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Semicolony ELI5 · comic

ACID transactions.

A bank transfer that cannot half-happen: either the whole thing lands, or none of it does.

  1. A −£100 B +£100 two steps
    1

    Moving £100 is really two steps: take it from one account, add it to the other.

  2. Where did my £100 go?!
    −£100 ? power cut!
    2

    If the power cuts between the two steps, the money must not vanish into thin air.

  3. −£100 +£100 one indivisible action
    3

    A transaction fences both steps so they behave as one indivisible action — atomicity.

  4. Both landed. Sealed.
    COMMIT
    4

    If every step succeeds, commit: lock it all in at once and it survives a crash a second later.

  5. Never mind — put it back.
    ROLLBACK as if it never happened
    5

    If anything fails, roll back: undo everything, as if it never happened.

  6. A atomic C consistent I isolated D durable
    6

    The four promises: atomic, consistent, isolated, durable — why a database feels trustworthy.

A money transfer that can’t half-happen — and the four promises behind it. (Bit moves £100.)
Semicolony semicolony.dev/eli5/acid/comic
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