Upstash supports two consistency modes:
Eventual Consistency: When the database is eventually consistent, persistence to disk and replication of the data are performed asynchronously while concurrently returning response to the client. Writes are not guaranteed to be durable even if the client receives a success response. In trade-off data safety in case of a zone crash, eventual consistency mode provides low latency & high throughput during writes.
Strong Consistency: When strong consistency is enabled, persistence to disk and replication to the majority of replicas are performed before returning response to the client. Writes are guaranteed to be durable and replicated when the client receives a success response. But while providing data safety even during a zone crash, enabling strong consistency mode may increase the latency of writes.
Eventual consistency is the default mode. You can enable strong consistency mode, by checking Strong Consistency Mode checkbox while creating the database.