Practice/Netflix/Auto-Expire Cache

Auto-Expire Cache

CodingMust

Problem Overview

Design and implement a key-value cache with automatic expiration. The system should allow storing key-value pairs that automatically expire after a specified duration.

This problem builds incrementally across multiple parts, with each part extending the previous functionality. It's commonly asked at Netflix as "Log Rate Limiter" and tests your understanding of caching strategies, memory management, and data structure design.

Important Notes

• There are multiple parts to this problem -- ask the interviewer how many parts there are to better manage your time
• Start with the simplest solution and discuss trade-offs before optimizing
• Each part addresses a specific concern: basic functionality, memory leaks, bounded capacity

Part 1: Basic Auto-Expire Cache

Implement an AutoExpireCache class with basic get/set operations and automatic expiration.

Example

` import time

cache = AutoExpireCache()

Set a value with 2 second TTL

cache.set("user_session", {"user_id": 123}, 2)

Immediately get - should return the value

print(cache.get("user_session")) # {"user_id": 123}

Wait for expiration

time.sleep(3)

After expiration - should return None

print(cache.get("user_session")) # None

Test with False as a valid value

cache.set("feature_flag", False, 60) result = cache.get("feature_flag") print(result) # False print(result is None) # False (it's actually the value False, not None) `

Requirements

• set(key, value, ttl_seconds): Store key-value pair that expires after TTL
• get(key): Return value if exists and not expired, None otherwise
• Handle any value type including False, None, and 0
• Overwrite existing keys with new value and TTL
• Use lazy deletion (clean up on access)

API Design Considerations

• Why return None for cache miss instead of False?

  • False can be a valid cached value
  • Could also raise KeyError, but None is more Pythonic

• Should set return anything?

  • Could return True/False for success
  • Keeping it simple (void) is usually fine

• Error handling for negative TTL or empty keys?