Execute a TTL Key-Value Store with Transactions

Problem Statement

Design a simple in-memory key-value store that supports basic operations, TTL-based expiration, and a single active transaction. You must maintain a logical clock (time) that increments with each operation.

Requirements

1. SET(key, value, ttl): Sets a key to a value that expires at current_time + ttl.
2. GET(key): Returns the value if the key exists and has not expired; otherwise returns NULL.
3. DELETE(key): Removes the key.
4. BEGIN(): Starts a transaction. Only one transaction can be active at a time.
5. COMMIT(): Persists all changes made since BEGIN().
6. ROLLBACK(): Discards all changes made since BEGIN().
7. TICK(): Increments the logical clock by 1.

Transactions are isolated: changes made during a transaction are not visible to GET unless the current scope is within the transaction. If a SET or DELETE is called outside a transaction, it takes effect immediately.

Interface

class KVStore:
    def __init__(self):
        self.time = 0
        
    def tick(self): ...
    def set(self, key, value, ttl): ...
    def get(self, key): ...
    def delete(self, key): ...
    def begin(self): ...
    def commit(self): ...
    def rollback(self): ...

Constraints

• All keys and values are strings.
• Time is a non-negative integer.
• ttl is a positive integer indicating duration from the current time.

Example

kv = KVStore()
kv.set("a", "1", 10)
kv.tick()
kv.get("a") # returns "1"
kv.begin()
kv.set("a", "2", 5)
kv.get("a") # returns "2"
kv.rollback()
kv.get("a") # returns "1" (original value persists)