Concurrency control for mixed transactions in real-time databases

Many recent studies have suggested that the optimistic concurrency control (OCC) protocols outperform the locking-based protocols in real-time database systems (RTDBS). However, the OCC protocols suffer from the problem of unnecessary transaction restarts that is detrimental to transactions meeting their deadlines. The problem is more intensified in mixed transaction environments. Firm transactions are more vulnerable to restarts when they are in conflict with hard transactions on data access. In this paper, we addressed the problem and devised an effective OCC protocol with dynamic adjustment of serialization order, called OCC-DA, for RTDBS with mixed transactions. This protocol can avoid unnecessary transaction restarts by dynamically adjusting the serialization order of the conflicting transactions with respect to the validating transaction. As a result, much resource can be saved and more firm transactions can meet their deadlines without affecting the execution of hard transactions. The characteristics of the OCC-DA protocol were examined in detail by simulation. The results show that the performance of the OCC-DA protocol was consistently better than the other two popular protocols, OCC with forward validation and OCC with Wait-50, over a wide range of system settings. In particular, the OCC-DA protocol provides a more significant performance gain in mixed transaction environments.