Token bucket algorithm in Rest API - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of token bucket refill calculation

Consider a token bucket with capacity 10 tokens and refill rate 2 tokens per second. Initially, the bucket is full. After 3 seconds, how many tokens are in the bucket if 5 tokens were consumed immediately after start?

Rest API

capacity = 10
refill_rate = 2  # tokens per second
initial_tokens = 10
consumed = 5
elapsed_time = 3

# Calculate tokens after elapsed time
new_tokens = min(capacity, initial_tokens - consumed + refill_rate * elapsed_time)
print(new_tokens)

A 10

B 9

C 11

D 7

💡 Hint

Remember the bucket cannot hold more tokens than its capacity.

After consuming 5 tokens, 5 remain. After 3 seconds, 2 tokens per second refill adds 6 tokens. 5 + 6 = 11, but capacity is 10, so final tokens = 10.

🧠 Conceptual

intermediate

Understanding token bucket burst capacity

Which statement best describes the burst capacity in a token bucket algorithm?

A The maximum number of tokens that can be consumed instantly without waiting.

B The rate at which tokens are added to the bucket per second.

C The minimum number of tokens required to start the bucket.

D The time interval between token refills.

💡 Hint

Think about how many requests can be handled at once without delay.

Burst capacity is the bucket's maximum token count, allowing that many requests to be served immediately.

🔧 Debug

advanced

Identify the error in token bucket token consumption code

What error will this code produce when trying to consume tokens from the bucket? ```python capacity = 5 tokens = 3 consume = 4 if tokens >= consume: tokens -= consume else: print("Not enough tokens") ```

A No error, tokens become -1

B Prints 'Not enough tokens' and tokens remain 3

C Raises a TypeError

D SyntaxError due to missing colon

💡 Hint

Check the condition and what happens when tokens are insufficient.

Since tokens (3) are less than consume (4), the else branch runs, printing the message and tokens stay unchanged.

📝 Syntax

advanced

Syntax error in token bucket refill function

Which option contains the correct syntax for a Python function that refills tokens in a bucket?

Rest API

def refill_tokens(current_tokens, capacity, refill_rate, elapsed_time):
    new_tokens = current_tokens + refill_rate * elapsed_time
    if new_tokens > capacity:
        return capacity
    return new_tokens

A

def refill_tokens(current_tokens, capacity, refill_rate, elapsed_time):
    new_tokens = current_tokens + refill_rate * elapsed_time
    if new_tokens > capacity:
    return capacity
    return new_tokens

B

def refill_tokens(current_tokens, capacity, refill_rate, elapsed_time)
    new_tokens = current_tokens + refill_rate * elapsed_time
    if new_tokens > capacity:
        return capacity
    return new_tokens

C

def refill_tokens(current_tokens, capacity, refill_rate, elapsed_time):
    new_tokens = current_tokens + refill_rate * elapsed_time
    if new_tokens > capacity
        return capacity
    return new_tokens

D

def refill_tokens(current_tokens, capacity, refill_rate, elapsed_time):
    new_tokens = current_tokens + refill_rate * elapsed_time
    if new_tokens > capacity:
        return capacity
    return new_tokens

💡 Hint

Check for colons and indentation.

Option D has correct function definition, colons, and proper indentation.

🚀 Application

expert

Calculate tokens after multiple consumption and refill events

A token bucket has capacity 8 tokens and refills at 1 token per second. Initially, it is full. The following events happen: - At 0 seconds, 3 tokens are consumed. - At 2 seconds, 4 tokens are consumed. - At 5 seconds, no consumption. What is the number of tokens in the bucket at 5 seconds?

A 4

B 7

C 6

D 5

💡 Hint

Calculate tokens after each event considering refill and consumption.

Start: 8 tokens At 0s consume 3 → tokens = 5 Refill 2s *1 = 2 tokens → tokens = min(8,5+2)=7 At 2s consume 4 → tokens = 3 Refill 3s *1 = 3 tokens → tokens = min(8,3+3)=6 At 5s no consumption, tokens = 6 But careful: consumption at 2s happens after refill at 2s? Usually consumption happens at event time, so refill from 0 to 2s is 2 tokens, then consume 4 tokens. Tokens at 2s after refill: 7, after consumption: 3 From 2s to 5s refill 3 tokens → 3+3=6 tokens at 5s. So final tokens = 6.