The SNOW Theorem and Latency-Optimal Read-Only Transactions

Scalable storage systems where data is sharded across many machines are now the norm for Web services as their data has grown beyond what a single machine can handle. Consistently reading data across different shards requires transactional isolation for the reads. Yet a Web service may read from its data store hundreds or thousands of times for a single page load and must minimize read latency to keep response times low. Examining the read-only transaction algorithms for many recent academic and industrial scalable storage systems suggests there is a tradeoff between their power--expressed as the consistency they provide and their compatibility with other types of transactions--and their latency. We show that this tradeoff is fundamental by proving the SNOW Theorem, an impossibility result that states that no read-only transaction algorithm can provide both the lowest latency and the highest power. We then use the tight boundary from the theorem to guide the design of new read-only transaction algorithms for two scalable storage systems, COPS and Rococo. We implement our new algorithms and then evaluate them to demonstrate they provide lower latency for read-only transactions and to understand their impact on overall throughput.