In Search of an Understandable Consensus Algorithm

Diego Ongaro, John Ousterhout · USENIX Annual Technical Conference · 2014 · computer-science · record ongaro2014-raft
tag layer · index · concepts · authors · עברית

Where this record's text came from. faceted 2026-08 by reading the locally held text (pdftext); every evidence string verified as an exact substring of that text before merging. One method facet cannot hold this paper: it is simultaneously an algorithm/system design, a controlled behavioural experiment (43 students, paired quizzes, paired t-test, linear regression model), and a partly mechanised formal proof in TLA+. I chose system-or-tool-building because the deliverable is the algorithm and its implementation; the user-study evidence sentence 'A user study with 43 students at two universities shows that Raft is significantly easier to understand than Paxos: after learning both algorithms, 33 of these students were able to answer questions about Raft better than questions about Paxos.' is verbatim in the text if a behavioural-experiment reading is preferred. The Paxos relation is filed as 'contradicts' because the paper argues against Paxos as a foundation rather than merely differing from it. Replication status is untested: the user study is the authors' own and no independent replication is reported. Extraction is clean prose apart from figure and table text (Figure 2's algorithm summary) being interleaved into the body; no quotes were taken from those regions. 2026-08-14 and 2026-08-15 schema-0.3 passes: every quote verified as an exact substring of the staged text and anchored at merge time; concept terms verified present; every edge endpoint an existing node. Update, 15 September 2026: EXCEPTION, measured with the merge gate's own predicate (Paper.locate): 1 quote(s) on this record are not found as one exact string in the current staged text - content_tags.findings[0].evidence. The sentence above does not hold for them as written. Where a row records why - the quote is grounded in another copy of the paper, joins two passages with an ellipsis, or differs from the staged text by extraction noise - that note stays on the row.

1 quote on this page was cut from a different copy of this article. This project holds two extractions of this paper: the plain text every other quote here is anchored in, and the publisher’s own HTML edition, which we are not licensed to republish. It is verbatim in the second and cannot be found in the first — not because the tagging is wrong, but because the extraction damaged the passage: a page number dropped into the middle of a number, a formula moved out of its sentence, a ligature where two letters belong. The record names the copy for each one. The quote is the paper’s; the extraction is not.

How to read this page. The article itself is not reproduced here — this paper is not cleared for redistribution, so what follows is the tag layer alone. The evidence sentences are withheld with the article. The table at the foot lists every tag the record carries; each row names the facet it belongs to, and the whose words column says whether the tag records the paper's own wording or the tagger's sentence about the paper. Every tag links to each other paper in the corpus carrying it. The machine-readable edition of the same thing is the TEI file beside this one, where the tags live in standOff, and the article is at its publisher via the link below. When we have changed our mind. Some entries in the table begin with a word in capitals and the reason for it. EVIDENCE CORRECTED — the finding still stands, but we had quoted the wrong sentence from the paper, and the quotation has been replaced with the right one. RETRACTED — the entry itself was wrong; it stays on the page, marked, with the reason, rather than being deleted. WITHDRAWN AND REPLACED — the entry was wrong, and a corrected version of it appears in the entries below. Each mark carries its own reason, in brackets, because the reasons differ from one entry to the next. An entry we have WITHDRAWN is removed from this page and kept in the record, with its reason. A line reading Audit and a date is a later re-reading of the entry against its whole sentence: DOUBTFUL means it was kept, and the reason says what is loose about it. Quotation marks inside the tagger's sentence. Where a tag is the tagger's own sentence, a phrase in quotation marks inside it is borrowed from the paper — the tagger's wording around it is not. The paper's own sentence is the evidence quote for that row; on this page it is withheld with the article, and the article is at its publisher. Nothing inside quotation marks anywhere on this page is ours: if it is quoted, it is the paper's. Whose words a concept carries. A concept entry records a term the paper itself uses, so the term, the sense given for it, and its evidence are the paper's own words, quoted where the paper fixes them. This is the opposite of a sense entry in the tag table, which is the tagger's sentence about how the paper uses a term. Where a concept entry could not be grounded in the paper's words, the entry says so on its own row.

