Give

Consensus: agreeing despite failures

Why agreement is hard, and how majorities let a cluster agree anyway.

Back to Coding · Replication and consensus

Planned. This module is part of the plan and is not written yet.

COCO-DST-REP-0404Level16

Why it matters

A mobile-money ledger kept on several servers is only safe if every server agrees on the order of transactions, even when one crashes. Consensus is how they agree.

Draft outline · in review
For
Level 16 · 300 Level
Inside
6 lessons and a test · about 120 minutes
Topic
Replication and consensus · advanced, 4 of 4
Plate CO-DST-REP-04: planned.

Inside the module.

6 lessons in order, about 120 minutes in all, then a short test. Learners go at their own pace; the minutes are a guide, not a timetable.

Draft outline · in review

  1. Lesson 1: The consensus problem

    Several nodes must agree on one value, or one order of values, despite crashes and delays.

    Explain20 min, from minute 0

  2. Lesson 2: Majorities and quorums

    Why a cluster of 2f + 1 nodes can survive f crashes.

    Explain20 min, from minute 20

  3. Lesson 3: Any two majorities overlap

    Show that two majorities must share a node, and why that prevents two different decisions.

    Worked example20 min, from minute 40

  4. Lesson 4: Electing a leader

    Terms, votes, one vote per node per term, and randomised timeouts.

    Explain25 min, from minute 60

  5. Lesson 5: Committing entries

    A leader commits an entry from its own term once a majority has stored it; the subtlety for older terms comes in Raft step by step.

    Explain25 min, from minute 85

  6. Lesson 6: Check your skills

    Quorum sizes and failure cases.

    Review10 min, from minute 110

  7. End-of-module test

    Explain the consensus problem and why majorities help, and describe how a cluster elects a leader and commits entries.

    Mastered at 85% or more. Retake it until you pass; nothing is lost, and each try gives feedback.

    Already know it? A Challenge Test (planned) clears the whole module: 90% overall, no objective below 75%.

    Last stageafter about 120 min

Practicalities

Draft outline · in review

  • Built to run offline

    Planned to run on the Lantern's own computer, with no internet needed.

  • Languages

    Planned first in English; every module aims to reach all of the Lantern's languages.

Try a question.

One question from the module, as a learner would meet it. Have a go, then open the worked solution a step at a time.

Draft outline · in review

A cluster of 5 nodes uses majority quorums. How many nodes can crash while it still makes progress? And why can't two different leaders both win an election in the same term?

Work it out on paper first, then show the answer and mark it yourself.

Particulars

Where it sits

Coding · Distributed computing · Replication and consensus

What a learner can do after it

  1. Explain the consensus problem and why majorities (quorums) help.
  2. Describe how a cluster elects a leader and commits entries.

Needs first

Unlocks

Level

Level 16 · 300 Level

Show levels in

Level mappings are approximate. A qualified teacher or lecturer in each country must check them before they appear in the app. Levels 13–18 describe the content, not the learner: “Level 15” means material usually taught in a second-year university course, whoever chooses to learn it.

Difficulty for its level

4 of 4 · Advanced

Time

About 120 minutes

Alignments

  • Aligned to CS2023partial · in reviewCS2023 Parallel and Distributed Computing: consensus (descriptive ref)
  • Aligned to Typical university coursepartial · in reviewTypical third-year distributed systems course: consensus

Alignment in review until a qualified teacher or lecturer checks it. No exam body, university or vendor endorses us; learners still sit the real exam. University-level modules give knowledge, not university credit.

Challenge Test

Planned: a Challenge Test to prove you know it and skip it. Pass at 90% overall, with no objective below 75%.

University courses it previews

Computer Science, Software Engineering

On these pathways

The route here.

Everything this module builds on, drawn as a route from its foundations. Solid lanes must be mastered first, or cleared by a Challenge Test; dashed lanes are suggestions that never lock anything.

