Part 4 — ACID Transactions: System Design & Interview Q&A (Staff/Principal Level)

maang.io System Design Series

What is this?

The first three parts built the knowledge; this part teaches you to perform it. An ACID question in a senior interview is rarely "define atomicity" — it's "design the payment flow for a marketplace," and ACID is the lens you reason through. The interviewer isn't checking whether you memorized four letters. They're checking whether you know when correctness is worth its cost, and when it isn't.

The one-line idea: at the staff/principal level, ACID isn't a feature you switch on — it's a budget you allocate, spending strong guarantees where being wrong is expensive and relaxing them everywhere else. Every answer below is really an argument about where to spend.

We'll run ten questions as mock interviews. For each: what they're really asking, a model answer that builds intuition, and the senior line — the one sentence that signals you've actually shipped this.