Couchbase Server 6.5 introduces Distributed Multi-document ACID Transactions, which combine ACID guarantees with the scalability, high availability, performance, and flexibility that Couchbase is known for. These transactions are easy to implement using the Couchbase SDK and offer automatic commit or rollback, retries for transient errors, and configurable duration to resolve deadlocks. They ensure that only committed data is readable by consumers and use a combination of optimistic and pessimistic locking to manage conflicts. Transactions can span multiple documents across different nodes and databases in a Couchbase cluster, providing atomicity, consistency, isolation, and durability. The new synchronous replication protocol enhances durability by allowing tunable levels of data protection against failures, and these features are also applicable to Ephemeral buckets for caching use cases. Overall, Couchbase's innovative approach to ACID transactions in a distributed system expands potential applications for NoSQL databases.