Abstract: In a distributed real time database system, the only way to ensure transaction atomicity is to investigate and develop a real time atomic commit protocol. An optimistic real time commit protocol, i.e. , Double Space Commit （2SC） is proposed, which is specifically designed for the high performance distributed real time transaction. 2SC allows transactions to optimistically access the data held by the committing transaction in a controlled manner. The controlled access reduces the data inaccessibility and the priority inversion that is inherent in distributed real time commit processing. When the prepared transaction aborts, only the transactions in its abort dependency set abort while the transactions in its commit dependency set will execute as normal. Extensive simulation experiments have been performed to compare the performance of the 2SC with other protocols such as the base protocol, the permits reading of modified prepared data for timeliness （PROMPT）, and the deadline driven conflict resolution （DDCR）. The simulation results show that 2SC has the best performance. Furthermore, it can seamlessly integrate with concurrency control protocol in existence, which can ensure transaction serialization and atomic commitment.