Other fields define these differently the same word, defined another way in another discipline
consensus 6 fields · replication 3 fields · decomposition 2 fields · statistical-significance 2 fields · linear regression 2 fields

Tags on this paper

Every tag the record carries, each linking to every other paper in the corpus that carries it. A tag appears here whether or not its words occur in the article — the spans below are only the ones a sentence in this paper licenses. Of the 10 concept tags on this paper, 5 are grounded in a quoted sentence.

distributed consensus replicated state machine leader election fault tolerance replicated log understandability Paxos decomposition nondeterminism state space design alternatives are chosen by asking two stated questions about UNDERSTANDABILITY - how hard is each to explain, and how easily can a reader completely grasp it and its implications - and the authors state plainly that this analysis is highly subjective the first technique is problem decomposition: the problem is divided into pieces that can be solved, explained AND understood relatively independently, and the paper names the four it used the second technique reduces the STATE SPACE rather than the code - fewer states to consider, more coherence, nondeterminism removed where possible - and it is made concrete by two rules about what logs are not allowed to do NONDETERMINISM IS DELIBERATELY ADDED WHERE IT SHRINKS THE STATE SPACE: randomised approaches handle all possible choices in one fashion - choose any, it does not matter - so randomisation is used to simplify leader election even though the general policy was to eliminate it the comparison is a within-subject crossover: every participant learns both algorithms and takes both quizzes, with the ORDER reversed for about half of them, so that individual differences and the experience gained from the first half are both accounted for the two ways the experiment FAVOURED THE COMPETITOR are stated by the authors rather than left to a reader: some participants had prior experience with the rival algorithm, and the rival's teaching video was longer the regression model's estimate of the quiz effect is reported as LARGER than the raw score difference, and the gap between the two is explained by prior experience with the rival algorithm rather than set aside the participants' own preference is collected AND THEN DISCOUNTED by the authors, who name both reasons it may be unreliable: self-report against measured scores, and the participants' possible knowledge of the hypothesis being tested the correctness argument is anchored to a formal specification that is itself the subject of the proof, and the paper states exactly how far the mechanisation reaches - one property mechanically proven, resting on invariants that have not been mechanically checked consensus Raft replication leader safety log understandable majority consensus algorithm regression linear regression statistical-significance t-test system-or-tool-building

Legend

relation to other work	`claim_relations`	how this paper stands to another: agrees, contradicts, extends, qualifies
claim type	`claim_type`	the shape of the claim: existence, mechanism, method, critique, prediction, null
concept edge	`concept_edges`	an asserted relation between two concepts, with the sentence asserting it
concept	`concepts`	an idea the paper works with, quoted where the paper uses it
content tag	`content_tags`	a finding, definition, quantity, statistic or variable named in the paper
control	`controls`	an experiment run to eliminate a rival explanation
limitation	`limitations`	what the paper says its evidence cannot support
mechanism	`mechanisms`	how the reported effect is said to come about
method	`method`	how the paper produced its evidence
phenomenon	`phenomenon`	the effect or regularity under study
sense	`senses`	a distinct use of a term, quoted where the paper fixes it

The paper

The full text is not reproduced here. This article is not cleared for redistribution, so the page carries the tag layer only. Its evidence sentences are withheld; the grounding status remains with each tag, and the article itself is at https://raft.github.io/raft.pdf. The reading view exists for the papers whose licence permits it.

Every tag on this page

claim_relations 3 · claim_type 1 · concept_edges 2 · concepts 5 · content_tags 8 · controls 1 · limitations 9 · mechanisms 9 · method 1 · phenomenon 1 · senses 3

Relations on this page carry a quotation for the claim. Where a relation was separately evidenced — and so tested against the vocabulary for its type — the row says so; 1 of 2 relations here carries that mark. The rest are the tagger’s reading of the sentence quoted beside them.

