Design Contact Tracing - System Design | WinJob

Problem statement: privacy-preserving exposure notification

National-scale BLE proximity logging without collecting identity or GPS.

Numbers to state early

Metric A: 180M installed apps
Metric B: 14-day TEK window
Metric C: 2m RSSI threshold

Mechanism

Frame the product as decentralized matching: phones hold encounter logs locally; the server only distributes diagnosis keys after voluntary positive report. Never promise real-time location tracking—that violates the Apple/Google privacy model interviewers expect.

Failure and edge cases

False positives when RSSI threshold is too loose in crowded transit
Clock skew breaking TEK day boundaries across time zones
Orphaned diagnosis keys if upload succeeds but CDN purge fails

When discussing Problem statement: privacy-preserving exposure notification, never place a central encounter warehouse—phones match TEKs locally. Cite 180M installed apps before drawing components. Flow emphasis: Citizen phone → BLE scanner → Exposure server.

Design pressure specific to exposure notification

Operators running BLE scanner under this section must honor opt-out wipes, 14-day TTL eviction, and signed bundle freshness. Battery caps on 2m RSSI threshold drive duty-cycle tuning.

Java

javaOne Dark Pro

1 public final class RollingProximityId {
2   private final byte[] rpid;
3   private final Instant intervalStart;
4   private final int intervalNumber;
5  
6   public boolean matchesInterval(Instant encounterAt, Duration tekPeriod) {
7     long day = encounterAt.getEpochSecond() / tekPeriod.getSeconds();
8     return day == intervalNumber;
9   }
10 }

Python

pythonOne Dark Pro

1 @dataclass(frozen=True)
2 class RollingProximityId:
3     rpid: bytes
4     interval_start: datetime
5     interval_number: int
6  
7     def matches_interval(self, encounter_at: datetime, tek_period_hours: int = 24) -> bool:
8         day = int(encounter_at.timestamp() // (tek_period_hours * 3600))
9         return day == self.interval_number

TypeScript

typescriptOne Dark Pro

1 export interface RollingProximityId {
2   rpid: Uint8Array;
3   intervalStart: Date;
4   intervalNumber: number;
5 }
6  
7 export function matchesInterval(
8   id: RollingProximityId,
9   encounterAt: Date,
10   tekPeriodHours = 24,
11 ): boolean {
12   const day = Math.floor(encounterAt.getTime() / (tekPeriodHours * 3600 * 1000));
13   return day === id.intervalNumber;
14 }

Why interviewers care

Contact Tracing interviews reward crisp scope, explicit trade-offs, and failure stories—not generic microservice diagrams.

Interview checkpoint

Name one failure story for Problem statement: privacy-preserving exposure notification that proves you understand real outages, not happy-path diagrams.

Key Highlights

•180M installed apps
•Citizen phone → BLE scanner → Exposure server
•National-scale BLE proximity logging without collecting identity or GPS.

1	public final class RollingProximityId {
2	private final byte[] rpid;
3	private final Instant intervalStart;
4	private final int intervalNumber;
5
6	public boolean matchesInterval(Instant encounterAt, Duration tekPeriod) {
7	long day = encounterAt.getEpochSecond() / tekPeriod.getSeconds();
8	return day == intervalNumber;
9	}
10	}

1	@dataclass(frozen=True)
2	class RollingProximityId:
3	rpid: bytes
4	interval_start: datetime
5	interval_number: int
6
7	def matches_interval(self, encounter_at: datetime, tek_period_hours: int = 24) -> bool:
8	day = int(encounter_at.timestamp() // (tek_period_hours * 3600))
9	return day == self.interval_number

1	export interface RollingProximityId {
2	rpid: Uint8Array;
3	intervalStart: Date;
4	intervalNumber: number;
5	}
6
7	export function matchesInterval(
8	id: RollingProximityId,
9	encounterAt: Date,
10	tekPeriodHours = 24,
11	): boolean {
12	const day = Math.floor(encounterAt.getTime() / (tekPeriodHours * 3600 * 1000));
13	return day === id.intervalNumber;
14	}