Rate limiting with sliding window in Redis - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to add the current timestamp to the sorted set for rate limiting.

Redis

redis.call('ZADD', 'user:[1]:requests', current_time, current_time)

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

A123

Blimit

Ctimestamp

Duser123

Attempts:

3 left

💡 Hint

Common Mistakes

Using a generic word like 'timestamp' instead of a user ID.

Using 'user123' which results in malformed key 'user:user123:requests'.

The key for the sorted set should uniquely identify the user, such as '123'.

2fill in blank

medium

Complete the code to remove timestamps older than the sliding window duration.

Redis

redis.call('ZREMRANGEBYSCORE', 'user:123:requests', 0, current_time [1] window_size)

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

A-

B+

C*

D/

Attempts:

3 left

💡 Hint

Common Mistakes

Using addition instead of subtraction.

Using multiplication or division which are incorrect here.

We subtract the window size from the current time to remove old timestamps.

3fill in blank

hard

Fix the error in the code to count requests within the sliding window.

Redis

local count = redis.call('ZCOUNT', 'user:123:requests', current_time [1] window_size, current_time)

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

A*

B+

C-

D/

Attempts:

3 left

💡 Hint

Common Mistakes

Using addition instead of subtraction in the range.

Using multiplication or division which are invalid here.

We count timestamps from current_time - window_size up to current_time.

4fill in blank

hard

Fill both blanks to complete the Lua script that enforces rate limiting with sliding window.

Redis

if count [1] limit then
  return [2]
end

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

A>=

Btrue

C<=

Dfalse

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<=' instead of '>=' for the condition.

Returning true to block the request instead of false.

If the count is greater than or equal to the limit, return false to block the request.

5fill in blank

hard

Fill all three blanks to complete the Lua script that adds the request timestamp, cleans old entries, and returns true if allowed.

Redis

redis.call('ZREMRANGEBYSCORE', 'user:123:requests', 0, current_time [1] window_size)
local count = redis.call('ZCOUNT', 'user:123:requests', current_time [2] window_size, current_time)
if count [3] limit then
  return false
end
redis.call('ZADD', 'user:123:requests', current_time, current_time)
return true

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

A-

B+

C<=

D>=

Attempts:

3 left

💡 Hint

Common Mistakes

Using addition instead of subtraction for score ranges.

Using '<=' instead of '>=' in the if condition.

We subtract window_size to remove old timestamps and count requests in the sliding window. If count is greater or equal to limit, block the request.

Practice

1. What is the main purpose of using a sliding window in rate limiting with Redis?

easy

A. To count actions within a moving time frame for fair limits

B. To store user passwords securely

C. To permanently block users after one action

D. To sort data alphabetically

Solution

1. Step 1: Understand sliding window concept

   Sliding window rate limiting counts actions in a recent time frame that moves forward as time passes.

2. Step 2: Identify purpose in Redis context

   Redis uses sorted sets to track timestamps of actions to enforce limits fairly over time.

3. Final Answer:

   To count actions within a moving time frame for fair limits -> Option A

4. Quick Check:

   Sliding window = moving time frame count [OK]

Hint: Sliding window means counting recent actions, not all time [OK]

Common Mistakes:

• Thinking sliding window blocks permanently
• Confusing with password storage
• Assuming it sorts data alphabetically

2. Which Redis command is essential to add a timestamp to a sorted set for sliding window rate limiting?

easy

A. ZADD

B. GET

C. HSET

D. LPUSH

Solution

1. Step 1: Identify command to add to sorted set

   ZADD adds members with scores (timestamps) to a sorted set in Redis.

2. Step 2: Confirm other commands' purpose

   GET retrieves string values, HSET sets hash fields, LPUSH adds to lists, none add to sorted sets.