facet	tag	field	conf.	whose words
claim_type	method Quotation withheld — this record is not cleared for redistribution. The sentence is in the paper at the link above.	`claim_type`		the paper's words
phenomenon	distributed consensus for replicated logs Quotation withheld — this record is not cleared for redistribution. The sentence is in the paper at the link above.	`phenomenon`		the paper's words
method	system-or-tool-building Quotation withheld — this record is not cleared for redistribution. The sentence is in the paper at the link above. also in: system-or-tool-building	`method`		the paper's words
mechanisms	mechanisms Quotation withheld — this record is not cleared for redistribution. The sentence is in the paper at the link above. also in: design alternatives are chosen by asking two stated questions about UNDERSTANDABILITY - how hard is each to explain, and how easily can a reader completely grasp it and its implications - and the authors state plainly that this analysis is highly subjective	`mechanisms[0]`	0.95	the tagger's words
mechanisms	mechanisms Quotation withheld — this record is not cleared for redistribution. The sentence is in the paper at the link above. also in: the first technique is problem decomposition: the problem is divided into pieces that can be solved, explained AND understood relatively independently, and the paper names the four it used	`mechanisms[1]`	0.95	the tagger's words
mechanisms	mechanisms Quotation withheld — this record is not cleared for redistribution. The sentence is in the paper at the link above. also in: the second technique reduces the STATE SPACE rather than the code - fewer states to consider, more coherence, nondeterminism removed where possible - and it is made concrete by two rules about what logs are not allowed to do	`mechanisms[2]`	0.95	the tagger's words
mechanisms	mechanisms Quotation withheld — this record is not cleared for redistribution. The sentence is in the paper at the link above. also in: NONDETERMINISM IS DELIBERATELY ADDED WHERE IT SHRINKS THE STATE SPACE: randomised approaches handle all possible choices in one fashion - choose any, it does not matter - so randomisation is used to simplify leader election even though the general policy was to eliminate it	`mechanisms[3]`	0.95	the tagger's words
mechanisms	mechanisms Quotation withheld — this record is not cleared for redistribution. The sentence is in the paper at the link above. also in: the comparison is a within-subject crossover: every participant learns both algorithms and takes both quizzes, with the ORDER reversed for about half of them, so that individual differences and the experience gained from the first half are both accounted for	`mechanisms[4]`	0.95	the tagger's words
mechanisms	mechanisms Quotation withheld — this record is not cleared for redistribution. The sentence is in the paper at the link above. also in: the two ways the experiment FAVOURED THE COMPETITOR are stated by the authors rather than left to a reader: some participants had prior experience with the rival algorithm, and the rival's teaching video was longer	`mechanisms[5]`	0.95	the tagger's words
mechanisms	mechanisms Quotation withheld — this record is not cleared for redistribution. The sentence is in the paper at the link above. also in: the regression model's estimate of the quiz effect is reported as LARGER than the raw score difference, and the gap between the two is explained by prior experience with the rival algorithm rather than set aside	`mechanisms[6]`	0.95	the tagger's words
mechanisms	mechanisms Quotation withheld — this record is not cleared for redistribution. The sentence is in the paper at the link above. also in: the participants' own preference is collected AND THEN DISCOUNTED by the authors, who name both reasons it may be unreliable: self-report against measured scores, and the participants' possible knowledge of the hypothesis being tested	`mechanisms[7]`	0.95	the tagger's words
mechanisms	mechanisms Quotation withheld — this record is not cleared for redistribution. The sentence is in the paper at the link above. also in: the correctness argument is anchored to a formal specification that is itself the subject of the proof, and the paper states exactly how far the mechanisation reaches - one property mechanically proven, resting on invariants that have not been mechanically checked	`mechanisms[8]`	0.95	the tagger's words
limitations	Quotation withheld Quotation withheld — this record is not cleared for redistribution. The sentence is in the paper at the link above. what this limits: the liveness argument: Quotation withheld	`limitations[0]`		the tagger's words
limitations	The mechanised safety proof rests on invariants that were never mechanically checked Quotation withheld — this record is not cleared for redistribution. The sentence is in the paper at the link above. what this limits: the safety proof as evidence: THIS PROOF RELIES ON INVARIANTS THAT HAVE NOT BEEN MECHANICALLY CHECKED (FOR EXAMPLE, WE HAVE NOT PROVEN THE TYPE SAFETY OF THE SPECIFICATION)	`limitations[1]`		the tagger's words
limitations	The participants' self-reported preference for Raft may be unreliable and biased by knowledge of the authors' hypothesis Quotation withheld — this record is not cleared for redistribution. The sentence is in the paper at the link above. what this limits: the understandability claim, which is the paper's central selling point: PARTICIPANTS MAY HAVE BEEN BIASED BY KNOWLEDGE OF OUR HYPOTHESIS THAT RAFT IS EASIER TO UN[derstand], and the self-reports MAY BE LESS RELIABLE THAN ... QUIZ SCORES	`limitations[2]`		the tagger's words
