User Creation Endpoint: Implement and Validate a REST API Endpoint for User Registration

Question

## Problem

Implement a `POST /users` endpoint for user registration. The request body is JSON. Your implementation must:

1. Validate required fields: `email` (valid format), `password` (min 8 chars, at least one digit), `username` (alphanumeric, 3-20 chars).
2. Check that the email is not already registered (query a mock DB).
3. Hash the password before storing (do not store plaintext).
4. Return `201 Created` with `{"id": ..., "email": ..., "username": ...}` on success, or `400 Bad Request` with validation errors, or `409 Conflict` if email exists.

```python
from flask import Flask, request, jsonify
import hashlib, re

app = Flask(__name__)
USERS_DB = {}

@app.route('/users', methods=['POST'])
def create_user():
    data = request.get_json()
    # Validate, check duplicate, hash, store, respond
    ...
```

```
POST /users {"email":"a@b.com","password":"pass1234","username":"alice"}
-> 201 {"id":"uuid","email":"a@b.com","username":"alice"}

POST /users {"email":"a@b.com", ...}  (duplicate)
-> 409 {"error":"email already registered"}
```

## Follow-ups

1. `hashlib.md5` is not suitable for passwords. What should you use instead and why (bcrypt/argon2)?
2. How do you return multiple validation errors in one response rather than stopping at the first?
3. Rate-limit this endpoint to 5 requests per IP per minute. Where does that logic live?
4. How would you write an integration test for this endpoint that covers the happy path and both error cases?

User Creation Endpoint: Implement and Validate a REST API Endpoint for User Registration

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