Design Smart Locker System - System Design | WinJob

Problem Statement: Networked Smart Locker Banks

Mechanism (1)

Locker Command service owns reservation lifecycle; route geometry is a derived projection keyed by reservation_id.

Operational signals

Watch drop_success_rate, compartment_fault_rate, allocator_latency_p95, pickup_delay_after_drop, reservation_expire_churn. Page when compartment_fault_rate > 1.5% for 20 minutes in a locker bank.

Domain narrative

At dinner peak, locker bank SM-441 receives 1,200 inbound drop requests in ten minutes. The reservation scorer allocates compartment for parcel O-9182 (small parcel, 12-min parcel clock) with O-9201 (medium parcel, same locker bank LB-2204). Validator rejects when projected overflow walk exceeds 8 minutes or fit score crosses tier threshold—offer never surfaces to carrier driver C-5521.

Deep dive

Peak drop command rate ≈ 4,500/s globally with partition key locker_bank_id. Never let analytics consumers mutate reservation state; only Locker Command commits transitions. Customer-facing ETA must recompute on every reservation mutation, including silent reservation release after drop.

javaOne Dark Pro

1 public final class LockerCommand1 {
2   private final UUID reservationId;
3   private final long expectedVersion;
4   private final String lockerBankId;
5   private final int maxOverflowWalkMinutes;
6   public UUID reservationId() { return reservationId; }
7 }

pythonOne Dark Pro

1 @dataclass(frozen=True)
2 class LockerCommand1:
3     reservation_id: str
4     expected_version: int
5     locker_bank_id: str
6     max_overflow_walk_minutes: int

typescriptOne Dark Pro

1 interface LockerCommand1 {
2   reservationId: string;
3   expectedVersion: number;
4   lockerBankId: string;
5   maxOverflowWalkMinutes: number;
6 }

Why interviewers care

Smart Locker System interviews reward crisp scope, explicit trade-offs, and failure stories—not generic microservice diagrams.

Interview checkpoint

Name one failure story for Problem Statement: Networked Smart Locker Banks that proves you understand real outages, not happy-path diagrams.

Key Highlights

•Anchor 1-A: locker command owns writes
•Anchor 1-B: bank-local compartment scoring
•Anchor 1-C: compartment fit + dwell-time caps
•Anchor 1-D: honest ETA on mutation

1	public final class LockerCommand1 {
2	private final UUID reservationId;
3	private final long expectedVersion;
4	private final String lockerBankId;
5	private final int maxOverflowWalkMinutes;
6	public UUID reservationId() { return reservationId; }
7	}

1	@dataclass(frozen=True)
2	class LockerCommand1:
3	reservation_id: str
4	expected_version: int
5	locker_bank_id: str
6	max_overflow_walk_minutes: int

1	interface LockerCommand1 {
2	reservationId: string;
3	expectedVersion: number;
4	lockerBankId: string;
5	maxOverflowWalkMinutes: number;
6	}