Design a rate limiter in HLD - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define the main component responsible for limiting requests.

HLD

class [1]Limiter:
    def __init__(self, max_requests, window_seconds):
        self.max_requests = max_requests
        self.window_seconds = window_seconds
        self.requests = {}  # user_id -> list of timestamps

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ARate

BRequest

CLimit

DThrottle

Attempts:

3 left

💡 Hint

Common Mistakes

Using generic names like 'RequestLimiter' which is less common.

Using 'Throttle' which is related but not the standard class name.

The main class is commonly called RateLimiter as it controls the rate of requests.

2fill in blank

medium

Complete the code to check if a user can make a request within the rate limit window.

HLD

def is_allowed(self, user_id, current_time):
    if user_id not in self.requests:
        self.requests[user_id] = []
    self.requests[user_id] = [t for t in self.requests[user_id] if t > current_time - [1]]
    return len(self.requests[user_id]) < self.max_requests

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Aself.window_seconds

Bself.max_requests

Ccurrent_time

Duser_id

Attempts:

3 left

💡 Hint

Common Mistakes

Using max_requests instead of window_seconds for filtering timestamps.

Using current_time directly without subtracting the window.

The window to keep requests is defined by self.window_seconds, which is the time frame for rate limiting.

3fill in blank

hard

Fix the error in the code that adds a new request timestamp after checking allowance.

HLD

def add_request(self, user_id, current_time):
    if self.is_allowed(user_id, current_time):
        self.requests[user_id].[1](current_time)
        return True
    return False

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Aadd

Bappend

Cinsert

Dextend

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'add' which is for sets, not lists.

Using 'extend' which expects an iterable, not a single element.

To add a single timestamp to the list, use append. Other methods are incorrect for this purpose.

4fill in blank

hard

Fill both blanks to implement a sliding window rate limiter that removes old requests and checks the count.

HLD

def is_allowed(self, user_id, current_time):
    self.requests[user_id] = [t for t in self.requests.get(user_id, []) if t [1] current_time - self.window_seconds]
    return len(self.requests[user_id]) [2] self.max_requests

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A>

B<

C>=

D<=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '>=' instead of '>' which may keep one extra old timestamp.

Using '>=' or '>'' for the count comparison which would block one extra request.

We keep timestamps greater than the window start (t > current_time - window) and allow if count is less than max_requests.

5fill in blank

hard

Fill all three blanks to implement a dictionary comprehension that tracks request counts per user within the window.

HLD

request_counts = {user: len([t for t in times if t [1] current_time - window]) for user, times in [2].items() if len(times) [3] 0}

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A>

Brequests

Dself.requests

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'requests' instead of 'self.requests' which is undefined in this scope.

Using '>=' instead of '>' which may include old timestamps.

Using 'len(times) >=' 0 which is always true and unnecessary.

We filter timestamps greater than the window start, iterate over self.requests, and only include users with more than zero requests.