Implement a Recoverable In-Memory Key-Value Store

Problem Statement

Design a thread-safe, in-memory key-value store that supports basic get(key) and put(key, value) operations. To ensure durability, the system must append every put operation to a local file (an Write-Ahead Log or WAL) before modifying the in-memory state. Upon process restart, the system must reconstruct the latest state by replaying the log.

Requirements

1. Thread Safety: Multiple threads may call get and put simultaneously. get operations should return the latest committed value.
2. Persistence: Ensure that a put is safely flushed to disk. You must handle the scenario where the system crashes mid-write (torn writes).
3. Recovery: Implement a load() function that reads the WAL and populates the in-memory store.
4. Data Format: Keys and values are arbitrary byte arrays.

Interface

class PersistentKV:
    def __init__(self, log_path: str):
        """Initialize the store and recover from log_path if it exists."""

    def put(self, key: bytes, value: bytes) -> None:
        """Store key-value pair and ensure durability."""

    def get(self, key: bytes) -> bytes:
        """Retrieve the value associated with key."""

Example

If the log contains: [Length: 3][Key: 'foo'][Length: 3][Val: 'bar'] [Length: 3][Key: 'foo'][Length: 3][Val: 'baz']

After load(), get('foo') should return b'baz'.

Constraints

• The log file grows indefinitely; focus on the recovery logic first.
• Handle potential partial writes at the end of the file caused by a crash.