Rate limiter with INCR and EXPIRE in Redis - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to increment the request count for a user key.

Redis

redis.call('INCR', [1])

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

Auser_key

Bexpire_time

Climit

Dtimestamp

Attempts:

3 left

💡 Hint

Common Mistakes

Using expire_time instead of the key to increment

Using limit which is a number, not a key

The INCR command increments the count for the given user key.

2fill in blank

medium

Complete the code to set the expiration time for the user key.

Redis

redis.call('EXPIRE', [1], 60)

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

Acount

Btimestamp

Cuser_key

Dlimit

Attempts:

3 left

💡 Hint

Common Mistakes

Setting expiration on a number or limit instead of the key

Using wrong key name

The EXPIRE command sets the expiration time on the user key to 60 seconds.

3fill in blank

hard

Fix the error in the code to check if the key exists before setting expiration.

Redis

if redis.call('EXISTS', [1]) == 1 then redis.call('EXPIRE', [1], 60) end

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

Alimit

Buser_key

Ccount

Dtimestamp

Attempts:

3 left

💡 Hint

Common Mistakes

Checking existence of wrong key

Using count or limit instead of key

We check if the user_key exists before setting expiration on it.

4fill in blank

hard

Fill both blanks to create a Lua script that increments the user key and sets expiration if it's a new key.

Redis

local current = redis.call('INCR', [1])
if current == 1 then redis.call('EXPIRE', [2], 60) end
return current

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

Auser_key

Blimit

Dtimestamp

Attempts:

3 left

💡 Hint

Common Mistakes

Using different keys for increment and expire

Using limit or timestamp instead of user_key

The script increments the user_key and sets expiration on the same key if it's the first increment.

5fill in blank

hard

Fill all three blanks to complete the Lua script that increments the user key, sets expiration, and checks if the limit is exceeded.

Redis

local current = redis.call('INCR', [1])
if current == 1 then redis.call('EXPIRE', [2], 60) end
if current > [3] then return 0 else return 1 end

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

Auser_key

Climit

Dtimestamp

Attempts:

3 left

💡 Hint

Common Mistakes

Using timestamp instead of user_key for increment

Mixing keys and limit values

Not comparing count to limit

The script increments the user_key, sets expiration on it, and compares the count to the limit to decide if the request is allowed.

Practice

1. What is the main purpose of using the INCR command in a Redis rate limiter?

easy

A. To increase the count of actions performed by a user

B. To set the expiration time for a key

C. To delete a key from Redis

D. To reset the count to zero

Solution

1. Step 1: Understand the role of INCR in counting

   The INCR command increments the integer value stored at a key by one, which is used to count user actions.

2. Step 2: Differentiate from other commands

   Commands like EXPIRE set timeouts, and DEL deletes keys, but INCR specifically increases counts.

3. Final Answer:

   To increase the count of actions performed by a user -> Option A

4. Quick Check:

   INCR counts actions = A [OK]

Hint: INCR always adds 1 to a key's value [OK]

Common Mistakes:

• Confusing INCR with EXPIRE
• Thinking INCR resets counts
• Assuming INCR deletes keys

2. Which Redis command syntax correctly sets a key to expire after 60 seconds?

easy

A. EXPIRE key '60s'

B. EXPIRE 60 key

C. EXPIRE key 60

D. SETEX key 60 value

Solution

1. Step 1: Recall EXPIRE command syntax

   The correct syntax is EXPIRE key seconds, where seconds is an integer.

2. Step 2: Evaluate options

   EXPIRE key 60 matches the syntax. EXPIRE 60 key reverses arguments. SETEX key 60 value is a different command combining SET and EXPIRE. EXPIRE key '60s' uses invalid string format for seconds.

3. Final Answer:

   EXPIRE key 60 -> Option C

4. Quick Check:

   EXPIRE key seconds = D [OK]

Hint: EXPIRE command uses key then seconds [OK]

Common Mistakes:

• Swapping key and seconds arguments
• Using string with units for seconds
• Confusing SETEX with EXPIRE

3. Given the following Redis commands executed in order:

INCR user:123
EXPIRE user:123 10
INCR user:123
TTL user:123

What will the TTL user:123 command return immediately after these commands?

medium

A. 10

B. 9 or less

C. -1

D. 0

Solution

1. Step 1: Analyze command sequence

   First INCR creates or increments the key. Then EXPIRE sets TTL to 10 seconds. Another INCR increments the count but does not reset TTL.

2. Step 2: Understand TTL behavior

   TTL counts down from 10 seconds after EXPIRE. The second INCR does not affect TTL, so TTL will be less than or equal to 9 seconds depending on execution time.

3. Final Answer:

   9 or less -> Option B

4. Quick Check:

   TTL after EXPIRE and INCR = B [OK]

Hint: INCR does not reset TTL, so TTL decreases over time [OK]

Common Mistakes:

• Assuming INCR resets TTL
• Expecting TTL to remain exactly 10
• Confusing TTL -1 (no expire) with positive TTL

4. You implemented a rate limiter using INCR and EXPIRE, but the count never resets after the expiration time. What is the most likely cause?

medium

A. The key was deleted manually before expiration

B. INCR command resets the TTL unintentionally

C. Redis server is out of memory

D. EXPIRE was never set on the key

Solution

1. Step 1: Identify why count doesn't reset

   If the count never resets, it means the key never expired, so the expiration was not set properly.

2. Step 2: Evaluate other options

   INCR does not reset TTL, manual deletion would remove the key, and out of memory causes errors but not this behavior.

3. Final Answer:

   EXPIRE was never set on the key -> Option D

4. Quick Check:

   No EXPIRE means no reset = C [OK]

Hint: Always set EXPIRE once to enable automatic reset [OK]

Common Mistakes:

• Assuming INCR resets TTL
• Ignoring that EXPIRE must be set
• Blaming Redis memory issues incorrectly

5. You want to implement a rate limiter that allows 100 requests per user per minute using Redis with INCR and EXPIRE. Which approach is best to avoid resetting the expiration time on every request?

hard

A. Set EXPIRE only when INCR returns 1

B. Call EXPIRE every time after INCR

C. Use SETEX instead of INCR

D. Delete the key manually after 60 seconds

Solution

1. Step 1: Understand TTL reset problem

   Calling EXPIRE every time resets TTL, causing the window to slide indefinitely, which is incorrect for fixed window rate limiting.

2. Step 2: Best practice for setting EXPIRE

   Set EXPIRE only when the key is created (when INCR returns 1), so TTL counts down properly without resetting.

3. Final Answer:

   Set EXPIRE only when INCR returns 1 -> Option A

4. Quick Check:

   Set EXPIRE once at first INCR = A [OK]

Hint: Set EXPIRE only on first INCR to avoid TTL reset [OK]

Common Mistakes:

• Resetting EXPIRE on every request
• Using SETEX which overwrites count
• Manually deleting keys instead of TTL