L2AW theorem

Local Reads and Linearizable Asynchronous Replication

What is the L2AW theorem ?

Ideally crash-tolerant replication protocols, which are at the heart of read-intensive reliable datastores, must offer: 1) local reads (for performance), 2) linearizability (i.e., strong consistency), and 3) ensure safety under asynchrony.

However, existing crash-tolerant replication protocols provide up to two of the three above features:

1. RC protocols

May provide Asynchronous and Local Reads but can return stale values thus relax consistency.

1. LS protocols

Come with Linearizable and Local reads but cannot tolerate Asynchrony.

2. RA protocols

Ensure Linearizability under Asynchrony but have costly reads involving multiple replicas.

This hints a fundamental tradeoff between consistency, asynchrony, and performance in crash-tolerant protocols. Inspired by the above we introduce and prove the L2AW impossibility which asserts that: any Linearizable Asynchronous read/write register protocol that tolerates a crash (Without blocking reads or writes), has no Local reads.

Almost Local Reads (ALRs)

Guided by the L2AW impossibility, we introduce the idea of almost-local reads (ALRs), which can be implemented in a crash-tolerant and linearizable manner under asynchrony. ALRs inevitably have higher latency than local (single-node) reads, but they are very lightweight, with computation and network costs close to local reads.

When applied to the protocols in all three corners of the design space (i.e., RA, RC, LS), ALRs add the missing piece: 1) they improve the throughput for RA protocols 2) ensure linearizability for RC protocols and 3) allow LS protocols to safely operate under asynchrony.

Eurosys '23 Poster

Related Projects

HERMES: a high-performance single-key fault-tolerant and strongly consistent replication protocol.

ZEUS: locality-aware distributed (multi-key) transactions with high performance and reliability.


The code and accompanied materials are freely distributed to the public under the Apache 2.0 License.


If you have any questions regarding the L2AW theorem or ALRs do not hesitate
to contact Antonis Katsarakis ( antonios.katsarakis {at} huawei.com ).