limitations	Raft's strong-leader design rules out some performance optimisations available to leaderless algorithms Quotation withheld — this record is not cleared for redistribution. The sentence is in the paper at the link above. what this limits: the performance comparison: Raft's strong-leader design rules out optimisations available to leaderless algorithms	`limitations[3]`		the tagger's words
limitations	Optimisations known for other consensus algorithms were not applied or evaluated here Quotation withheld — this record is not cleared for redistribution. The sentence is in the paper at the link above. what this limits: any performance comparison against optimised implementations: VARIOUS OPTIMIZATIONS HAVE BEEN PROPOSED IN THE LITERATURE FOR OTHER ALGORITHMS; MANY OF THESE COULD BE APPLIED TO RAFT, BUT WE LEAVE THIS TO FUTURE WORK	`limitations[4]`		the tagger's words
limitations	The design criterion the paper is built around is the authors' own judgement about how hard things are to explain. Quotation withheld — this record is not cleared for redistribution. The sentence is in the paper at the link above. what this limits: the claim that this algorithm is more understandable by design: the alternatives were judged by how hard each is to explain and how easy it would be for a reader to understand, which are the authors' own assessments	`limitations[5]`	1.0	the tagger's words
limitations	The understandability result is a quiz given to students on two named courses. Quotation withheld — this record is not cleared for redistribution. The sentence is in the paper at the link above. what this limits: the finding that this algorithm is more understandable than Paxos: it was measured by an experimental study using upper-level undergraduate and graduate students in an Advanced Operating Systems course at Stanford and a distributed computing course elsewhere	`limitations[6]`	1.0	the tagger's words
limitations	The equivalence claimed with the algorithm it replaces is on result and efficiency, with the structure explicitly different. Quotation withheld — this record is not cleared for redistribution. The sentence is in the paper at the link above. what this limits: treating results about Paxos as results about this algorithm: the paper states it produces an equivalent result and is as efficient, but that its structure is different	`limitations[7]`	1.0	the tagger's words
limitations	Removing nondeterminism was the general aim, and the paper records where that aim was abandoned. Quotation withheld — this record is not cleared for redistribution. The sentence is in the paper at the link above. what this limits: a description of this algorithm as deterministic by design: the authors state that although they tried in most cases to eliminate nondeterminism, in some situations nondeterminism actually improves understandability	`limitations[8]`	1.0	the tagger's words
controls	counterbalancing the order of the Paxos and Raft portions of the study, half the participants each way Quotation withheld — this record is not cleared for redistribution. The sentence is in the paper at the link above. eliminates: that the difference in quiz scores reflects individual differences in ability or learning carried over from whichever algorithm was studied first, rather than the algorithms' understandability · outcome: held	`controls[0]`	0.9	the tagger's words
concepts	understandability Quotation withheld — this record is not cleared for redistribution. The sentence is in the paper at the link above. also in: understandability	`concepts[5]`		the paper's words
concepts	Paxos Quotation withheld — this record is not cleared for redistribution. The sentence is in the paper at the link above. also in: Paxos	`concepts[6]`		the paper's words
concepts	decomposition Quotation withheld — this record is not cleared for redistribution. The sentence is in the paper at the link above. also in: decomposition	`concepts[7]`		the paper's words
concepts	nondeterminism Quotation withheld — this record is not cleared for redistribution. The sentence is in the paper at the link above. also in: nondeterminism	`concepts[8]`		the paper's words
concepts	state space Quotation withheld — this record is not cleared for redistribution. The sentence is in the paper at the link above. also in: state space	`concepts[9]`		the paper's words
senses	understandability AT DESIGN TIME - AS TWO QUESTIONS THE AUTHORS ASK THEMSELVES: how hard is each alternative to explain, including how complex its state space is and whether it has subtle implications, and how easy it will be for a reader to understand it completely Quotation withheld — this record is not cleared for redistribution. The sentence is in the paper at the link above.	`senses[0]`	1.0	the tagger's words
senses	understandability AT EVALUATION TIME - AS A QUIZ SCORE: the same word is measured by testing upper-level undergraduate and graduate students on two named university courses, so the paper's central term has one meaning while designing and a different, countable one while evaluating Quotation withheld — this record is not cleared for redistribution. The sentence is in the paper at the link above.	`senses[1]`	1.0	the tagger's words
senses	the equivalence with Paxos - ON RESULT AND EFFICIENCY ONLY: the paper states the result is equivalent and the efficiency comparable while the structure is different, so a claim proved about one is not automatically a claim about the other Quotation withheld — this record is not cleared for redistribution. The sentence is in the paper at the link above.	`senses[2]`	1.0	the tagger's words
claim_relations	extends → Paxos consensus algorithm Quotation withheld — this record is not cleared for redistribution. The sentence is in the paper at the link above.	`claim_relations[0]`		the tagger's words
