E-Commerce Inventory & Order Processing: Interview with a FAANG Engineer

System Design: E-Commerce Inventory & Order Processing

Watch someone solve the amazon prime day order processing system problem in an interview with a FAANG engineer and see the feedback their interviewer left them. Explore this problem and others in our library of interview replays.

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 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...

Feedback about Declarative Dragon (the interviewer)
Would you want to work with this person? Yes
How excited would you be to work with them? 4/4
How good were the questions? 4/4
How helpful was your interviewer in guiding you to the solution(s)? 4/4

My interviewer could not have been more helpful. The advice was awesome.

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...
Dialectic Thunderstorm: ...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.
Declarative Dragon: Sure. ...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, great. All right, so we're gonna do an Amazon question. Okay. So on Prime Day, Amazon processes over a million orders per minute. Each order must automatically reserve inventory, charge the customer, and confirm the order, all while preventing overselling, which is selling more units than exist, avoiding duplicate charges, and maintaining a real-time count visible to other buyers? ...this is similar to many systems design questions you get where the actual action is pretty simple, right?
Dialectic Thunderstorm: Yeah, that makes sense.

Declarative Dragon: So my initial feedback is that every question you get is going to be half a question... So from your reading of the question, I think you got it right where you were like, the inventory is kind of what we're looking to handle here.
Dialectic Thunderstorm: Okay, okay. So yeah, it sounds like our real concern here is that inventory contention and managing, uh, transactions when a customer goes to reserve and check out.

Declarative Dragon: ...So, my feedback is... your workflow diagram should reflect all the bits and pieces, you know?
Dialectic Thunderstorm: Okay, that makes sense.

Declarative Dragon: So you want to ensure there's consistency over availability.
Dialectic Thunderstorm: Sounds good.

Declarative Dragon: ...Let’s think about that in the context of failure modes, avoiding contention issues, especially for high-traffic scenarios.
Dialectic Thunderstorm: Right.

Declarative Dragon: ...So the point of that is that you ask questions, and especially for a big company like Amazon, you're gonna be able to use services that already exist to handle questions you don't want to build from scratch.

Dialectic Thunderstorm: Okay.

Declarative Dragon: We want to understand that we have specific inventory and management procedures across multiple regions to ensure that orders are processed effectively.

Dialectic Thunderstorm: Right.

Declarative Dragon: ...Make decisions and then changing them if they are wrong is a good judgment skill to have.

Non-Functional Requirements

  1. Consistency Over Availability: Deciding that for high contention scenarios, we want to ensure the consistency of inventory checks without overselling.
  2. Scalability: The system should handle 1 million orders per minute effectively by scaling across global regions.
  3. Resilience: In case of a service failure, fallback protocols should be established to maintain transaction integrity.

Modeling Requirements

  1. Order Model: To represent user orders, each record contains:

    • userID
    • productID
    • quantityRequested
    • quantityFulfilled
    • status (in-cart, in checkout, completed)

  2. Inventory Model: To represent available products in the system:

    • productID
    • totalAvailable
    • totalReserved

The primary goal is to ensure orders can be placed efficiently while managing high contention for inventory items—giving customer-facing assurance of product availability and swift transaction processing.

Conclusion

In situations where inventory services are down or when handling myriad requests concurrently, leveraging messaging systems can be vastly beneficial.
...This is important for providing a seamless transactions experience for customers while ensuring the organization can scale effectively.