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Common Problems
Inventory Management
Understanding the Problem
📦 What is an Inventory Management System?
An inventory management system tracks product stock across multiple warehouse locations. When inventory arrives, the system records it. When orders ship, the system deducts stock. The system can also transfer inventory between locations and alert managers when stock runs low.
Requirements
When the interview starts, you'll get something like this:
"Design an inventory management system that tracks products across multiple warehouses. The system needs to handle adding and removing inventory, transferring stock between locations, and alerting when inventory runs low."
That's a start, but there's a lot of room for interpretation. Before touching the whiteboard, spend a few minutes clarifying what you're actually building.
Clarifying Questions
Structure your questions around four themes: what operations does the system support, what can go wrong, what's in scope, and what might we extend later?
Here's how the conversation might go:
You: "When you say 'multiple warehouses,' are we talking about a fixed set of warehouses configured at startup, or can warehouses be added dynamically?"Interviewer: "Keep it simple. Fixed set of warehouses. You can assume they're configured when the system initializes."
Good. We're not building warehouse lifecycle management.
You: "For the low-stock alerts you mentioned, how do those work? Do we configure a threshold per product, or is it more granular?"Interviewer: "It should be per product per warehouse. Different warehouses might need different thresholds for the same product. When stock drops below the threshold, trigger a notification."
That's important. Alerts are warehouse-specific, not global. A product could be low in Warehouse A but fine in Warehouse B.
You: "How should the notification happen? Are we sending emails, calling a webhook, or just returning something to the caller?"Interviewer: "Good question. Let's keep it pluggable. The system should call some callback interface when stock is low. What happens after that - email, webhook, logging - is someone else's problem."
Perfect. We're building the alert mechanism, not the notification delivery.
You: "What about invalid operations? Should we allow negative inventory, or reject operations that would take stock below zero?"Interviewer: "Reject them. If someone tries to remove 100 units but we only have 50, that should fail. Same with transfers, validate before moving anything."
The system enforces invariants. Stock can't go negative.
You: "What about concurrent access? If two processes are trying to modify the same warehouse's inventory at the same time, do we need to handle that?"Interviewer: "Yes, concurrency is important here. Multiple operations could be happening simultaneously - one warehouse receiving a shipment while another is fulfilling an order for the same product. Make sure operations are thread-safe."
Good. We need to think about synchronization from the start.
You: "Last one. What's out of scope? Are we managing product catalogs, handling orders, that kind of thing?"Interviewer: "Yes. Products exist externally. Orders and payments are handled upstream. Keep the focus on the inventory tracking logic."
Final Requirements
After that back-and-forth, you'd write this on the whiteboard:
Final Requirements
Requirements:
1. Track inventory for products across multiple warehouses
2. Add stock to a specific warehouse (receiving shipments)
3. Remove stock from a specific warehouse (fulfilling orders)
4. Check availability: given a product and quantity, return which warehouses can fulfill it
5. Transfer stock between warehouses
6. Low-stock alerts
7. Reject operations that would result in negative inventory
8. System must be thread-safe to handle concurrent operations
Out of Scope:
- Product catalog management (products exist externally)
- Order processing / payment / serviceability
- PersistencePerfect. We've scoped out the problem and have concrete requirements to work from. The next step is identifying what objects we need to build this system.
Core Entities and Relationships
Now that the requirements are clear, we need to figure out what objects make up the system. The trick is scanning the requirements for nouns that represent things with behavior or state. We start by considering each of the nouns in our requirements as candidates, pruning until we have a list of entities that make sense to model.
Class Design
InventoryManager
Warehouse
AlertConfig
AlertListener
Final Class Design
Implementation
InventoryManager
Warehouse
AlertConfig
AlertListener
Complete Code Implementation
Verification
Test Case 1: Alert threshold crossing behavior
Test Case 2: Atomic transfer with proper locking
Test Case 3: Validation prevents negative inventory
Extensibility
"How do you prevent overselling when orders are in progress?"
"How would you handle inventory that's being shipped between warehouses?"
What is Expected at Each Level?
Junior
Mid-level
Senior
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On This Page
Understanding the Problem
Requirements
Clarifying Questions
Final Requirements
Core Entities and Relationships
Class Design
InventoryManager
Warehouse
AlertConfig
AlertListener
Final Class Design
Implementation
InventoryManager
Warehouse
AlertConfig
AlertListener
Complete Code Implementation
Verification
Extensibility
"How do you prevent overselling when orders are in progress?"
"How would you handle inventory that's being shipped between warehouses?"
What is Expected at Each Level?
Junior
Mid-level
Senior
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