Simulating Escrow Transactions

Long running transactions, such as online purchases, severely reduce throughput under standard locking protocols such as two-phase locking. We investigated the use of escrow transactions to remove the need to wait for locks, and thus reduced the average transaction runtime. Escrow transactions allow transactions to borrow a quantity from the record until the transaction aborts or commits and to update the record appropriately. By escrowing some quantity away from the record, the record itself remains available for other transactions to read and escrow from as well. Patrick O'Neil's paper suggested this approach would theoretically reduce transaction run time, so we built a simulator to test the idea. The simulator models transactions under various workloads interacting with locks, a disk manager, and an escrow journal to compare the performance of escrow transactions against reads and writes using twophase locking. We found that escrow transactions significantly outperformed the two-phase-locking protocol. The locking scheme often spent nearly half its time waiting for locks, while the escrows were only blocked by the availability of the disk. When lock contention was high and transactions were likely to deadlock when using two-phase-locking, escrow transactions average run time was nearly 25 times faster than two-phase locking. Future work will involve extending the simulator to model a distributed database over a wide area network. We will investigate using escrow transactions and data redistribution schemes to reduce network communications and hence cut down the overall transaction run time.