A Commit Protocol in Distributed Real-Time Databases

Traditional transaction commit protocols are unsuitable in the real time context. And the few real time commit protocols proposed in the literatures restrict the concurrency of transaction execution and affect timing properties of transactions. This paper proposes the 2SC commit protocol for distributed real time database systems, which can increase the concurrency of transactions and help to meet real time transactions' deadlines. In the paper, a history model of distributed real time transactions is presented, and commit dependencies and abort dependencies between transactions by conflicting operations are described. The paper gives the 2SC (double space commit) protocol rules, in which distributed real time transaction commit processing is conducted in two spaces: The local transaction space at a node and the global transaction space for the system. In the local transaction space, when a participator in preparing (to commit) is aborted, only the transactions in its abort dependency set, but not in its commit dependency set, are aborted. A preparing participator allows its locked data to be accessed by other transactions in a certain conditions. In the global transaction space, a participator can actively abort only before entering its preparing phase. Once a transaction misses its deadline before entering its preparing phase, the coordinator and all the participators will silently kill. The paper proves the correctness of the 2SC protocol, commit atomicity of distributed real time transactions and analyzes its advantages: improving the concurrency and decreasing cascading aborts. Furthermore, 2SC protocol can incorporate with existing concurrency control strategies to ensure both serializability and commit atomicity of distributed real time transactions, and the performance, as shown in the experimental system, is better than that of the HOPT protocol.