Hydra: Serialization-Free Network Ordering for Strongly Consistent Distributed Applications


Inho Choi, National University of Singapore; Ellis Michael, University of Washington; Yunfan Li, National University of Singapore; Dan R. K. Ports, Microsoft Research; Jialin Li, National University of Singapore


Many distributed systems, e.g., state machine replication and distributed databases, rely on establishing a consistent order of operations on groups of nodes in the system. Traditionally, this ordering has been established by application-level protocols like Paxos or two-phase locking. Recent work has shown significant performance improvements are attainable by making ordering a network service, but current network sequencing implementations require routing all requests through a single sequencer – leading to scalability, fault tolerance, and load balancing limitations.

Our work, Hydra, overcomes these limitations by using a distributed set of network sequencers to provide network ordering. Hydra leverages loosely synchronized clocks on network sequencers to establish message ordering across them, per-sequencer sequence numbers to detect message drops, and periodic timestamp messages to enforce progress when some sequencers are idle. To demonstrate the benefit of Hydra, we co-designed a state machine replication protocol and a distributed transactional system using the Hydra network primitive. Compared to serialization-based network ordering systems, Hydra shows equivalent performance improvement over traditional approaches in both applications, but with significantly higher scalability, shorter sequencer failover time, and better network-level load balancing.

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@inproceedings {285190,
author = {Inho Choi and Ellis Michael and Yunfan Li and Dan R. K. Ports and Jialin Li},
title = {Hydra: {Serialization-Free} Network Ordering for Strongly Consistent Distributed Applications},
booktitle = {20th USENIX Symposium on Networked Systems Design and Implementation (NSDI 23)},
year = {2023},
isbn = {978-1-939133-33-5},
address = {Boston, MA},
pages = {293--320},
url = {https://www.usenix.org/conference/nsdi23/presentation/choi},
publisher = {USENIX Association},
month = apr
Choi Paper (Prepublication) PDF

Presentation Video