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System Design & Architecture Interview Question 1: Cap Partition Inventory

System Design & Architecture Interview Questions · Test 1 · question 1 of 10

CAP & Consistency Modelsmedium

A concert-ticketing service stores seat inventory in a database replicated across three sites. The business rule is absolute: a seat must never be sold to two people, and the business accepts that some customers will see booking errors if needed to guarantee that.

During a network partition, one site loses connectivity to the other two, but clients can still reach every site. How should the system behave while the partition lasts?

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Answer:

  • B. Keep accepting bookings only on the side that holds a majority of replicas, and have the isolated site reject booking writes until the partition heals

The CAP theorem says that when a network partition happens, a replicated system must choose between consistency (every read sees the latest write, even if requests have to block or fail) and availability (every non-failing node answers). "Never double-sell, even at the cost of errors" is an explicit choice of consistency over availability (CP). The usual way to get that is a majority quorum. The two connected sites form a majority and can keep committing, because any two majorities share at least one replica. The isolated site cannot tell whether the majority has already sold a seat, so it must refuse writes. That makes it unavailable, but it stays correct.

Option A is the availability-first (AP) choice. Both sides can sell the same seat during the partition, and "reconcile afterwards" means one customer's confirmed ticket gets cancelled. The stem forbids exactly that.

Option C makes things worse. With asynchronous replication, each site acknowledges a booking before the others know about it, so two sites can commit the same seat. It gives up consistency for latency, which is the opposite of the requirement.

Option D serves writes from stale state. The isolated site's copy of the inventory does not include sales the majority made after the partition began, so it will double-sell.

Reference: Relational vs. NoSQL data - CAP and PACELC theorems

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