Why booking tests availability and concurrency in LLD - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to ensure the booking system checks availability before confirming.

LLD

if slot.is_available() and not slot.[1]():
    slot.book()

Drag options to blanks, or click blank then click option'

Ais_locked

Bis_full

Cis_reserved

Dis_open

Attempts:

3 left

💡 Hint

Common Mistakes

Using is_full instead of is_locked causes race conditions.

The slot must not be locked by another process to allow booking, so is_locked is the correct check.

2fill in blank

medium

Complete the code to lock the slot before booking to handle concurrency.

LLD

def book_slot(slot):
    if slot.[1]():
        slot.lock()
        slot.book()

Drag options to blanks, or click blank then click option'

Ais_locked

Bis_booked

Cis_reserved

Dis_available

Attempts:

3 left

💡 Hint

Common Mistakes

Locking without checking availability leads to errors.

We must check if the slot is available before locking it to prevent double booking.

3fill in blank

hard

Fix the error in the concurrency control to prevent double booking.

LLD

def book_slot(slot):
    if slot.is_available():
        slot.[1]()
        slot.book()

Drag options to blanks, or click blank then click option'

Areserve

Bunlock

Clock

Dcancel

Attempts:

3 left

💡 Hint

Common Mistakes

Unlocking before booking causes race conditions.

Locking the slot before booking prevents other processes from booking the same slot concurrently.

4fill in blank

hard

Fill both blanks to implement optimistic concurrency control in booking.

LLD

def book_slot(slot, version):
    if slot.version == [1]:
        slot.book()
        slot.version = [2]

Drag options to blanks, or click blank then click option'

Aversion

Bversion + 1

Cversion - 1

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Decrementing version causes incorrect concurrency control.

Check the current version matches, then increment version after booking to detect conflicts.

5fill in blank

hard

Fill all three blanks to implement a thread-safe booking with locking and availability check.

LLD

def safe_book(slot):
    with slot.[1]():
        if slot.[2]():
            slot.[3]()

Drag options to blanks, or click blank then click option'

Alock

Bis_available

Cbook

Dunlock

Attempts:

3 left

💡 Hint

Common Mistakes

Checking availability outside lock causes race conditions.

Use a lock context to ensure thread safety, check availability, then book the slot.

Practice

1. Why is it important to handle concurrency when booking test slots in a system?

easy

A. To allow unlimited bookings for the same slot

B. To slow down the booking process intentionally

C. To prevent multiple users from booking the same slot at the same time

D. To avoid showing available slots to users

Solution

1. Step 1: Understand concurrency in booking

   Concurrency means multiple users try to book the same slot simultaneously.

2. Step 2: Identify the problem caused by concurrency

   If concurrency is not handled, multiple users can book the same slot, causing double bookings.

3. Final Answer:

   To prevent multiple users from booking the same slot at the same time -> Option C

4. Quick Check:

   Concurrency handling = prevent double bookings [OK]

Hint: Concurrency means multiple users booking simultaneously [OK]

Common Mistakes:

• Thinking concurrency allows unlimited bookings
• Ignoring the need to prevent double bookings
• Assuming concurrency slows down the system intentionally

2. Which of the following is a correct way to ensure availability checks during booking in a system?

easy

A. Check slot availability after booking confirmation

B. Lock the slot before confirming the booking

C. Allow booking without checking availability

D. Ignore concurrency and rely on user honesty

Solution

1. Step 1: Understand locking in booking systems

   Locking a slot means reserving it temporarily to prevent others from booking it simultaneously.

2. Step 2: Identify when to check availability

   Availability must be checked and locked before confirming booking to avoid conflicts.

3. Final Answer:

   Lock the slot before confirming the booking -> Option B

4. Quick Check:

   Lock before confirm = correct availability check [OK]

Hint: Lock slot before booking to avoid conflicts [OK]

Common Mistakes:

• Checking availability after booking causes errors
• Ignoring availability checks leads to double bookings
• Relying on user honesty is not a system design

3. Consider this simplified booking flow code snippet:

def book_slot(slot_id):
    if is_available(slot_id):
        reserve(slot_id)
        confirm_booking(slot_id)
        return 'Booked'
    else:
        return 'Unavailable'

What issue can arise if two users call book_slot at the same time for the same slot_id?

medium

A. Both users might get 'Booked' causing double booking

B. The system crashes due to race condition

C. Both users get 'Unavailable' response

D. Only one user can call the function at a time automatically

Solution

1. Step 1: Analyze the code flow for concurrency

   Both users check availability before reservation without locking, so both may see the slot as available.

2. Step 2: Understand race condition effect

   Without locking, both reserve and confirm booking, causing double booking.

3. Final Answer:

   Both users might get 'Booked' causing double booking -> Option A

4. Quick Check:

   Race condition = double booking risk [OK]

Hint: Check-then-act without lock causes double booking [OK]

Common Mistakes:

• Assuming system crashes automatically
• Thinking both get 'Unavailable' response
• Believing function serializes calls automatically

4. In a booking system, the code uses a simple availability check without locking:

if check_availability(slot):
    book(slot)

Users report double bookings. What is the best fix?

medium

A. Add a lock or transaction around availability check and booking

B. Remove availability check to speed up booking

C. Increase server hardware to handle more requests

D. Notify users to book slower

Solution

1. Step 1: Identify the cause of double bookings

   Without locking, multiple users can pass availability check simultaneously causing conflicts.

2. Step 2: Apply concurrency control

   Using locks or transactions ensures only one booking proceeds at a time for the same slot.

3. Final Answer:

   Add a lock or transaction around availability check and booking -> Option A

4. Quick Check:

   Locking fixes concurrency issues [OK]

Hint: Use locks or transactions to fix concurrency bugs [OK]

Common Mistakes:

• Removing availability check causes more errors
• Upgrading hardware does not fix concurrency logic
• Telling users to slow down is not a system fix

5. You are designing a test booking system that must handle thousands of users booking slots concurrently. Which design approach best ensures availability and prevents double bookings?

hard

A. Show all slots as available and accept bookings first come, first served

B. Allow users to book without checks and fix conflicts later manually

C. Use a single global lock for all bookings to serialize requests

D. Use optimistic locking with retries and real-time slot availability updates

Solution

1. Step 1: Understand scalability needs

   Thousands of users require a scalable approach that avoids bottlenecks.

2. Step 2: Evaluate locking strategies

   Single global lock serializes all requests causing delays; manual fixes cause poor user experience.

3. Step 3: Choose optimistic locking with retries

   This approach allows concurrent attempts, detects conflicts, retries, and updates availability promptly.

4. Final Answer:

   Use optimistic locking with retries and real-time slot availability updates -> Option D

5. Quick Check:

   Optimistic locking + updates = scalable concurrency [OK]

Hint: Optimistic locking scales better than global locks [OK]

Common Mistakes:

• Using global lock causes slow system
• Ignoring concurrency leads to double bookings
• Manual conflict fixes harm user experience