https://raft.github.io The Raft Consensus Algorithm Raft logo Quick Links * Raft paper * raft-dev mailing list * Raft implementations What is Raft? Raft is a consensus algorithm that is designed to be easy to understand. It's equivalent to Paxos in fault-tolerance and performance. The difference is that it's decomposed into relatively independent subproblems, and it cleanly addresses all major pieces needed for practical systems. We hope Raft will make consensus available to a wider audience, and that this wider audience will be able to develop a variety of higher quality consensus-based systems than are available today. Hold on--what is consensus? Consensus is a fundamental problem in fault-tolerant distributed systems. Consensus involves multiple servers agreeing on values. Once they reach a decision on a value, that decision is final. Typical consensus algorithms make progress when any majority of their servers is available; for example, a cluster of 5 servers can continue to operate even if 2 servers fail. If more servers fail, they stop making progress (but will never return an incorrect result). Consensus typically arises in the context of replicated state machines, a general approach to building fault-tolerant systems. Each server has a state machine and a log. The state machine is the component that we want to make fault-tolerant, such as a hash table. It will appear to clients that they are interacting with a single, reliable state machine, even if a minority of the servers in the cluster fail. Each state machine takes as input commands from its log. In our hash table example, the log would include commands like set x to 3. A consensus algorithm is used to agree on the commands in the servers' logs. The consensus algorithm must ensure that if any state machine applies set x to 3 as the n^th command, no other state machine will ever apply a different n^th command. As a result, each state machine processes the same series of commands and thus produces the same series of results and arrives at the same series of states. Raft Visualization Here's a Raft cluster running in your browser. You can interact with it to see Raft in action. Five servers are shown on the left, and their logs are shown on the right. We hope to create a screencast soon to explain what's going on. This visualization (RaftScope) is still pretty rough around the edges; pull requests would be very welcome. The Secret Lives of Data is a different visualization of Raft. It's more guided and less interactive, so it may be a gentler starting point. Publications This is "the Raft paper", which describes Raft in detail: In Search of an Understandable Consensus Algorithm (Extended Version) by Diego Ongaro and John Ousterhout. A slightly shorter version of this paper received a Best Paper Award at the 2014 USENIX Annual Technical Conference. Diego Ongaro's Ph.D. dissertation expands on the content of the paper in much more detail, and it includes a simpler cluster membership change algorithm. More Raft-related papers: * Doug Woos, James R. Wilcox, Steve Anton, Zachary Tatlock, Michael D. Ernst, and Thomas Anderson. Planning for Change in a Formal Verification of the Raft Consensus Protocol. Certified Programs and Proofs (CPP), January 2016. * James R. Wilcox, Doug Woos, Pavel Panchekha, Zachary Tatlock, Xi Wang, Michael D. Ernst, and Thomas Anderson. Verdi: A Framework for Implementing and Verifying Distributed Systems. Programming Language Design and Implementation (PLDI), June 2015. * Hugues Evrard and Frederic Lang. Automatic Distributed Code Generation from Formal Models of Asynchronous Concurrent Processes. Parallel, Distributed, and Network-Based Processing (PDP), March 2015. * Heidi Howard, Malte Schwarzkopf, Anil Madhavapeddy, and Jon Crowcroft. Raft Refloated: Do We Have Consensus?. SIGOPS Operating Systems Review, January 2015. * Heidi Howard. ARC: Analysis of Raft Consensus. University of Cambridge, Computer Laboratory, UCAM-CL-TR-857, July 2014. Talks These talks serve as good introductions to Raft: * Talk on Raft at CS@Illinois Distinguished Lecture Series by John Ousterhout, August 2016: preview of the video Video YouTube Slides PDF with RaftScope visualization * Talk on Raft and its TLA+ spec as part of Dr. TLA+ Series by Jin Li, July 2016: preview of the video Video YouTube Slides SlideShare * Talk on Raft at Build Stuff 2015 by Diego Ongaro, November 2015: preview of the video Video InfoQ Slides HTML PDF with RaftScope visualization * Talks on Rust, Raft, and distributed systems at Rust Bay Area Meetup by Yvonne Coady, Diego Ongaro, Andrew Hobden, Dan Burkert, and Alex Newman, August 2015: preview of the video Video Air Mozilla Slides Diego: PDF with RaftScope visualization * Talk on Raft at CoreOS Fest 2015 by Diego Ongaro, May 2015: preview of the video Video YouTube Slides PDF with RaftScope visualization * Talk on Raft at Sourcegraph meetup by Diego Ongaro, April 2015: preview of the video Video YouTube Slides PDF with RaftScope visualization * Talk on Raft at LinkedIn by Diego Ongaro, September 2014: preview of the video Video YouTube Slides PDF PPTX with RaftScope visualization * Talk on Raft at USI 2014 and /dev/summer 2014 by Arnaud Bailly, July 2014: preview of the video Video YouTube (French) Slides Speaker Deck (English) * Talk on Raft at 2014 USENIX Annual Technical Conference by Diego Ongaro, June 2014: preview of the video Video USENIX Slides RaftScope visualization * Talk on Raft at CraftConf 2014 by Diego Ongaro, April 2014: preview of the video Video Ustream Slides PDF PPTX * Talk on Raft at