Design a delivery cost aggregator

Q: Design a delivery cost aggregator

This is a System Design interview question from Rippling for Software Engineer roles. View the full question and solution on PracHub.

Q: How do I approach System Design interview questions?

System Design questions require understanding of core concepts and practice. PracHub provides solutions with explanations to help you master system design interviews.

Question

In-Memory Driver Payments Aggregator

Goal

Design and implement an in-memory component that tracks driver payments and exposes one metric: the total cost of all deliveries (for a live dashboard).

APIs to Provide

addDriver(driverId: int, usdHourlyRate: double)
recordDelivery(driverId: int, startTime, endTime)
getTotalCost(): double

Requirements and Assumptions

Drivers have different hourly rates.
Drivers may have overlapping deliveries and are paid for each delivery independently.
Each delivery is recorded immediately after completion and lasts no more than 3 hours.
Time fields must support at least 1-second precision.
getTotalCost returns the aggregate cost across all drivers for a live dashboard.
All inputs are valid; exact output formatting is not required.

Clarifications to Make Explicit

Time representation: Use UTC Unix epoch seconds (int64/long) for startTime and endTime to avoid timezone/DST issues and meet the "≥ 1-second precision" requirement.
Currency arithmetic: Avoid accumulating money in double to prevent rounding drift on the live dashboard.
Thread-safety: Assume the component may be called concurrently (dashboard + event ingestion). Make the implementation thread-safe.