Design an EV Charging Station Locator & Booking System - System Design | WinJob

Problem Statement: EV Charging Locator and Booking

Phase understanding for EV Charging Station Locator & Booking (ChargePoint / PlugShare / Tesla discovery class).

Mechanism (1)

PlugShare-class locator aggregates 50k+ public sites across ChargePoint, EVgo, and proprietary feeds. Drivers need sub-200ms nearby search, honest connector-level availability, optional holds, and unified checkout when the CPO allows it.

Scenario anchor

Interviewers probe driver opens map on I-95 with 12% SoC and stale green pins—answer with data structures and SLO numbers, not vendor slogans.

Operational signals

Track nearby_search_p95_ms, connector_staleness_p95_sec, booking_hold_expired_rate, ingest_lag_sec, payment_capture_fail_rate, and tile_invalidation_lag_sec. Page when nearby_search_p95_ms > 250ms for 5 minutes or hold_expired_rate > 8%.

Language anchors

discovery read path owns catalog projections
booking command owns hold and checkout writes
geohash/H3 tiles partition map hot cells
freshness SLA with observed_at on every pin

javaOne Dark Pro

1 public record BookingHoldId(String value) { } // sec-001 locator

pythonOne Dark Pro

1 def rank_score(distance_m: float, price_cents: int, kw: int, wait_min: float) -> float:
2     return (kw * 10) - distance_m * 0.02 - price_cents * 0.01 - wait_min * 5  # sec-001

typescriptOne Dark Pro

1 export function isStale(observedAtMs: number, nowMs: number, slaSec: number): boolean {
2   return nowMs - observedAtMs > slaSec * 1000;  // sec-001
3 }

Why interviewers care

an EV Charging Station Locator & Booking System interviews reward crisp scope, explicit trade-offs, and failure stories—not generic microservice diagrams.

Interview checkpoint

Name one failure story for Problem Statement: EV Charging Locator and Booking that proves you understand real outages, not happy-path diagrams.

Key Highlights

•Anchor 1-A: discovery read path owns catalog projections
•Anchor 1-B: booking command owns hold and checkout writes
•Anchor 1-C: geohash/H3 tiles partition map hot cells
•Anchor 1-D: freshness SLA with observed_at on every pin

1	def rank_score(distance_m: float, price_cents: int, kw: int, wait_min: float) -> float:
2	return (kw * 10) - distance_m * 0.02 - price_cents * 0.01 - wait_min * 5 # sec-001

1	export function isStale(observedAtMs: number, nowMs: number, slaSec: number): boolean {
2	return nowMs - observedAtMs > slaSec * 1000; // sec-001
3	}