From Level 1, Instructions in order to Level 12, Why version control?
  1. CO-BLK-SEQ-01 Instructions in order, Level 1–4, Coding. Planned.
  2. CO-BLK-SEQ-01Instructions in orderCoding · Primary 1 to Primary 4 (Level 1–4) · Planned
  3. CO-BLK-LOP-01 Repeat blocks, Level 3–5, Coding. Sample lesson in the demo.
  4. CO-BLK-LOP-01Repeat blocksCoding · Primary 3 to Primary 5 (Level 3–5) · Sample lesson in the demo Needs Instructions in order (required).Try the sample lesson: Repeat blocks
  5. DS-CMP-TYP-01 The home row, Level 3–6, Digital skills. Planned.
  6. DS-CMP-TYP-01The home rowDigital skills · Primary 3 to Primary 6 (Level 3–6) · Planned · recommended
  7. CO-BLK-CND-01 If this, then that, Level 3–6, Coding. Planned.
  8. CO-BLK-CND-01If this, then thatCoding · Primary 3 to Primary 6 (Level 3–6) · Planned · recommended
  9. CO-BLK-LOP-02 Patterns with loops, Level 4–6, Coding. Planned.
  10. CO-BLK-LOP-02Patterns with loopsCoding · Primary 4 to Primary 6 (Level 4–6) · Planned Needs Repeat blocks (required).
  11. DS-FIL-ORG-01 Folders that make sense, Level 4–7, Digital skills. Planned.
  12. DS-FIL-ORG-01Folders that make senseDigital skills · Primary 4 to JSS 1 (Level 4–7) · Planned · recommended
  13. MA-ARI-ORD-01 Order of operations, Level 5–6, Maths. Planned.
  14. MA-ARI-ORD-01Order of operationsMaths · Primary 5 to Primary 6 (Level 5–6) · Planned · recommended
  15. CO-BLK-LOP-03 Loops inside loops, Level 5–7, Coding. Planned.
  16. CO-BLK-LOP-03Loops inside loopsCoding · Primary 5 to JSS 1 (Level 5–7) · Planned · recommended
  17. CO-PRG-PYB-01 Your first Python program, Level 7–9, Coding. Planned.
  18. CO-PRG-PYB-01Your first Python programCoding · JSS 1 to JSS 3 (BECE) (Level 7–9) · Planned Needs Patterns with loops (required), The home row (recommended).
  19. CO-PRG-PYB-02 Variables, Level 7–9, Coding. Sample lesson in the demo.
  20. CO-PRG-PYB-02VariablesCoding · JSS 1 to JSS 3 (BECE) (Level 7–9) · Sample lesson in the demo Needs Your first Python program (required).Try the sample lesson: Variables
  21. CO-PRG-PYB-03 Numbers and calculations, Level 7–9, Coding. Planned.
  22. CO-PRG-PYB-03Numbers and calculationsCoding · JSS 1 to JSS 3 (BECE) (Level 7–9) · Planned Needs Variables (required), Order of operations (recommended).
  23. MA-ALG-FRM-01 Substituting into formulae, Level 7–8, Maths. Planned.
  24. MA-ALG-FRM-01Substituting into formulaeMaths · JSS 1 to JSS 2 (Level 7–8) · Planned · recommended
  25. CO-NET-HOW-01 Packets, addresses and routes, Level 7–10, Coding. Planned.
  26. CO-NET-HOW-01Packets, addresses and routesCoding · JSS 1 to SSS 1 (Level 7–10) · Planned
  27. CO-PRG-PYB-05 Decisions with if, Level 8–10, Coding. Planned.
  28. CO-PRG-PYB-05Decisions with ifCoding · JSS 2 to SSS 1 (Level 8–10) · Planned Needs Numbers and calculations (required), If this, then that (recommended).
  29. CO-PRG-PYB-06 Loops in Python, Level 8–10, Coding. Planned.
  30. CO-PRG-PYB-06Loops in PythonCoding · JSS 2 to SSS 1 (Level 8–10) · Planned Needs Decisions with if (required), Loops inside loops (recommended).
  31. CO-PRG-PYF-01 Defining functions, Level 8–11, Coding. Planned.
  32. CO-PRG-PYF-01Defining functionsCoding · JSS 2 to SSS 2 (Level 8–11) · Planned Needs Loops in Python (required).
  33. CO-PRG-PYB-07 Lists, Level 8–10, Coding. Planned.
  34. CO-PRG-PYB-07ListsCoding · JSS 2 to SSS 1 (Level 8–10) · Planned Needs Loops in Python (required).
  35. CO-SYS-OPS-01 What an operating system does, Level 8–11, Coding. Planned.
  36. CO-SYS-OPS-01What an operating system doesCoding · JSS 2 to SSS 2 (Level 8–11) · Planned · recommended
  37. CO-PRG-PYF-02 Parameters and return values, Level 9–11, Coding. Planned.
  38. CO-PRG-PYF-02Parameters and return valuesCoding · JSS 3 (BECE) to SSS 2 (Level 9–11) · Planned Needs Defining functions (required), Substituting into formulae (recommended).
