Basic Tier Memorystore for Redis instances experience a full cache flush during maintenance. If your application cannot withstand a full cache flush, create it with a Standard Tier instance.
Memorystore for Redis instances undergo maintenance approximately once every six months to include the latest security patches and updates. Memorystore for Redis does not currently allow you to specify maintenance windows to indicate when maintenance updates to your Redis instance can occur.
Standard Tier instances preserve data using rolling upgrades, updating the replica node first, initiating a failover, then upgrading the primary node. To learn more about the failover process for Redis instances, see High Availability. For more details about the difference between the Basic Tier and the Standard Tier, see Redis Tier Capabilities.