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Optimal Lab

System Design · interactive

CAP: consistency or availability

Networks split. When one replica can't reach the others, it has two choices for a request: refuse it, or answer with what it has, which may be out of date. You have to pick one. Try both.

Replica A

x = 1

up to date

Replica B

x = 1

up to date

Replica C

x = 1

up to date

Network healthy

All three replicas agree: x = 1.

During a partition, prefer

step 1 of 5
  1. 1

    Three copies of the data, so one machine can fail without losing it.

    A, B and C all hold x = 1

  2. 2

  3. 3

  4. 4

In production

Tools and technologies

Every replicated database makes this choice; most let you tune it per request.

Not tied to one language — use these names when you search docs or design a system.