Improving predictability of transaction execution times in real-time databases

We present a design for multi-version concurrency control and recovery in a main memory database, and describe logical and physical versioning schemes that allow read-only transactions to execute without obtaining data item l ocks or system latches. Our schemes enable a system to provide the guarante e that updaters will never interfere with read-only transactions, and read- only transactions will not be delayed due to data contention. Consequently, transaction executions become more predictable-this partially alleviates a major problem in real-time database system (RTDBS) scheduling, namely, sig nificant unpredictability in transaction execution times. As a result, in a ddition to a transaction's deadline, a more accurate estimate of its execut ion time can also be taken into account, thus facilitating better schedulin g decisions. Our contributions include several space saving techniques for the main-memory implementation, including improved methods for logical agin g of data items and the introduction of physical aging for low-level struct ures. Some of these schemes have been implemented on a widely-used software platform within Lucent, and the full scheme is implemented in the Dali mai n-memory storage manager.