Rubyconf 2013 by Patrick Van Stee, November 2013: preview of the video Video YouTube Slides Speaker Deck * Talk on Raft at RICON West 2013 by Diego Ongaro, October 2013: preview of the video Video YouTube Slides PDF PPTX * Talk on Raft at Strange Loop 2013 by Ben Johnson, September 2013: preview of the video Video InfoQ Slides Speaker Deck * Talk on Raft and Rafter at the Erlang NYC Meetup by Tom Santero and Andrew Stone, August 2013: preview of the video Video Vimeo Slides Speaker Deck * Talk on Raft (venue unknown) by Patrick Van Stee, July 2013: preview of the video Slides Speaker Deck * Lecture for the Raft User Study by John Ousterhout, March 2013: preview of the video Video (screencast) YouTube MP4 Slides PDF PPTX Courses teaching Raft This is a list of courses that include lectures or programming assignments on Raft. This might be useful for other instructors and for online learners looking for materials. If you know of additional courses, please submit a pull request or an issue to update it. * University of California, San Deigo, CSE224: Graduate Networked Systems, George Porter. Includes lecture on Raft and programming assignment (Go) to develop a fault-tolerant Dropbox clone. (Winter 2023/2022, Fall 2020/2019/2018 ...) * Technical University of Munich, IN2259: Distributed Systems, Pramod Bhatotia and Martin Kleppmann. Includes lecture on Raft. (Winter 2022/2023, ...) * University of Illinois Urbana-Champaign, CS425: Distributed Systems, Indranil Gupta, Nikita Borisov, and Radhika Mittal. Includes Raft programming assignment in Go. (Spring 2021, ...) * The Chinese University of Hong Kong, CSCI4160: Distributed and Parallel Computing (Private), Eric Lo. Includes lecture on Paxos and Raft as well as Raft programming assignment in Java/Go (Fall 2019, Spring 2021, ...) * Indian Institute of Technology Delhi, COL 819: Distributed Systems, Smruti R. Sarangi. Includes lecture on Raft (video) ( PPTX) (Winter 2020, ...) * Carnegie Mellon University, CS 440: Distributed Systems, Yuvraj Agarwal, Dave Andersen. Includes Raft programming assignment in Go. (Fall 2019, ...) * Northeastern University, CS 3700: Networks and Distributed Systems, Christo Wilson, Long Lu. Includes an assignment to build a replicated key-value store based on the Raft protocol. (Fall 2018, ...) * Institute Technology Bandung, IF3230. Raft assignments with testcases for showcasing edge cases and scoring (2018) * Rose-Hulman Institute of Technology, CS 403: Programming Language Paradigms, Buffalo Hewner. Includes Raft programming assignment in Erlang (assignments). (Winter 2017, ...) * Princeton University, COS-418: Distributed Systems, Mike Freedman and Kyle Jamieson. Includes lecture on Raft (PPTX) and programming assignments to build a Raft-based key-value store. (Fall 2016, ...) * University of Washington, CSE 452: Distributed Systems, Tom Anderson. Includes lecture on Raft, though they call it Paxos. (Winter 2016, ...) * University of Colorado, Boulder, CSCI 5673: Distributed Systems, Shivakant Mishra. Includes assignment to download a Raft implementation and build a fault-tolerant data structure with it. (Fall 2015, ...) * University of Utah, CS 6963: Distributed Systems, Ryan Stutsman ( @rstutsman). Will include something about Raft (TBD). (Fall 2015, ...) * San Jose State University, CMPE 275 Enterprise Application Development, John Gash. Includes project to make a distributed filesystem using Raft. (Spring 2015, ...) * Brown, CS 138: Distributed Computer Systems, Tom Doeppner, Rodrigo Fonseca (@rodrigo_fonseca). Includes Raft programming assignment in Go. (Spring 2015, ...) * MIT, 6.824: Distributed Systems, Robert Morris. Includes lecture on Raft (lecture notes). See Jon Gjengset's posts for instructors and students. (Spring 2015, ...) * University of San Francisco, CS 636: Graduate Operating Systems, Greg Benson (@gregorydbenson). Includes lecture on Raft. (Spring 2015, ...) * Harvard, CS 261: Research Topics in Operating Systems, Margo Seltzer. Includes lecture on Raft (lecture notes). (Fall 2014, ...) * University of Houston, COSC 6360: Operating Systems, Jehan-Francois Paris (@jehanfrancois). Includes lecture on Raft ( PPT). (Fall 2014, ...) * Stanford, CS 244b: Distributed Systems, Dawson Engler, David Mazieres (@dmazieres). Included guest lecture on Raft by Diego Ongaro. Several students chose to work on Raft-based final projects. (Fall 2014) * NUST-SEECS, CS 332: Distributed Computing, Tahir Azim (@TahirAzim ). Includes lecture on Raft based on user study materials (tweet ). (Fall 2014, ...) * Duke, CPS 512: Distributed Systems, Bruce Maggs. Includes guest lecture on Raft (PPTX) by Landon Cox (@lpcox). (Spring 2014, Spring 2015, ...) * IIT Bombay, CS 733: Cloud Computing, Sriram Srinivasan. Includes Raft programming assignment in Go (assignments). (Spring 2014, Spring 2015, ...) Where can I ask questions? The best place to ask questions about Raft and its implementations is the raft-dev Google group. Some of the implementations also have their own mailing lists; check their READMEs. Where can I get Raft? There are many implementations of Raft available in various stages of development. This table lists the implementations we know about with source code available. The most popular and/or recently updated implementations are towards the top. This information will inevitably get out of date; please submit a pull request or an issue to update it. Leader Stars Name Primary Language License Election + Persistence? Membership Log Authors Log Changes? Compaction? 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