Design a Memory Cache Database (OOD)

Problem

Design the object model behind HubSpot's incremental cache-database OA: a key-value store with TTL and transactions.

Requirements

• set/get/delete, TTL expiry
• begin/commit/rollback (nested)
• Optional: scan/filter by field

What evaluators look for

• Clean separation: store, expiry, transaction log
• Lazy vs active expiry
• Rollback via change-log stack

Extension

Add LRU eviction under a memory cap and discuss its interaction with TTL.