Distributed transactional reads: the strong, the quick, the fresh \& the impossible

TL;DR

This paper explores the fundamental trade-offs in distributed storage systems between read consistency, latency, and data freshness, demonstrating that achieving all three simultaneously is impossible and proposing a modified protocol to optimize for minimal delay.

Contribution

The paper formally characterizes the trade-offs among consistency, delay, and freshness, and introduces a protocol that ensures minimal-delay reads under certain consistency guarantees.

Findings

Impossible to achieve all three: order-preserving, minimal delay, and maximum freshness simultaneously.

Atomic or causally consistent reads at minimal delay require reading stale data.

Order-preserving minimal-delay reads can be fresher than atomic reads.

Abstract

This paper studies the costs and trade-offs of providing transactional consistent reads in a distributed storage system. We identify the following dimensions: read consistency, read delay (latency), and data freshness. We show that there is a three-way trade-off between them, which can be summarised as follows: (i) it is not possible to ensure at the same time order-preserving (e.g., causally-consistent) or atomic reads, Minimal Delay, and maximal freshness; thus, reading data that is the most fresh without delay is possible only in a weakly-isolated mode; (ii) to ensure atomic or order-preserving reads at Minimal Delay imposes to read data from the past (not fresh); (iii) however, order-preserving minimal-delay reads can be fresher than atomic; (iv) reading atomic or order-preserving data at maximal freshness may block reads or writes indefinitely. Our impossibility results hold independently of other features of the database, such as update semantics (totally ordered or not) or data model (structured or unstructured). Guided by these results, we modify an existing protocol to ensure minimal-delay reads (at the cost of freshness) under atomic-visibility and causally-consistent semantics. Our experimental evaluation supports the theoretical results.