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Semicolony ELI5 · comic

The CAP theorem.

When the network splits, you can keep answering or keep everyone in sync, but not both.

  1. in sync
    1

    Two clerks in two cities keep copies of the same ledger, syncing every change as it lands.

  2. Hello? …anyone?
    line dead
    2

    One day the phone line between the cities goes dead. Neither clerk can reach the other.

  3. Can you update my balance?
    update
    3

    A customer walks up and asks to make a change. Now the clerk is stuck.

  4. Sorry — come back later.
    CONSISTENT refuse — never disagree
    4

    Choose consistency: refuse until the line is back, so the two ledgers can never disagree.

  5. Done — we’ll reconcile later.
    AVAILABLE £40 £50 answered — now they differ
    5

    Choose availability: take the change anyway, knowing the two ledgers now tell different stories.

  6. £50 £50 line healthy — both, free
    6

    When the line is healthy you get both. CAP is only the choice you make during the split.

Two clerks, one ledger each. When the line between them dies, they must pick a side.
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