Reservation and hold system in LLD - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define the method that reserves an item.

LLD

def reserve_item(self, item_id):
    if self.inventory.[1](item_id):
        self.inventory.remove(item_id)
        return True
    return False

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

Ais_available

Bcheck_availability

Chas_item

Dcontains

Attempts:

3 left

💡 Hint

Common Mistakes

Using a method that removes the item instead of checking availability.

The method is_available checks if the item is free to reserve.

2fill in blank

medium

Complete the code to hold an item for a user.

LLD

def hold_item(self, item_id, user_id):
    if self.inventory.[1](item_id):
        self.holds[item_id] = user_id
        return True
    return False

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

Ais_reserved

Bis_locked

Cis_held

Dis_available

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to hold an item that is already reserved or held.

The method is_available ensures the item is free before holding it.

3fill in blank

hard

Fix the error in the method that releases a hold on an item.

LLD

def release_hold(self, item_id):
    if item_id in self.holds:
        [1] self.holds[item_id]
        return True
    return False

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

Adelete

Bpop

Cremove

Ddiscard

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'remove' which is a list method, not dictionary.

The pop method removes the key and returns its value safely.

4fill in blank

hard

Fill both blanks to create a dictionary comprehension that maps item IDs to user IDs for all held items.

LLD

held_items = { [1]: [2] for [1], [2] in self.holds.items() }

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

Aitem_id

Buser_id

Ckey

Dvalue

Attempts:

3 left

💡 Hint

Common Mistakes

Using the same variable name for both key and value.

Use item_id and user_id as variable names for keys and values.

5fill in blank

hard

Fill all three blanks to filter held items where the user ID matches and the hold is still valid.

LLD

valid_holds = { [1]: [2] for [1], [2] in self.holds.items() if self.is_hold_valid([3]) }

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

Aitem_id

Buser_id

Dhold_id

Attempts:

3 left

💡 Hint

Common Mistakes

Passing the wrong variable to the validity check function.

The comprehension uses item_id and user_id for keys and values, and checks validity by passing item_id.

Practice

1. What is the primary purpose of a hold in a reservation and hold system?

easy

A. To delete all reservations from the system

B. To permanently reserve a resource without expiration

C. To cancel a confirmed reservation immediately

D. To temporarily block a resource before final booking

Solution

1. Step 1: Understand the role of a hold

   A hold temporarily blocks a resource to prevent others from booking it while the user decides.

2. Step 2: Differentiate hold from reservation

   A reservation is permanent until canceled, while a hold expires if not confirmed.

3. Final Answer:

   To temporarily block a resource before final booking -> Option D

4. Quick Check:

   Hold = Temporary block [OK]

Hint: Holds are temporary blocks, not permanent reservations [OK]

Common Mistakes:

• Confusing hold with permanent reservation
• Thinking holds never expire
• Assuming holds cancel reservations

2. Which data structure is best suited to track holds with expiration times efficiently?

easy

A. Simple array without ordering

B. Linked list without timestamps

C. Hash map with timestamps and a priority queue for expirations

D. Stack data structure

Solution

1. Step 1: Identify requirements for hold tracking

   We need fast lookup by hold ID and efficient expiration handling.

2. Step 2: Choose data structures

   A hash map allows quick hold lookup; a priority queue orders holds by expiration for timely removal.

3. Final Answer:

   Hash map with timestamps and a priority queue for expirations -> Option C

4. Quick Check:

   Hash map + priority queue = efficient hold tracking [OK]

Hint: Use hash map for lookup and priority queue for expirations [OK]

Common Mistakes:

• Using unordered arrays causing slow expiration checks
• Choosing stack which is LIFO, not suitable for expirations
• Ignoring timestamps in data structure

3. Consider this pseudo-code for confirming a hold:

if hold.exists(hold_id) and not hold.is_expired(hold_id):
    reservation.create(hold.resource)
    hold.remove(hold_id)
    return "Confirmed"
else:
    return "Failed"

What will be the output if the hold has expired?

medium

A. "Failed"

B. "Confirmed"

C. Error due to missing hold

D. "Confirmed" but resource is double booked

Solution

1. Step 1: Check hold existence and expiration

   The code confirms only if hold exists and is not expired.

2. Step 2: Analyze expired hold case

   If hold is expired, condition fails and returns "Failed" without creating reservation.

3. Final Answer:

   "Failed" -> Option A

4. Quick Check:

   Expired hold = "Failed" confirmation [OK]

Hint: Expired holds cause confirmation to fail [OK]

Common Mistakes:

• Assuming expired holds confirm successfully
• Expecting errors instead of failure message
• Ignoring hold expiration check

4. A developer wrote this code to release expired holds:

for hold in holds:
    if hold.expiration_time < current_time:
        holds.remove(hold)

What is the main issue with this code?

medium

A. Holds should not be removed, only marked expired

B. Modifying a list while iterating causes skipped elements or errors

C. Expiration time comparison is incorrect

D. Loop should use while instead of for

Solution

1. Step 1: Understand iteration and modification

   Removing items from a list while iterating over it causes skipping or runtime errors.

2. Step 2: Identify correct approach

   Use a separate list to collect expired holds or iterate over a copy to safely remove.

3. Final Answer:

   Modifying a list while iterating causes skipped elements or errors -> Option B

4. Quick Check:

   Remove during iteration = skipped elements [OK]

Hint: Never remove items from list while looping over it [OK]

Common Mistakes:

• Ignoring iteration modification side effects
• Assuming expiration comparison is wrong
• Thinking loop type causes the issue

5. You need to design a scalable reservation and hold system for a popular event with thousands of simultaneous users. Which approach best ensures no double booking and timely hold expiration?

hard

A. Use distributed locks on resources, store holds with TTL in a distributed cache, and confirm with atomic transactions

B. Store all holds in a single database table without expiration, confirm by updating status

C. Allow multiple holds per resource and resolve conflicts manually later

D. Use client-side timers to expire holds and update server asynchronously

Solution

1. Step 1: Prevent double booking with distributed locks

   Distributed locks ensure only one user can hold a resource at a time across servers.

2. Step 2: Use TTL in distributed cache for hold expiration

   TTL automatically expires holds after timeout, preventing indefinite blocking.

3. Step 3: Confirm holds atomically

   Atomic transactions guarantee reservation creation without race conditions.

4. Final Answer:

   Use distributed locks on resources, store holds with TTL in a distributed cache, and confirm with atomic transactions -> Option A

5. Quick Check:

   Distributed locks + TTL + atomic confirm = scalable, safe system [OK]

Hint: Combine distributed locks, TTL cache, and atomic confirm for scale [OK]

Common Mistakes:

• Ignoring concurrency causing double booking
• Relying on client-side expiration only
• Not using atomic operations for confirmation