Design a Contact Tracing System: Track Exposure Chains in a Population Graph

Question

## Round 1 - System Design

## Problem

Design a contact tracing system used during a disease outbreak. The system must:

1. Record a contact event: `log_contact(person_a, person_b, timestamp, duration_minutes)` — bidirectional.
2. Report exposure risk: `get_exposed(person_id, days_back, exposure_depth)` — return all people who came in contact (directly or through a chain of depth `exposure_depth`) with `person_id` in the last `days_back` days.
3. Mark a person as confirmed positive: `report_positive(person_id, test_date)`.
4. Automatically generate notifications for anyone within exposure_depth=2 of a confirmed positive.

Walk through your data model, graph traversal strategy, and scalability concerns.

```
log_contact(A, B, t1, 30)
log_contact(B, C, t2, 15)
report_positive(A)
get_exposed(A, days_back=14, exposure_depth=2) -> {B, C}
```

## Follow-ups

1. The exposure graph could have millions of nodes. How do you make BFS/DFS efficient? What indexes does your DB need?
2. Privacy is critical — people's contact histories are sensitive. How do you design the system to minimize data retention and exposure?
3. A person is contacted as a potential exposure but later tests negative. How does your notification system handle false positives?
4. How would Bluetooth-based proximity detection (e.g., Apple/Google API) integrate with your backend contact log?

Design a Contact Tracing System: Track Exposure Chains in a Population Graph

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