Availability checking in LLD - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is availability checking in system design?

Availability checking is the process of verifying if a resource, service, or system component is ready and able to handle requests or tasks at a given time.

beginner

Why is availability checking important in distributed systems?

It ensures that requests are sent only to components that are up and responsive, improving reliability and user experience by avoiding failures or delays.

intermediate

Name two common methods used for availability checking.

1. Heartbeat signals: periodic messages to confirm a component is alive.
2. Health checks: active tests like ping or API calls to verify responsiveness.

beginner

What is a heartbeat in availability checking?

A heartbeat is a regular signal sent by a system component to indicate it is operational and reachable.

intermediate

How does availability checking help in load balancing?

Load balancers use availability checks to route traffic only to healthy servers, preventing requests from going to down or overloaded machines.

What does availability checking verify in a system?

A If a component is cost-effective

B If a component has enough storage

C If a component is secure

D If a component is ready to handle requests

Availability checking confirms readiness to handle requests, not storage, security, or cost.

Which method is commonly used for availability checking?

A Data compression

B Encryption

C Heartbeat signals

D Caching

Heartbeat signals are used to confirm if a component is alive and responsive.

How does availability checking improve user experience?

A By avoiding requests to down components

B By increasing data storage

C By reducing network speed

D By encrypting data

It prevents sending requests to unavailable components, reducing errors and delays.

What role does availability checking play in load balancing?

A Increases server storage

B Routes traffic to healthy servers only

C Encrypts server data

D Compresses server responses

Load balancers use availability checks to send traffic only to servers that are up and healthy.

Which of these is NOT a typical availability check?

A User authentication

B Heartbeat signal

C Health check API call

D Ping test

User authentication is unrelated to availability checking.

Explain availability checking and why it is critical in system design.

Think about how systems know if parts are working before sending requests.

Describe how availability checking integrates with load balancing.

Consider how traffic is distributed only to servers that respond well.

Practice

1. What is the main purpose of availability checking in system design?

easy

A. To create backups of system data

B. To increase the speed of data processing

C. To encrypt user data for security

D. To determine if a resource is free or ready to use

Solution

1. Step 1: Understand the concept of availability checking

   Availability checking is about verifying if a resource like a room, item, or slot is free to be used or booked.

2. Step 2: Identify the main goal

   The main goal is to know if the resource is ready or free, not about speed, security, or backups.

3. Final Answer:

   To determine if a resource is free or ready to use -> Option D

4. Quick Check:

   Availability checking = resource readiness [OK]

Hint: Availability checking means resource is free or not [OK]

Common Mistakes:

• Confusing availability with performance optimization
• Mixing availability with security features
• Thinking availability means data backup

2. Which of the following code snippets correctly checks if a room is available given a list of booked rooms booked_rooms = [101, 102, 103] and a requested room requested_room = 104?

easy

A. if requested_room in booked_rooms:
print('Available')

B. if requested_room == booked_rooms:
print('Available')

C. if requested_room not in booked_rooms:
print('Available')

D. if requested_room > booked_rooms:
print('Available')

Solution

1. Step 1: Understand the list and requested room

   booked_rooms contains rooms already taken: 101, 102, 103. requested_room is 104.

2. Step 2: Check correct condition for availability

   Room is available if requested_room is NOT in booked_rooms. So, 'if requested_room not in booked_rooms' is correct.

3. Final Answer:

   if requested_room not in booked_rooms: print('Available') -> Option C

4. Quick Check:

   Not in booked_rooms means available [OK]

Hint: Check 'not in' to confirm availability [OK]

Common Mistakes:

• Using 'in' instead of 'not in' to check availability
• Comparing equality of a number to a list
• Using greater than operator on list

3. Given the following code, what will be the output?

booked_slots = {"9AM": True, "10AM": False}
requested_slot = "10AM"
if not booked_slots.get(requested_slot, False):
    print("Slot Available")
else:
    print("Slot Booked")

medium

A. Slot Available

B. Slot Booked

C. KeyError

D. No output

Solution

1. Step 1: Understand the dictionary and requested slot

   booked_slots maps times to True (booked) or False (free). "10AM" is False, meaning free.

2. Step 2: Evaluate the condition

   booked_slots.get("10AM", False) returns False. 'not False' is True, so it prints "Slot Available".

3. Final Answer:

   Slot Available -> Option A

4. Quick Check:

   False means free, so output is Slot Available [OK]

Hint: False means free slot, so print available [OK]

Common Mistakes:

• Assuming True means available instead of booked
• Expecting KeyError when key exists
• Ignoring default value in get()

4. Identify the bug in the following availability check code:

def is_available(stock, requested):
    if requested > stock:
        return True
    else:
        return False

print(is_available(5, 10))

medium

A. The function should return False when requested is greater than stock

B. The function is correct and returns True

C. The condition should be 'requested <= stock' to return True

D. The function should compare 'stock > requested' instead

Solution

1. Step 1: Analyze the condition logic

   Current code returns True if requested > stock, meaning more requested than available stock.

2. Step 2: Correct logic for availability

   Availability means stock should be enough or more than requested. So, if requested > stock, return False.

3. Final Answer:

   The function should return False when requested is greater than stock -> Option A

4. Quick Check:

   Requested > stock means not available [OK]

Hint: Availability means stock >= requested, else False [OK]

Common Mistakes:

• Returning True when requested exceeds stock
• Confusing greater than with less than
• Not testing with example values

5. You are designing an availability checking system for a hotel booking platform. Which approach best ensures high availability and scalability when checking room availability in real-time?

hard

A. Use a centralized database with locking to check and update availability synchronously

B. Cache availability data in memory with periodic sync to the database and use optimistic concurrency

C. Check availability by scanning all booking records on every request without caching

D. Allow double booking and resolve conflicts manually later

Solution

1. Step 1: Understand requirements for high availability and scalability

   System must respond quickly and handle many requests without blocking.

2. Step 2: Evaluate options for real-time availability checking

   Cache availability data in memory with periodic sync to the database and use optimistic concurrency uses caching and optimistic concurrency, reducing database load and avoiding locks, improving scalability and availability.

3. Final Answer:

   Cache availability data in memory with periodic sync to the database and use optimistic concurrency -> Option B

4. Quick Check:

   Caching + optimistic concurrency = scalable availability [OK]

Hint: Cache data and use optimistic concurrency for scalable availability [OK]

Common Mistakes:

• Using locking causing bottlenecks
• Scanning all records causing slow response
• Allowing double booking causing user issues