Implement Nested Transactions in an In-Memory Database

Problem Statement

Design an in-memory key-value store that supports nested transactions. Your database should support the following operations:

• SET(key, value): Stores a value for a key. If a transaction is active, this change is associated with the current transaction level.
• GET(key): Returns the value associated with the key. It should return the value from the most recent active transaction level; if not present, look in outer levels, and finally the base database.
• BEGIN(): Starts a new nested transaction level.
• ROLLBACK(): Discards all changes made in the most recent transaction level and returns to the previous level. If no transaction is active, return an error.
• COMMIT(): Persists all changes from the current transaction level to the parent level (or the base database if no parent exists).

Constraints

• Keys are strings; values are integers.
• The database must handle multiple nested levels (e.g., BEGIN called multiple times).

Example

# db = TransactionalDB()
db.SET("a", 10)
db.BEGIN()
db.SET("a", 20)
db.COMMIT()
print(db.GET("a")) # Output: 20

db.BEGIN()
db.SET("a", 30)
db.ROLLBACK()
print(db.GET("a")) # Output: 20

Implementation Signature

class TransactionalDB:
    def __init__(self): ...
    def set(self, key, value): ...
    def get(self, key): ...
    def begin(self): ...
    def rollback(self): ...
    def commit(self): ...