  39. CO-ALG-SRT-01 Linear search, Level 9–11, Coding. Planned.
  40. CO-ALG-SRT-01Linear searchCoding · JSS 3 (BECE) to SSS 2 (Level 9–11) · Planned Needs Parameters and return values (required), Lists (required).
  41. CO-PRG-PYF-03 Scope, Level 9–11, Coding. Planned.
  42. CO-PRG-PYF-03ScopeCoding · JSS 3 (BECE) to SSS 2 (Level 9–11) · Planned Needs Parameters and return values (required).
  43. CO-PRG-PYF-04 Breaking problems into functions, Level 9–12, Coding. Planned.
  44. CO-PRG-PYF-04Breaking problems into functionsCoding · JSS 3 (BECE) to SSS 3 (WASSCE) (Level 9–12) · Planned Needs Parameters and return values (required), Lists (required).
  45. CO-ALG-SRT-03 Simple sorts, Level 10–12, Coding. Planned.
  46. CO-ALG-SRT-03Simple sortsCoding · SSS 1 to SSS 3 (WASSCE) (Level 10–12) · Planned Needs Linear search (required).
  47. CO-ALG-SRT-02 Binary search, Level 10–12, Coding. Planned.
  48. CO-ALG-SRT-02Binary searchCoding · SSS 1 to SSS 3 (WASSCE) (Level 10–12) · Planned Needs Linear search (required).
  49. CO-PRG-PYF-05 Recursion, Level 11–13, Coding. Planned.
  50. CO-PRG-PYF-05RecursionCoding · SSS 2 to A level / JUPEB / foundation year (Level 11–13) · Planned Needs Scope (required).
  51. CO-ALG-SRT-04 Merge sort, Level 11–13, Coding. Planned.
  52. CO-ALG-SRT-04Merge sortCoding · SSS 2 to A level / JUPEB / foundation year (Level 11–13) · Planned Needs Simple sorts (required), Recursion (required).
  53. CO-SWD-GIT-01 Why version control?, Level 12–13, Coding. Planned.
  54. CO-SWD-GIT-01Why version control?Coding · SSS 3 (WASSCE) to A level / JUPEB / foundation year (Level 12–13) · Planned Needs Parameters and return values (required), Folders that make sense (recommended).
  55. CO-SWD-GIT-02Commits and historyCoding · SSS 3 (WASSCE) to A level / JUPEB / foundation year (Level 12–13) · Planned Needs Why version control? (required).
  56. CO-ALG-SRT-05Comparing algorithmsCoding · SSS 3 (WASSCE) to A level / JUPEB / foundation year (Level 12–13) · Planned Needs Binary search (required), Merge sort (required).
  57. From here on: Beyond school · university level
    CO-SWD-GIT-03Branches and mergingCoding · A level / JUPEB / foundation year to 100 Level (Level 13–14) · Planned Needs Commits and history (required).
  58. CO-SWD-GIT-04Working together with remotesCoding · A level / JUPEB / foundation year to 100 Level (Level 13–14) · Planned Needs Branches and merging (required).
  59. CO-DST-CCR-01Doing many things at onceCoding · 100 Level to 200 Level (Level 14–15) · Planned Needs Breaking problems into functions (required), Comparing algorithms (required), What an operating system does (recommended).
  60. CO-DST-FND-01What makes a system distributedCoding · 200 Level (Level 15) · Planned Needs Doing many things at once (required), Packets, addresses and routes (required), Working together with remotes (required).
  61. CO-DST-REP-01Why replicate?Coding · 200 Level to 300 Level (Level 15–16) · Planned Needs What makes a system distributed (required).
  62. CO-DST-REP-02Leaders and followersCoding · 200 Level to 300 Level (Level 15–16) · Planned Needs Why replicate? (required).
  63. CO-DST-REP-03Time, clocks and orderingCoding · 200 Level to 300 Level (Level 15–16) · Planned Needs Why replicate? (required).
  64. CO-DST-REP-04Consensus: agreeing despite failuresCoding · 300 Level (Level 16) · Planned · the destination Needs Leaders and followers (required), Time, clocks and ordering (required).
37 stops across Coding, Digital skills and Maths. 2 have a sample lesson in the demo; the other 35 are planned and not yet written.

Help light the room where this is learned.

Every route on this map starts in a classroom with 24 stations, a tutor that works offline and a facilitator upstairs. Four are planned and waiting for light.

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