High availability for Memorystore for Redis

This page describes high availability (HA) for Memorystore for Redis instances in the Standard Tier.

The Standard Tier protects a Redis instance from common failures by replicating data to one or more replicas, and by providing fast, automatic failover to a replica.

The Standard Tier is provisioned with one primary instance and one or more replicas. A Standard Tier instance that has the readReplicaMode setting deactivated has a single, non-read replica. A Standard Tier instance that has this setting enabled has one to five read replicas. To determine whether the setting is enabled, see View read replica information for your instance .

Memorystore for Redis provides high availability by replicating a primary instance to one or more replicas. Memorystore for Redis uses the asynchronous replication protocol to copy any changes that you make to the data on the primary instance to the replicas. Because of the asynchronous nature of replication, and depending on the write rate of the primary instance, replicas can lag behind the instance.

If the primary instance fails, then the instance fails over automatically to a replica. For instances that have more than one replica, the instance fails over automatically to a healthy replica with the least replication lag.

If you configure an instance to have only one non-read replica, then Memorystore for Redis directs all application connections to the primary endpoint. If you configure the instance to use read replicas, then applications can also use the read endpoint to distribute read queries across all replicas.

When a failover occurs

A failover occurs when the primary instance fails. During a failover, the primary instance and read endpoint redirect automatically to the new primary instance and replicas. Memorystore for Redis drops all connections to the primary endpoint. Memorystore for Redis also drops read endpoint connections to the read replica that's promoted.

How a failover affects your application

When the primary instance fails over to the replica, Memorystore for Redis drops existing connections to the primary endpoint of the instance. The instance is unavailable for an average of 30 seconds during automated repairs, and 15 seconds for maintenance events. Upon reconnection, your application is redirected automatically to the new primary instance by using the same connection string or IP address. You don't need to update your application after a failover.

During a failover, if there are connections to the read endpoint, then Memorystore for Redis drops the connections to the replica that's being promoted to the primary instance. Memorystore for Redis continues to serve the connections to the other replicas. After the failover is complete and the new replica is available, Memorystore for Redis redirects connections to the new replica.

Retry the instance connection after a failover

When a failover happens, Memorystore for Redis drops all connections from the primary endpoint. Depending on the number of replicas, Memorystore for Redis might also drop some read connections.

Because of this connection loss, your application must retry to reestablish the connection. We recommend that the retry logic use exponential backoff to ensure that you don't overload your instance with too many retry requests. In addition to including retry logic, we recommend that you test how a failover affects your application by testing the application with a manual failover .

Most Redis clients have built-in retry capabilities. If a connection drop occurs because of a failover, then we recommend that you use these retry capabilities.

A failover occurs when you perform the following tasks:

If you implement retry logic in your application to handle connection drops because of failovers, then your instance probably won't see a significant performance impact.

View the status for high availability

You can see high availability metrics for your Redis instance by using Cloud Monitoring . For information about the metrics that Cloud Monitoring provides for Memorystore for Redis, see Monitor Redis Instances and Supported monitoring metrics for Memorystore for Redis .

To see the built-in replication status that Redis provides, use the INFO command.