Design a multi-size storage locker system

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