Token bucket algorithm in Rest API - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to initialize the token bucket with the correct capacity.

Rest API

token_bucket = {"capacity": [1], "tokens": 0, "rate": 5}

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

A1

B5

C10

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Setting capacity to 0 or rate instead of capacity.

The capacity defines the maximum number of tokens the bucket can hold. Here, it should be 10.

2fill in blank

medium

Complete the code to add tokens to the bucket without exceeding capacity.

Rest API

token_bucket["tokens"] = min(token_bucket["tokens"] + [1], token_bucket["capacity"])

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

Atoken_bucket["rate"]

Btoken_bucket["capacity"]

C1

Dtoken_bucket["tokens"]

Attempts:

3 left

💡 Hint

Common Mistakes

Adding capacity or tokens instead of rate.

Tokens are added at the rate specified by token_bucket["rate"].

3fill in blank

hard

Fix the error in the code to check if enough tokens are available for a request.

Rest API

if token_bucket["tokens"] >= [1]:
    token_bucket["tokens"] -= [1]
    allowed = True
else:
    allowed = False

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

Arequest_tokens

Btoken_bucket["capacity"]

Ctoken_bucket["rate"]

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Using rate or capacity instead of request_tokens.

The code should check if the bucket has enough tokens for the requested amount, which is request_tokens.

4fill in blank

hard

Fill both blanks to create a dictionary comprehension that tracks tokens per user if tokens are above zero.

Rest API

user_tokens = {user: [1] for user, tokens in token_data.items() if tokens [2] 0}

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

Atokens

B>

C<

Duser

Attempts:

3 left

💡 Hint

Common Mistakes

Using user as value or wrong comparison operator.

The comprehension keeps users with tokens greater than zero and maps user to tokens.

5fill in blank

hard

Fill all three blanks to create a dictionary of users with tokens less than capacity and map user to tokens plus one.

Rest API

updated_tokens = { [1]: [2] + 1 for [3], tokens in token_data.items() if tokens < token_bucket["capacity"] }

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

Auser

Btokens

Dtoken

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong variable names or mixing key and value.

The comprehension maps each user to their tokens plus one if tokens are less than capacity.

Practice

1.

What is the main purpose of the token bucket algorithm in REST APIs?

easy

A. To encrypt API responses

B. To store user data securely

C. To control the rate of incoming requests by using tokens

D. To manage database connections

Solution

1. Step 1: Understand the token bucket algorithm concept

   The token bucket algorithm limits how many requests can be processed by controlling tokens that refill over time.

2. Step 2: Identify the purpose in REST APIs

   It helps prevent too many requests at once, protecting the server from overload.

3. Final Answer:

   To control the rate of incoming requests by using tokens -> Option C

4. Quick Check:

   Token bucket controls request rate = C [OK]

Hint: Token bucket limits request rate using tokens [OK]

Common Mistakes:

• Confusing token bucket with data storage
• Thinking it encrypts data
• Assuming it manages database connections

2.

Which of the following is the correct way to represent a token bucket refill rate in pseudocode?

1. refill_rate = tokens_per_second
2. refill_rate = seconds_per_token
3. refill_rate = max_tokens * time
4. refill_rate = tokens / max_tokens

easy

A. refill_rate = seconds_per_token

B. refill_rate = tokens_per_second

C. refill_rate = max_tokens * time

D. refill_rate = tokens / max_tokens

Solution

1. Step 1: Understand refill rate meaning

   The refill rate is how many tokens are added per second to the bucket.

2. Step 2: Match with options

   refill_rate = tokens_per_second correctly shows tokens added per second, which is the refill rate.

3. Final Answer:

   refill_rate = tokens_per_second -> Option B

4. Quick Check:

   Refill rate = tokens per second [OK]

Hint: Refill rate means tokens added each second [OK]

Common Mistakes:

• Confusing refill rate with time per token
• Multiplying max tokens by time incorrectly
• Using ratios instead of rates

3.

Given a token bucket with max_tokens = 5, refill_rate = 1 token/second, and an empty bucket at time 0, what is the number of tokens available at time 3 seconds?

medium

A. 3 tokens

B. 5 tokens

C. 0 tokens

D. 1 token

Solution

1. Step 1: Calculate tokens refilled after 3 seconds

   Since refill rate is 1 token per second, after 3 seconds, 3 tokens are added.

2. Step 2: Check max tokens limit

   The bucket max is 5 tokens, so 3 tokens fit without exceeding the max.

3. Final Answer:

   3 tokens -> Option A

4. Quick Check:

   3 seconds * 1 token/sec = 3 tokens [OK]

Hint: Multiply seconds by refill rate, cap at max tokens [OK]

Common Mistakes:

• Assuming bucket fills instantly to max
• Ignoring max token limit
• Using refill rate incorrectly

4.

Consider this pseudocode snippet for token bucket check:
if tokens <= 0:
  reject_request()
else:
  tokens -= 1
  allow_request()
What is the bug in this logic?

medium

A. It should check if tokens > 0 before allowing

B. It should increase tokens instead of decreasing

C. It should reject when tokens > 0

D. It should check if tokens < 1, not <= 0

Solution

1. Step 1: Recall proper token bucket logic

   To consume 1 token, check if tokens >= 1 before decrementing (equivalent to reject if tokens < 1).

2. Step 2: Identify the bug

   The code rejects only if tokens <= 0. For fractional tokens (common in real implementations), if 0 < tokens < 1, it allows the request, decrementing to negative, which is incorrect.

3. Final Answer:

   It should check if tokens < 1, not <= 0 -> Option D

4. Quick Check:

   Reject if tokens < 1 [OK]

Hint: Allow only if tokens >= 1 [OK]

Common Mistakes:

• Using <= 0 instead of < 1 causes off-by-one errors
• Increasing tokens on request instead of decreasing
• Rejecting requests when tokens are available

5.

You want to implement a token bucket that allows bursts of up to 10 requests and refills tokens at 2 tokens per second. If a client sends 15 requests instantly after being idle for 3 seconds, how many requests will be allowed immediately?

hard

A. 6 requests

B. 5 requests

C. 15 requests

D. 10 requests

Solution

1. Step 1: Calculate tokens available after 3 seconds idle

   Refill rate is 2 tokens/second, so after 3 seconds: 2 * 3 = 6 tokens. Max tokens allowed is 10, so bucket fills to 6 tokens.

2. Step 2: Consider burst capacity

   Since the bucket max is 10, if it was full before idle, it would have 10 tokens. But starting empty, after 3 seconds it has 6 tokens.

3. Step 3: Determine allowed requests

   The client sends 15 requests instantly, but only 6 tokens are available, so only 6 requests allowed immediately.

4. Final Answer:

   6 requests -> Option A

5. Quick Check:

   3 sec * 2 tokens/sec = 6 tokens available [OK]

Hint: Tokens = min(max_tokens, refill_rate * idle_time) [OK]

Common Mistakes:

• Assuming bucket always full at max tokens
• Allowing more requests than tokens available
• Ignoring refill rate and idle time