Fauna's ACID transaction engine is a strongly consistent distributed system that processes transactions in a partitioned, globally distributed environment. It guarantees atomicity, consistency, isolation, and durability properties of database systems. The engine is inspired by Calvin and uses a snapshot time to prevent stale reads. Transactions are executed in two phases: an execute phase where the node executes the core logic and dispatches reads to the closest data partitions, and a commit phase where the write effects are committed to the transaction log. Fauna provides a strong guarantee of strict serializability while allowing for perfect processing parallelism in the absence of contention among transactions. The system also allows for tradeoffs between performance and isolation levels, with read-only transactions running at snapshot isolation and read-write transactions being strictly serializable. Index reads can be opted out of optimistic locking to improve performance but may introduce write skew, which can lead to conflicts if not managed properly. Overall, Fauna's ACID transaction engine is a key feature that enables the system to provide strong consistency and durability guarantees in a globally distributed environment.