This page describes high availability for Memorystore for Redis instances in the Standard Tier. The Standard Tier provides high availability through replication and automatic failover capability. Memorystore for Redis does not use Redis Sentinel for high availability.
What high availability is
Memorystore for Redis provides high availability by replicating a master instance to a replica. A replica is a copy of the master that reflects changes to the master instance. Note: The replica is used only for enabling high availability. You cannot connect to the replica and use the replica as a read replica.
Each Memorystore for Redis instance in the Standard Tier is configured automatically as a master and replica pair. The replica serves as a standby and must be in a different zone than the master. Application requests are directed to the master instance. Changes made to the data on the master instance are copied to the replica using the Redis asynchronous replication protocol.
What high availability provides
If the master instance fails, the Memorystore for Redis service triggers a failover. The service promotes the replica to the master instance and, after recovery, configures the failed master as the replica of the new master.
To tolerate zone failures, the master instance and replica are located in different zones within the same region. Replication tolerates the loss of either the master instance or the replica at one time. If the master instance and replica fail at the same time due to multi-zone failure, data cannot be recovered.
When a failover is triggered
A failover occurs when the master Redis instance fails. During failover, all requests to the new master are redirected automatically to the replica, and the Memorystore for Redis instance continues to respond to your application.
How failover affects your applications
When a master instance fails over to the replica, existing connections to Memorystore for Redis are dropped. However, on reconnect, your application is redirected automatically to the new master instance using the same connection string or IP address. You do not need to update your application after a failover.
While the Memorystore for Redis service promotes the replica to master, your Memorystore for Redis instance is temporarily unavailable. Each node is located in a single zone, so zonal failures might cause prolonged recovery time. During this time, there is only one copy of the data.
How you view the status for high availability
You can see high availability metrics for your Redis instance by using Google Cloud's operations suite. For information about the metrics that Google Cloud's operations suite provides for Memorystore for Redis, see Monitoring Redis Instances. For more information about using Google Cloud's operations suite with Google Cloud, see the Stackdriver Monitoring documentation.
To see the native replication status that Redis provides, you can issue the Redis INFO command to the Memorystore for Redis instance.