Design locker allocation service
Overview
This question evaluates a candidate's understanding of stateful system design, resource allocation strategies, concurrency control, API and data modeling, and fault tolerance in the context of real-time locker assignment.
Amazon
Sep 6, 2025, 12:00 AM
Software Engineer
Technical Screen
System Design
0
0
Design a Best-Fit Locker Assignment Component
Context
You are designing the locker-assignment component for a Locker OS used at physical locker banks (one or more locations). Lockers and packages have sizes: Small (S), Medium (M), Large (L). Each locker holds at most one package. Couriers arrive with packages and need to be assigned the best-fitting LockerID quickly and safely under concurrency.
Assume:
- The component may run per location (preferred) or centrally with per-location sharding.
- Locker devices report available/occupied states; the service is the source of truth for assignment decisions.
Requirements
Design a component that:
- Returns the best-fitting locker for a given package size.
- Defines a clear "best fit" policy (exact match preferred; upsizing rules and priorities).
-
Exposes core operations:
- Register lockers
- Mark locker available/occupied/broken
- Assign locker to package (for courier)
- Release on pickup
- Query status and counts
- Specifies data structures and time/space complexity.
- Addresses concurrency (simultaneous couriers), idempotency and retries, race conditions, fairness/starvation.
- Describes persistence and failure recovery.
- Provides APIs, schema, and pseudocode for assign(packageSize) and release(lockerId).
- Considers extensions: reservations, per-location sharding, priorities/SLA tiers, and scaling.
Solution
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