Senior Backend Developer

Sam want to read exactly ‘TARGET’ number of pages.

He has an array ‘BOOK’ containing the number of pages for ‘N’ books.

Return YES/NO, if it is possible for him to read any 2 books and he can meet his ‘TARGET’ number of pages.

Example:

Input: ‘N’ = 5, ‘TARGET’ = 5
‘BOOK’ = [4, 1, 2, 3, 1] 

Output: YES
Explanation:
Sam can buy 4 pages book and 1 page book.

Design a URL-shortener service like Bitly. Explain the system architecture, database design, API structure, hashing/encoding logic, redirection mechanism, and how you would handle scalability (millions of requests), caching, and rate limiting.

Design a Booking API for a travel/hotel system. Explain endpoints such as search rooms, select rate, lock rate, payment, and confirmation. Also describe how you would ensure idempotency, handle concurrency, and prevent duplicate bookings.

A Django view is taking too long to respond, and you need to identify the performance bottlenecks. Explain how you would debug:

• N+1 queries
• Missing indexes
• Inefficient queryset filtering
• Heavy serialization
• Using Django Debug Toolbar and query logging

Write a function that retries a failed API call up to 3 times with exponential backoff.
If it still fails, return an error response.
Discuss how to handle exceptions, timeouts, and logging.

Design a JWT-based authentication system for a Django REST API.
Explain:

• Access Token
• Refresh Token
• Token Expiry
• Blacklisting
• Security Best Practices (token rotation, short expiration, HTTPS)

Two users are trying to book the last available room at the same time.
Explain how you would handle this concurrency issue at the backend:

• Database transactions
• Row-level locking
• Optimistic vs. pessimistic locking
• Idempotency keys
• Queue-based reservation system