claim_relations	contradicts → Paxos (Lamport) Quotation withheld — this record is not cleared for redistribution. The sentence is in the paper at the link above.	`claim_relations[1]`		the tagger's words
claim_relations	builds-on → Oki and Liskov's Viewstamped Replication Quotation withheld — this record is not cleared for redistribution. The sentence is in the paper at the link above.	`claim_relations[2]`		the tagger's words
concept_edges	understandability —operationalised-as→ state space Quotation withheld — this record is not cleared for redistribution. The sentence is in the paper at the link above. strength of the relation: 1 Audit 2026-09-28: DOUBTFUL - read against its full span and kept, but the span gives state-space complexity as one example of how understandability was judged; operationalised-as is loose. also in: understandability · state space	`concept_edges[0]`		the tagger's words
concept_edges	decomposition —causes→ understandability Quotation withheld — this record is not cleared for redistribution. The sentence is in the paper at the link above. status: the authors' DESIGN INTENT, stated as what they applied - not a measured outcome · direction on the measure: increases · the relation itself is separately evidenced · strength of the relation: 1 · the relation stated: decomposition is one of the techniques APPLIED TO IMPROVE understandability also in: decomposition · understandability	`concept_edges[1]`		the tagger's words
content_tags	43 — user-study size comparing Raft vs Paxos learnability Quotation withheld — this record is not cleared for redistribution. The sentence is in the paper at the link above.	`content_tags.quantities[0]`		the tagger's words
content_tags	A user study showed Raft is easier for students to learn than Paxos; Raft also adds a mechanism for changing cluster membership that uses overlapping majorities to guarantee safety. Quotation withheld — this record is not cleared for redistribution. The sentence is in the paper at the link above. about this quote: QUOTE REPAIRED: the extractor split a hyphen-space and the quote was recorded faithfully with the damage in it, which made it unanchorable. The repaired string is an EXACT SUBSTRING of the staged text - that is why it was accepted, not because a rule produced it. The text as originally recorded is kept in evidence as extracted.	`content_tags.findings[1]`		the tagger's words
content_tags	understandability Quotation withheld — this record is not cleared for redistribution. The sentence is in the paper at the link above. also in: understandability	`content_tags.variables[1]`		the tagger's words
content_tags	consensus algorithm Quotation withheld — this record is not cleared for redistribution. The sentence is in the paper at the link above. about this quote: QUOTE REPAIRED: the extractor split a hyphen-space and the quote was recorded faithfully with the damage in it, which made it unanchorable. The repaired string is an EXACT SUBSTRING of the staged text - that is why it was accepted, not because a rule produced it. The text as originally recorded is kept in evidence as extracted. also in: consensus algorithm	`content_tags.definitions[0]`		the tagger's words
content_tags	replicated state machine Quotation withheld — this record is not cleared for redistribution. The sentence is in the paper at the link above. also in: replicated state machine	`content_tags.definitions[1]`		the tagger's words
content_tags	regression Quotation withheld — this record is not cleared for redistribution. The sentence is in the paper at the link above. about this quote: EXTRACTION, NOT THE PAPER: the quote as extracted ended 'and s t n a p…', which is 'participants' reversed. It runs on into 'i f o r e b m u n' ('number of' reversed) - together the rotated y-axis label of Figure 15, which the extractor read bottom-to-top and placed inside this sentence. The sentence resumes 250 characters later at 'the order in…', so the true third factor is the learning order. The quote is TRIMMED rather than mended: joining the two halves would produce a string that reads correctly and exists in no document. Found by an internal tool; this was the only live quote in the corpus reaching into such a region. also in: regression	`content_tags.statistics[0]`		the tagger's words
content_tags	statistical-significance Quotation withheld — this record is not cleared for redistribution. The sentence is in the paper at the link above. about this quote: QUOTE REPAIRED: the extractor split a hyphen-space and the quote was recorded faithfully with the damage in it, which made it unanchorable. The repaired string is an EXACT SUBSTRING of the staged text - that is why it was accepted, not because a rule produced it. The text as originally recorded is kept in evidence as extracted. also in: statistical-significance	`content_tags.statistics[1]`		the tagger's words
content_tags	t-test Quotation withheld — this record is not cleared for redistribution. The sentence is in the paper at the link above. also in: t-test	`content_tags.statistics[2]`		the tagger's words

Interpretive choices

Tags that could not be placed in the text:

content_tags.findings[0] — quotation withheld — the page has already reproduced as much of this paper as the rights budget allows
NOT MACHINE-VOUCHED: declared-ellipsis - An elided quote: its 2 pieces ('it produces a result equivalent to (mult' ... 'er election, log replication, and safety') are each in our copy of the paper (with line-break hyphens, spacing and punctuation set aside), and the quote marks the cut with an ellipsis, so the joined string is in no copy. Located mechanically, piece by piece.

What was deliberately not tagged, and why, is recorded per paper by the tagger. Where a paper has no such record, this page says so rather than implying that nothing was declined.