ThespisTRX: Causally-Consistent Read Transactions

Data consistency defines how usable a data set is. Causal consistency is the strongest type of consistency that can be achieved when data is stored in multiple locations, and fault tolerance is desired. Thespis is a middleware that innovatively leverages the Actor model to implement causal consistency over a DBMS, whilst abstracting complexities for application developers behind a REST interface. Following the evaluation of correctness, performance and scalability of Thespis, it is illustrated how a business application can be guaranteed causal consistency, but still encounter Time-To-Check-Time-To-Use (TOCTOU) race conditions. The design and implementation of ThespisTRX is given, which builds upon, and extends, the Thespis middleware to offer read-only transaction capabilities, allowing clients to read a causally-consistent version of multiple data entities. A correctness analysis illustrates how ThespisTRX avoids TOCTOU race conditions, and empirical performance tests show that this can be achieved with minimal overheads.