System requirements

Functional:

• User can bid an item.
• User can see the current highest bid for any item.
• If there is no new bid for 1 hour, user with highest bid will get notification to submit payment within 10 min. If payment success, the user win the bid.

Non-Functional:

• Low latency: The bid price should be low latency to view and submit bid.
• High availability and Reliability: The auction system should be highly available to users.
• Security: The bid and payment from users information must be secure.

Capacity estimation

Assume daily active user is 10m.

Read QPS: 10m / 100k = 100

Peak read QPS: 100 * 2 = 200

Assume 10% user bid 1 item per day

Write QPS: 10m * 10% / 100k = 10

DB usage capacity:

Each bid may take up to 1kb to save.

Daily DB usage: 10m * 10% * 1kb = 1GB

Yearly DB usage: 1GB * 400 = 400GB

We need 3 replica. So yearly DB usage: 400GB * 3 = 1.2 TB

API design

REST api:

1. POST bidItem
2. PUT updateDid
3. GET getItemBid

WebSocket message:

1. From Client to server: subscribeToItem
2. From server to client: getItemBid

Database design

I choose SQL db.

The main schema of Item table:

id (primary key)

name

highestBidId (reference key to bid table)

The main schema of bid table:

id (primary key)

itemId (reference key to item table)

status (enum: bidWon, bidLoss, bidComplete, bidFail)

price

createdAt

updatedAt

High-level design

1. Client: Web and mobiles applications to view and submit the bid.
2. Load balance: Load balance the traffic by path based and round-robin approach.
3. API Gateway: Authentication, rate limiting, security and keep an WebSocket connection with client.
4. Bid read service: check and response the item highest bid from DB cache.
5. WebSocket session NoSQL DB: Store which API Gateway keep the webSocket connection with which client.
6. bid write service: Write the latest bid to SQL DB.
7. SQL DB: Source of truth for the item and bid information.
8. DB Cache: Cache layer of the SQL DB for fast reading.
9. Job Scheduler: Create jobs when bids expire. Trigger a notification to user if the bid won through notification service.
10. Notification service: Send notification to users through SMS, email and push notifications if users win bids.

Request flows

Detailed component design

Dig deeper into 2-3 components and explain in detail how they work. For example, how well does each component scale? Any relevant algorithm or data structure you like to use for a component? Also you could draw a diagram using the diagramming tool to enhance your design...

Trade offs/Tech choices

Explain any trade offs you have made and why you made certain tech choices...

Failure scenarios/bottlenecks

Try to discuss as many failure scenarios/bottlenecks as possible.

Future improvements

What are some future improvements you would make? How would you mitigate the failure scenario(s) you described above?