FW-KV: improving read guarantees in PSI

We present FW-KV, a novel distributed transactional in-memory key-value store that guarantees the Parallel Snapshot Isolation (PSI) correctness level. FW-KV's primary goal is to allow its read-only transactions to access more up-to-date (fresher) versions of objects than Walter, the state-of-the-art implementation of PSI. FW-KV achieves that without assuming synchrony or a synchronized clock service. The improved level of freshness comes at no significant performance degradation, especially in low contention workloads, as assessed by our evaluation study including two standard OLTP benchmarks, YCSB and TPC-C. The performance gap between FW-KV and Walter is less than 5% in low contention scenarios, and less than 28% in high contention.