Consensus: Bridging Theory and Practice

01 Claim ledger

What the paper is allowed to say

Each row is a sentence already in the study. The status is the same on every row: a model claim, not a live measurement.

1. Claim 01 · Paper model

   The defect

   The Raft paper left membership change as a sketch. Changing who is in the majority while a log is live is where implementations fork the brain.

2. Claim 02 · Paper model

   The rule

   Membership changes one server at a time, or through joint consensus, so there is never a moment when two disjoint majorities can both commit.

3. Claim 03 · Paper model

   How it is put together

   A cold server does not vote until it has caught up. Joint consensus overlaps the old set and the new set. Leadership and membership are different decisions.

4. Claim 04 · Paper model

   Where the claim stops

   The dissertation is the careful version. Blog posts often skip the overlap.

02 Three cuts

Observation, model, falsifier

A desk does not stop at the summary. Each claim is cut three ways, using only this study's own assumptions and checks. Nothing here is a new figure.

1. 01 The defect

   Observation

   What the study says

   The Raft paper left membership change as a sketch. Changing who is in the majority while a log is live is where implementations fork the brain.

   Model

   What has to hold

   A chain that restakes every epoch is a different membership protocol.

   Falsifier

   What would retire it

   Can two configurations both hold a majority at once?

2. 02 The rule

   Observation

   What the study says

   Membership changes one server at a time, or through joint consensus, so there is never a moment when two disjoint majorities can both commit.

   Model

   What has to hold

   A chain that restakes every epoch is a different membership protocol.

   Falsifier

   What would retire it

   Does a new server vote before it has the log?

3. 03 How it is put together

   Observation

   What the study says

   A cold server does not vote until it has caught up. Joint consensus overlaps the old set and the new set. Leadership and membership are different decisions.

   Model

   What has to hold

   A chain that restakes every epoch is a different membership protocol.

   Falsifier

   What would retire it

   Does a new server vote before it has the log?

4. 04 Where the claim stops

   Observation

   What the study says

   The dissertation is the careful version. Blog posts often skip the overlap.

   Model

   What has to hold

   The dissertation is the careful version. Blog posts often skip the overlap.

   Falsifier

   What would retire it

   Can two configurations both hold a majority at once?

03 Sequence

One action, as an operating tape

1. 01Add or remove one voter.
2. 02Commit the configuration entry under the rules of the overlapping sets.
3. 03Only then start using the new majority alone.
4. 04Can two configurations both hold a majority at once?

04 Load-bearing

The argument, and where a pitch drops it

1. Why it is still on the desk

   The cut

   If a cluster can be grown by editing a config file and restarting, it is not this procedure.

   Why it carries weight

   If this cut is skipped, the paper's name is being used without the mechanism that makes the name mean anything.

   Where it is dropped

   A later client, parameter or reward formula is a different object from this paragraph.

2. After the text

   The cut

   etcd and Consul spent years on this chapter. The bugs were in membership, not in the log-matching property everyone quotes.

   Why it carries weight

   If this cut is skipped, the paper's name is being used without the mechanism that makes the name mean anything.

   Where it is dropped

   A chain that restakes every epoch is a different membership protocol.

05 Register

What has to be true

• Model · Not re-measured

  The dissertation is the careful version. Blog posts often skip the overlap.

• Stake and threshold · Not re-measured

  It is still crash fault tolerance, not Byzantine.

• Stake and threshold · Not re-measured

  A chain that restakes every epoch is a different membership protocol.

06 Divergence

What happened after the paper

etcd and Consul spent years on this chapter. The bugs were in membership, not in the log-matching property everyone quotes.

A later client, parameter set, or reward formula is a different object. Cite this paper for the mechanism. Cite a primary release for the network. This desk has not re-run the proof.

07 Pre-mortem

What to check before you use the idea

1. Kill test 1Can two configurations both hold a majority at once?
2. Kill test 2Does a new server vote before it has the log?
3. Kill test 3Is the configuration itself a log entry?
4. 0 of 3 marked on this browser. A mark is a reading note, not a pass, a rating, or a recommendation.

08 Anatomy

The paper, in the order a builder needs

09 Lexicon

Terms, opened into the record

Joint consensus

A majority that must include both the old set and the new set.

Configuration entry

A log entry whose payload is the set of voters.

10 Repository

Every linked record on this page

Underlined words open a page that already exists: a concept, a protocol profile, a failure record, or another paper. If a word is not underlined, this desk does not have a record for it.

11 Related

The next papers, as theses

• Raft · 2014

  In Search of an Understandable Consensus Algorithm

  A strong leader accepts entries, replicates them in order, and commits only when a majority of the current term has stored them.

• Paxos · 1998

  The Part-Time Parliament

  A value is chosen only when a majority has accepted it in some ballot, and every higher ballot must honour a value that majority might already have chosen.