3. Final Answer:

   ZADD -> Option A

4. Quick Check:

   ZADD adds to sorted set [OK]

Hint: Use ZADD to add timestamps to sorted sets [OK]

Common Mistakes:

• Using GET which only reads strings
• Confusing HSET (hashes) with sorted sets
• Using LPUSH which is for lists

3. Given this Redis Lua script snippet for sliding window rate limiting:

local now = tonumber(redis.call('TIME')[1])
redis.call('ZREMRANGEBYSCORE', key, 0, now - window)
local count = redis.call('ZCARD', key)
if count < limit then
  redis.call('ZADD', key, now, now)
  return 1
else
  return 0
end

What will be the return value if the current count is equal to the limit?

medium

A. 1

B. nil

C. 0

D. An error

Solution

1. Step 1: Analyze the condition count < limit

   If count is equal to limit, the condition count < limit is false.

2. Step 2: Determine return value when condition is false

   When false, the script returns 0, indicating rate limit reached.

3. Final Answer:

   0 -> Option C

4. Quick Check:

   count == limit returns 0 [OK]

Hint: If count equals limit, return 0 (limit reached) [OK]

Common Mistakes:

• Assuming count == limit returns 1
• Expecting an error instead of 0
• Thinking nil is returned

4. You have this Redis Lua script for sliding window rate limiting:

local now = tonumber(redis.call('TIME')[1])
redis.call('ZREMRANGEBYSCORE', key, 0, now - window)
local count = redis.call('ZCARD', key)
if count <= limit then
  redis.call('ZADD', key, now, now)
  return 1
else
  return 0
end

What is the bug in this script?

medium

A. ZREMRANGEBYSCORE is used incorrectly

B. Using <= allows one extra request beyond limit

C. ZADD should use a string member, not number

D. TIME command returns a string, not number

Solution

1. Step 1: Check condition count <= limit

   This condition allows adding a request when count equals limit, exceeding limit by one.

2. Step 2: Verify other commands usage

   ZREMRANGEBYSCORE usage is correct to remove old timestamps; ZADD accepts numbers as members; TIME returns array with string seconds converted to number.

3. Final Answer:

   Using <= allows one extra request beyond limit -> Option B

4. Quick Check:

   count <= limit allows limit+1 requests [OK]

Hint: Use < not <= to enforce strict limit [OK]

Common Mistakes:

• Thinking ZREMRANGEBYSCORE is wrong here
• Believing ZADD can't use numbers as members
• Confusing TIME return type

5. You want to implement a sliding window rate limiter in Redis that allows 100 requests per 60 seconds. Which sequence of Redis commands best enforces this limit efficiently?

hard

A. HSET to store count, GET to read count, INCR to increment count

B. LPUSH to add timestamp, LLEN to count, LTRIM to remove old timestamps

C. SETNX to set a flag, EXPIRE to reset after 60 seconds

D. ZREMRANGEBYSCORE to remove old timestamps, ZCARD to count, ZADD to add current timestamp

Solution

1. Step 1: Understand sliding window enforcement

   Use sorted sets to store timestamps, remove old ones, count current requests, and add new timestamp.

2. Step 2: Evaluate each option's commands

   ZREMRANGEBYSCORE to remove old timestamps, ZCARD to count, ZADD to add current timestamp uses ZREMRANGEBYSCORE, ZCARD, ZADD correctly for sliding window. LPUSH to add timestamp, LLEN to count, LTRIM to remove old timestamps uses lists which are less efficient for range removals. HSET to store count, GET to read count, INCR to increment count uses hashes and counters, not suitable for sliding window. SETNX to set a flag, EXPIRE to reset after 60 seconds uses simple flags, not counting requests.

3. Final Answer:

   ZREMRANGEBYSCORE, ZCARD, ZADD sequence -> Option D

4. Quick Check:

   Sorted sets + remove old + count + add = correct [OK]

Hint: Use sorted sets with ZREMRANGEBYSCORE, ZCARD, ZADD [OK]

Common Mistakes:

• Using lists which are inefficient for sliding windows
• Using counters without timestamps
• Using flags without counting requests