Design a device owner lookup service

Q: Design a device owner lookup service

This question evaluates a candidate's understanding of system design concepts such as real-time event ingestion, handling out-of-order and duplicate events, scalable storage and indexing, and consistency and conflict-resolution trade-offs.

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Question

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Scenario

Your server receives a stream of events from clients. Each event contains:

user_id
device_id
timestamp (event time)

A device may be associated with different users over time (e.g., shared device, logout/login, resale).

Core requirement

Build a backend service that supports:

get_device_owner(device_id) -> user_id
Returns the current owner of the device.

Assume “current owner” means the user_id from the latest event by timestamp for that device_id.

Constraints / concerns to address

Events may arrive out of order .
Clients may retry → duplicate events .
Large scale (many devices; high write QPS; low-latency reads).
Define behavior for ties (same timestamp) and missing devices.

Deliverables

API design (ingestion + query)
Data model
Storage and indexing
How you handle out-of-order/duplicate events
Consistency guarantees and trade-offs
Monitoring and operational considerations

Design a device owner lookup service

Quick Overview

Scenario

Core requirement

Constraints / concerns to address

Deliverables

Solution

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