Design a multi-size storage locker system
Company: Amazon
Role: Software Engineer
Category: System Design
Difficulty: medium
Interview Round: Onsite
Design a storage-locker system that supports **storing** and **retrieving** items when lockers come in multiple sizes.
## Scenario
A company operates self-service locker locations (like package lockers). Each location contains many individual locker compartments of different sizes (e.g., S/M/L, or dimension-based). Couriers arrive to deposit packages; customers later retrieve them.
## Functional requirements
1. **Store / check-in**: Given a package (dimensions or size class) and a target location, assign a locker and generate a pickup credential (PIN/QR/code).
2. **Optimal assignment**: Choose the **smallest** available locker that fits the package (minimize wasted space). If none fit, return an error.
3. **Retrieve / pickup**: Given a pickup credential at a location kiosk, open the correct locker door and mark the item as picked up.
4. **Expiration**: Packages expire after a configured time; expired packages may be removed by an operator.
5. **Operations/admin**: Mark lockers out-of-service; support manual overrides (e.g., door jam, lost code).
## Non-functional requirements
- Low latency at kiosk/courier check-in.
- Correctness under concurrency (multiple couriers arriving simultaneously).
- High availability (locations may have intermittent connectivity).
- Auditability and security (access logs).
## Deliverables
- APIs and data model
- Core services/components and how they interact
- Locker allocation strategy (including concurrency control)
- Storage/consistency choices, scaling strategy, and failure handling
Quick Answer: This question evaluates a candidate's system-design skills, including distributed systems architecture, resource allocation/bin-packing reasoning for multi-size lockers, concurrency control, availability and consistency trade-offs, API and data-model design, and operational concerns like auditing and failure handling.
