System Design: E-Commerce Inventory & Order Processing

Interview Summary

Problem type

Amazon Prime Day Order Processing System

Interview question

Design a system that handles over 1 million orders per minute during a peak shopping event. Each order must reserve inventory, charge the customer, and confirm the order — all while preventing overselling, avoiding duplicate charges, and maintaining real-time inventory counts visible to other buyers. The core challenge is managing high-contention inventory updates at a massive global scale while keeping the user experience fast and accurate.

Interview Feedback

Feedback about Dialectic Thunderstorm (the interviewee)

• Advance this person to the next round? No
• How were their technical skills? 3/4
• How was their problem-solving ability? 3/4
• What about their communication ability? 2/4

You seem to have the technical skills, but have some trouble communicating them. I recommend more practice for a real interview. Here are some of the things we talked about today. Ask questions to understand the scope of the question. You will always get half a scenario; they expect you to ask questions. You want to understand which services are provided to you vs. what you are building. That will help you understand what is important.

Interview Transcript

Declarative Dragon: Hello. Hello. How are you doing today?
Dialectic Thunderstorm: I'm doing very well. How are you?
Declarative Dragon: I'm good. I'm just opening up my notes. Um, so today is— it looks like you were looking for a more of a mentoring session rather than just like a straight interview.
Dialectic Thunderstorm: Yeah, I'm— what I was thinking was, you know, I wanted to somewhat mirror an interview environment, but in my opinion, I'm not super ready and this is my first one of these. So I'm really just hoping, like, I want you to challenge me and, you know, play devil's advocate like you would normally, but I also might just need a little bit of help.

Declarative Dragon: Sure. Have you— do you have a system design interview coming up?
Dialectic Thunderstorm: I do. I am interviewing with [REDACTED] with their system design interview on Wednesday.

Declarative Dragon: Okay. So do you have any— I like to ask just in case people do. Some people come with questions. If you don't have one that you'd like to practice with, I have a whole bunch that I can grab. I'm thinking this one. Okay, I have one. Would you like to just get started on an interview question and then we can kind of see how it goes?
Dialectic Thunderstorm: Yeah, I think that sounds good.
Declarative Dragon: Okay. And then I think because this is more of a coaching session and your first one, I will do the feedback kind of like during the interview as opposed to like normally I like do at the end. I think maybe for this one we'll do it in between and then for your next one maybe just do like a straight interview like it's the real thing.

Dialectic Thunderstorm: Yeah, that's my plan. I think that sounds good.

Declarative Dragon: Okay, great. Let's get started.

Dialectic Thunderstorm: Okay, so I guess I'm just going to use this plain text to put down my initial thoughts and then bring up the whiteboard once we get to the high-level design.

Declarative Dragon: Okay.

Dialectic Thunderstorm: Okay, so I— the, the first thing I want to lay out is the functional requirements. And, okay, so the main thing that we want to do is allow, we'll say customers can view items in the inventory along with, I guess, price, but more importantly availability. And then we also want to ensure that customers can reserve items. I, in other words, put them in their cart. And I think this is probably something we want to list as part of this functional requirement, but when they reserve an item, that item's availability reflects the reservation for other customers. And then that is probably kind of going to be sort of in a temporary phase where the item is reserved. So when a customer checks out, the inventory is updated to reflect it.

Declarative Dragon: Yeah. So my initial feedback is that every question you get is going to be half a question, right?
If you've watched Great British Bake Off, I always make it— I use that metaphor because it's like the second challenge they do where you get half a recipe and have to do it. So the point of that is that you ask questions, and especially for a big company like Amazon, you're going to be able to use services that already exist to take care of things that aren't the most important part of the question.

Dialectic Thunderstorm: Yeah, that makes sense.

Declarative Dragon: So does that, does that question make sense? Basically just a shopping cart for a very busy shopping day.

Dialectic Thunderstorm: It does. It pretty closely aligns with the kinds of things I've been studying for this. So I think it's perfect.

Declarative Dragon: Okay, great. Let's get started.

Key Points in Interview

• Functional Requirements: Key elements include:

  • Viewing items with prices and availability.
  • Reservation of items in the cart.

• Non-Functional Requirements:

  • Consistency should be prioritized over availability to avoid overselling.

• Checkout and Updates:

  • Checks on inventory before confirming orders.

Conclusion

This session highlighted many necessary factors in system design, from functional and non-functional requirements to handling failure modes and making critical design decisions, especially in a high-volume context like Amazon's Prime Day.