Design Proof of Delivery - System Design | WinJob

Problem Statement: Proof of Delivery at Carrier Scale

Design FedEx/UPS/Amazon-class proof of delivery: capture signatures and photos, prove drop-off with geofence and fraud checks, and expose tamper-evident records to merchants.

Staff candidates separate upload path from attestation commit with explicit idempotency.

Operational detail

Target upload ACK p95 <800ms on 4G, virus-scan completion p95 <12s, merchant webhook delivery p95 <3s after attestation commit. State AP for merchant dashboard cache and CP for billing-grade attestation row once hash chain is sealed.

Failure and edge cases

driver uploads POD while still 200m from geofence — quarantine until manual review

Interview checkpoints

javaOne Dark Pro

1 public record PodDedupeKey(String stopId, String clientUuid) {}
2 public String sealHash(String podId, String artifactHashes) {
3   return Hashing.sha256().hashString(podId + artifactHashes, UTF_8).toString();
4 }

pythonOne Dark Pro

1 def geofence_ok(point, polygon, buffer_m=25):
2     return distance_to_polygon_m(point, polygon) <= buffer_m
3  
4 def presign_ttl_seconds(service_level: str) -> int:
5     return 600 if service_level == "HIGH_VALUE" else 300

typescriptOne Dark Pro

1 export function podClientKey(stopId: string, clientUuid: string): string {
2   return `${stopId}:${clientUuid}`;
3 }
4 export function isReplay(status: number, existingPodId?: string): boolean {
5   return status === 409 && Boolean(existingPodId);
6 }

Why interviewers care

Proof of Delivery interviews reward crisp scope, explicit trade-offs, and failure stories—not generic microservice diagrams.

Interview checkpoint

Name one failure story for Problem Statement: Proof of Delivery at Carrier Scale that proves you understand real outages, not happy-path diagrams.

Key Highlights

•courier captures signature bitmap, geotagged photo, and barcode scan at the doorstep
•merchant and shipper need legally defensible delivery attestation within seconds
•disputes ('not received') require immutable artifacts, not mutable status columns
•FedEx/UPS/Amazon last-mile scale: millions of POD events during evening peaks

1	public record PodDedupeKey(String stopId, String clientUuid) {}
2	public String sealHash(String podId, String artifactHashes) {
3	return Hashing.sha256().hashString(podId + artifactHashes, UTF_8).toString();
4	}

1	def geofence_ok(point, polygon, buffer_m=25):
2	return distance_to_polygon_m(point, polygon) <= buffer_m
3
4	def presign_ttl_seconds(service_level: str) -> int:
5	return 600 if service_level == "HIGH_VALUE" else 300

1	export function podClientKey(stopId: string, clientUuid: string): string {
2	return `${stopId}:${clientUuid}`;
3	}
4	export function isReplay(status: number, existingPodId?: string): boolean {
5	return status === 409 && Boolean(existingPodId);
6	}