Can I just choose CA and avoid partitions?

Not in a distributed system. Partitions are network failures, and you cannot schedule the network to never fail. A single-node database is effectively CA because there is no network inside it, which is a fine answer at moderate scale. Once data is replicated across machines, partition behavior must be defined, so the real choice is CP versus AP.

Is the C in CAP the same as the C in ACID?

No. CAP consistency means linearizability: all nodes agree and reads see the latest write, as if one copy existed. ACID consistency means a transaction moves a single database between valid states, respecting constraints like foreign keys. They share a letter and almost nothing else.

Why do people say PACELC matters more than CAP?

Because partitions are rare and replication lag is constant. CAP only governs the few minutes a year the network is actually split. PACELC also covers the healthy case, where you choose between fast local writes with stale replicas or slow synchronous writes with strong consistency. That choice affects every request every day.

Fundamentalschapter 4 of 6 · 4 lessons

CAP Theorem

In a partition, choose: Consistency or Availability. You can't have both.

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Your database keeps three copies: one in Virginia, one in Ireland, one in Singapore. At 14:02 the link between Virginia and the other two drops. Not the machines, just the link. All three are running and all three are still taking requests.

Somebody in Ireland changes a price. A second later, somebody in Virginia reads that price.

Now choose. Virginia can hand back the old price, which is wrong, or refuse to answer until the link returns. There is no third option where it answers correctly, because the correct value is on the far side of a broken cable.

That is the whole theorem, and it is not pick two of three. Whichever database you already run has made this choice for you, and most teams cannot say which way it went.

Lessons

4 in this chapter

What C, A, and P Actually MeanEach letter has a precise meaning, and two of them are weaker than they sound.2 min
When the Network SplitsDuring a partition, a node can answer possibly-stale or refuse. There is no third option.2 min
Choosing a Side in PracticeThe right letter depends on what a wrong answer costs versus what no answer costs.2 min
PACELC: The Trade You Make Every DayPartitions are rare. The latency-versus-consistency trade runs every second.2 min