Highly Available Transactions: Virtues and Limitations

To minimize network latency and remain online during server fail-ures and network partitions, many modern distributed data storage systems eschew transactional functionality, which provides strong semantic guarantees for groups of multiple operations over multi-ple data items. In this work, we consider the problem of providing Highly Available Transactions (HATs): transactional guarantees that do not suffer unavailability during system partitions or incur high network latency. We introduce a taxonomy of highly avail-able systems and analyze existing ACID isolation and distributed data consistency guarantees to identify which can and cannot be achieved in HAT systems. This unifies the literature on weak trans-actional isolation, replica consistency, and highly available sys-tems. We analytically and experimentally quantify the availability and performance benefits of HATs—often two to three orders of magnitude over wide-area networks—and discuss their necessary semantic compromises.