinel instance and the Sentinel-managed Redis instance (in this case the source Redis instance) are running, it is important to confirm that the applications connect to them with the proper client. Take the go-redis library as an example:

import "github.com/redis/go-redis/v9"

// The <master-name> configuration has the value 'master-instance' in the 'sentinel.conf' file.
//
// There is only one Sentinel instance in our setup.
// In a production environment, multiple Sentinel instances with a reasonable 'quorum' value can be very important to achieve high availability.
client := redis.NewFailoverClient(&redis.FailoverOptions{
    MasterName:    "master-instance",
    SentinelAddrs: []string{"200.0.0.3:5000"},
})

As shown in the snippet above, clients connect to the Sentinel-managed Redis not directly but via Sentinel instance(s). The reason is that during a failover process, Sentinel needs to notify the clients about various of events and information such as:

  • The primary instance is offline.
  • Sentinel has promoted a new primary instance, and here is the network information of the new primary.

The Sentinel-to-clients notification mechanism is powered by Pub/Sub, you can read more about Sentinel internals here. Using a Sentinel-compatible client is essential to achieve the goal of minimal downtime migration.

3. Configure Replication​

With comprehensive introduction to Sentinel, the goal is still to migrate the Redis instance to a Dragonfly instance. Start a new Dragonfly instance and use the REPLICAOF command to instruct itself to replicate data from the source:

dragonfly:6380$> REPLICAOF 200.0.0.1 6379
"OK"

After the primary-replica relationship is established, you should see data replicated into the new Dragonfly instance. We can check the replication information again on both instances:

redis-source:6379$> INFO replication
# Replication
role:master
connected_slaves:1
slave0:ip=200.0.0.2,port=6380,state=online,offset=15693,lag=2
# ... more
# ... output
# ... omitted
dragonfly:6380$> INFO replication
# Replication
role:replica
master_host:200.0.0.1
master_port:6379
master_link_status:up
master_last_io_seconds_ago:8
master_sync_in_progress:0

Sentinel should also be aware of the primary Redis instance, as well as the new Dragonfly instance joining as a replica:

sentinel:5000$> SENTINEL GET-MASTER-ADDR-BY-NAME master-instance
1) "200.0.0.1"
2) "6379"
sentinel:5000$> SENTINEL REPLICAS master-instance
1)  1) "name"
    2) "200.0.0.2:6380"
    3) "ip"
    4) "200.0.0.2"
    5) "port"
    6) "6380"
# ... more
# ... output
# ... omitted

4. Promote Replica as Primary​

Now you have finished the setup. Let's recap the current topology:

  • There is a primary Redis instance (redis-source, 200.0.0.1:6379).
  • The new Dragonfly instance (dragonfly, 200.0.0.2:6380) is a replica of the primary Redis instance.
  • There is also a Sentinel instance (sentinel, 200.0.0.3:5000) monitoring the primary and the replica.

The server applications connect to the primary-replica via the Sentinel instance. If you force the Sentinel to failover, which means promoting the Dragonfly instance as the new primary, it will be a successful migration to Dragonfly.

There are multiple ways to trigger the failover/promotion, for instance:

  • Turn off the primary Redis instance directly. Sentinel will promote the Dragonfly instance as the primary.
  • Use the SENTINEL FAILOVER <master-name> command to force a failover as if the master was not reachable, then turn off the Redis instance.

Note that the methods above are based on the assumption that the Dragonfly instance is the only candidate that can be promoted, as there are no other replicas in the topology. Let's use the second method:

sentinel:5000$> SENTINEL FAILOVER master-instance
OK

sentinel:5000$> SENTINEL GET-MASTER-ADDR-BY-NAME master-instance
1) "200.0.0.2"
2) "6380"

sentinel:5000$> SENTINEL REPLICAS master-instance
1)  1) "name"
    2) "200.0.0.1:6379"
    3) "ip"
    4) "200.0.0.1"
    5) "port"
    6) "6379"
# ... more
# ... output
# ... omitted

As you can see, now the Dragonfly instance is the primary, and we can safely turn off the Redis instance, which is the replica. After turning off the Redis instance, Sentinel would not be able to promote it again as the primary. You have successfully migrated to Dragonfly.

Considerations​

Using Redis Sentinel to perform a migration introduces a set of considerations that are essential for ensuring a smooth and successful transition. Here are some key considerations to keep in mind:

  • Downtime: While Redis Sentinel is designed to minimize downtime, there might still be minor disruptions during failover/promotion. Evaluate your application's tolerance for downtime and plan accordingly.

  • Compatibility and Versioning: Ensure that the Redis and Sentinel versions you're using are compatible with each other and with Dragonfly. Mismatched versions could lead to unexpected behavior or issues during migration.

  • Network Latency: Since data needs to be synchronized between the source Redis primary and the Dragonfly replica, consider the network latency and bandwidth between these instances. Slow or unstable connections might affect data replication speed and stability.

  • Data Volume: Evaluate the size of your dataset and the rate at which data changes. Larger datasets or high write rates might require more time for data replication and synchronization.

  • Failover Testing: Test failover scenarios in a controlled environment to ensure that Sentinel behaves as expected. This helps identify and resolve any potential issues before migrating in a production setting.

  • Monitoring and Alerts: Implement robust monitoring for both the source Redis instance and the Dragonfly instance. Set up alerts to notify you of any anomalies or potential problems.

  • Load Balancing: Consider how your application's load balancer or DNS is configured. Sentinel-driven migration might require adjustments to your load balancing setup to ensure traffic is directed to the new primary node.

  • Backup Strategy: Have a solid backup strategy in place before starting the migration. While Redis Sentinel helps with failover/promotion, having backups as a safety net is crucial.

  • Rollback Plan: Despite careful planning, unforeseen issues can arise. Develop a rollback plan that allows you to revert to the previous setup if the migration encounters significant problems.

  • Documentation and Training: Ensure that your team is familiar with Redis Sentinel concepts, configuration, and failover procedures. Proper documentation and training can mitigate confusion during the migration process.

  • Testing Environment: Whenever possible, test the migration in a staging or testing environment that closely mimics your production setup. This helps identify and address any issues before performing the actual migration.

By taking these considerations into account and tailoring your migration plan accordingly, you can effectively leverage Redis Sentinel to execute a seamless migration from Redis to Dragonfly, while minimizing risks and ensuring a successful transition.