Thread safety in design in LLD - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a thread-safe counter using a lock.

LLD

class Counter:
    def __init__(self):
        self.value = 0
        self.lock = [1]

    def increment(self):
        with self.lock:
            self.value += 1

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Athreading.Event()

Bthreading.Condition()

Cthreading.Thread()

Dthreading.Lock()

Attempts:

3 left

💡 Hint

Common Mistakes

Using Thread instead of Lock causes errors because Thread is for creating threads, not synchronization.

Using Event or Condition without proper locking does not guarantee exclusive access.

Using threading.Lock() ensures that increments are thread-safe by allowing only one thread to modify the counter at a time.

2fill in blank

medium

Complete the code to safely read a shared variable with a read-write lock.

LLD

class SharedData:
    def __init__(self):
        self.data = 0
        self.rw_lock = [1]

    def read(self):
        self.rw_lock.acquire_read()
        value = self.data
        self.rw_lock.release_read()
        return value

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Athreading.Semaphore()

BReadWriteLock()

Cthreading.Lock()

Dthreading.RLock()

Attempts:

3 left

💡 Hint

Common Mistakes

Using a simple Lock blocks all threads, reducing concurrency.

RLock allows reentrant locking but does not differentiate read/write access.

A ReadWriteLock allows multiple readers or one writer at a time, making reads thread-safe without blocking each other.

3fill in blank

hard

Fix the error in the code to avoid race conditions when updating a shared list.

LLD

shared_list = []

lock = threading.Lock()

def add_item(item):
    [1]  # Fix: ensure thread safety
    shared_list.append(item)
    lock.release()

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Alock.acquire()

Block.release()

Clock.wait()

Dlock.notify()

Attempts:

3 left

💡 Hint

Common Mistakes

Calling release before acquire causes runtime errors.

Using wait or notify without proper locking does not prevent race conditions.

Acquiring the lock before modifying the shared list prevents race conditions by ensuring only one thread modifies the list at a time.

4fill in blank

hard

Fill both blanks to implement a thread-safe singleton pattern.

LLD

class Singleton:
    _instance = None
    _lock = [1]

    @classmethod
    def get_instance(cls):
        if cls._instance is None:
            with cls._lock:
                if cls._instance is None:
                    cls._instance = [2]  # Create instance
        return cls._instance

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Athreading.Lock()

Bthreading.Thread()

CSingleton()

Dobject()

Attempts:

3 left

💡 Hint

Common Mistakes

Using Thread instead of Lock causes synchronization issues.

Creating the instance with object() does not call the Singleton constructor.

Using threading.Lock() ensures only one thread creates the singleton instance. The instance is created by calling Singleton().

5fill in blank

hard

Fill all three blanks to implement a thread-safe producer-consumer queue.

LLD

import queue
import threading

class ProducerConsumer:
    def __init__(self):
        self.q = queue.Queue()
        self.lock = [1]
        self.not_empty = threading.Condition(self.lock)

    def produce(self, item):
        with self.lock:
            self.q.put(item)
            [2]  # Notify consumers

    def consume(self):
        with self.lock:
            while self.q.empty():
                [3]  # Wait for items
            return self.q.get()

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Athreading.Lock()

Bself.not_empty.notify()

Cself.not_empty.wait()

Dthreading.Event()

Attempts:

3 left

💡 Hint

Common Mistakes

Not using a lock causes race conditions.

Calling notify without holding the lock causes errors.

Waiting without a loop can cause missed signals.

The lock ensures mutual exclusion. notify() wakes waiting consumers when an item is produced. wait() blocks consumers until items are available.