Virtual Consensus in Delos


Mahesh Balakrishnan, Jason Flinn, Chen Shen, Mihir Dharamshi, Ahmed Jafri, Xiao Shi, Santosh Ghosh, Hazem Hassan, Aaryaman Sagar, Rhed Shi, Jingming Liu, Filip Gruszczynski, Xianan Zhang, Huy Hoang, Ahmed Yossef, Francois Richard, and Yee Jiun Song, Facebook, Inc.

Awarded Best Paper!


Consensus-based replicated systems are complex, monolithic, and difficult to upgrade once deployed. As a result, deployed systems do not benefit from innovative research, and new consensus protocols rarely reach production. We propose virtualizing consensus by virtualizing the shared log API, allowing services to change consensus protocols without downtime. Virtualization splits the logic of consensus into the VirtualLog, a generic and reusable reconfiguration layer; and pluggable ordering protocols called Loglets. Loglets are simple, since they do not need to support reconfiguration or leader election; diverse, consisting of different protocols, codebases, and even deployment modes; and composable, via RAID-like stacking and striping. We describe a production database called Delos which leverages virtual consensus for rapid, incremental development and deployment. Delos reached production within 8 months, and 4 months later upgraded its consensus protocol without downtime for a 10X latency improvement. Delos can dynamically change its performance properties by changing consensus protocols: we can scale throughput by up to 10X by switching to a disaggregated Loglet, and double the failure threshold of an instance without sacrificing throughput via a striped Loglet.

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@inproceedings {258856,
author = {Mahesh Balakrishnan and Jason Flinn and Chen Shen and Mihir Dharamshi and Ahmed Jafri and Xiao Shi and Santosh Ghosh and Hazem Hassan and Aaryaman Sagar and Rhed Shi and Jingming Liu and Filip Gruszczynski and Xianan Zhang and Huy Hoang and Ahmed Yossef and Francois Richard and Yee Jiun Song},
title = {Virtual Consensus in Delos},
booktitle = {14th {USENIX} Symposium on Operating Systems Design and Implementation ({OSDI} 20)},
year = {2020},
isbn = {978-1-939133-19-9},
pages = {617--632},
url = {},
publisher = {{USENIX} Association},
month = nov,

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