Performance-Optimal Read-Only Transactions (Extended Version)
Read-only transactions are critical for consistently reading
data spread across a distributed storage system but have
worse performance than simple, non-transactional reads. We
identify three properties of simple reads that are necessary
for read-only transactions to be performance-optimal, i.e.,
come as close as possible to simple reads. We demonstrate
a fundamental tradeoff in the design of read-only transactions by proving that performance optimality is impossible to
achieve with strict serializability, the strongest consistency.
Guided by this result, we present PORT, a performanceoptimal design with the strongest consistency to date. Central to PORT are version clocks, a specialized logical clock
that concisely captures the necessary ordering constraints.
We show the generality of PORT with two applications.
Scylla-PORT provides process-ordered serializability with
simple writes and shows performance comparable to its nontransactional base system. Eiger-PORT provides causal consistency with write transactions and significantly improves
the performance of its transactional base system.