Cache stampede prevention in Redis - Mini Project: Build & Apply

Cache Stampede Prevention with Redis

📖 Scenario: You are building a web application that uses Redis as a cache to speed up data retrieval. Sometimes, many users request the same data at once, causing a 'cache stampede' where the backend is overwhelmed with requests to regenerate the same cache entry.To prevent this, you will implement a simple cache stampede prevention technique using Redis locks.

🎯 Goal: Build a Redis-based cache system that prevents multiple processes from regenerating the same cache key simultaneously by using a lock key.

📋 What You'll Learn

Create a Redis key to store cached data with a specific name.

Create a Redis key to act as a lock for cache regeneration.

Implement logic to check if the cache exists before regenerating.

Use the lock key to ensure only one process regenerates the cache at a time.

💡 Why This Matters

🌍 Real World

Web applications often use Redis caching to speed up data access. Preventing cache stampedes ensures the backend is not overwhelmed when many users request the same data.

💼 Career

Understanding cache stampede prevention is important for backend developers and DevOps engineers working with high-traffic systems and caching layers.

1

Create the cache key with initial data

Create a Redis string key called cache:data with the value "initial value".

Redis

# Your code here

Hint

Use the SET command to create a key with a string value.

2

Create a lock key for cache regeneration

Create a Redis string key called cache:lock with the value "locked" to act as a lock indicator.

Redis

SET cache:data "initial value"
# Your code here

Hint

Use the SET command again to create the lock key.

3

Check if cache exists and acquire lock

Use the Redis GET command to check if cache:data exists. Then use SET cache:lock "locked" NX PX 30000 to acquire the lock only if it does not exist, with a 30-second expiration.

Redis

SET cache:data "initial value"
SET cache:lock "locked"
# Your code here

Hint

Use GET to read the cache and SET with NX and PX options to set the lock only if it does not exist.

4

Release the lock after cache regeneration

Delete the cache:lock key using the DEL command to release the lock after cache regeneration is done.

Redis

SET cache:data "initial value"
SET cache:lock "locked"
GET cache:data
SET cache:lock "locked" NX PX 30000
# Your code here

Hint

Use the DEL command to remove the lock key.

Practice

1. What is the main goal of cache stampede prevention in Redis?

easy

A. To allow multiple processes to update cache simultaneously

B. To increase the size of the cache automatically

C. To delete all cache entries periodically

D. To stop many requests from hitting the database at the same time

Solution

1. Step 1: Understand cache stampede problem

   Cache stampede happens when many requests try to refresh expired cache simultaneously, causing database overload.

2. Step 2: Identify the goal of prevention

   Prevention aims to allow only one process to refresh cache, reducing database hits.

3. Final Answer:

   To stop many requests from hitting the database at the same time -> Option D

4. Quick Check:

   Cache stampede prevention = stop many DB hits [OK]

Hint: Cache stampede means many requests hit DB; prevention stops this [OK]

Common Mistakes:

• Thinking cache stampede increases cache size
• Believing cache stampede deletes cache entries
• Assuming multiple processes update cache simultaneously

2. Which Redis command syntax correctly sets a lock with expiration to prevent cache stampede?

easy

A. SET key value NX PX 3000

B. SET key value XX EX 3000

C. GET key NX PX 3000

D. DEL key NX PX 3000

Solution

1. Step 1: Recall Redis SET options for locking

   SET with NX sets key only if it does not exist; PX sets expiration in milliseconds.

2. Step 2: Identify correct syntax

   SET key value NX PX 3000 uses SET with NX and PX correctly to create a lock with 3000ms expiry.

3. Final Answer:

   SET key value NX PX 3000 -> Option A

4. Quick Check:

   SET with NX and PX = lock with expiry [OK]

Hint: Use SET with NX and PX for lock with expiry [OK]

Common Mistakes:

• Using GET instead of SET for locking
• Using XX instead of NX option
• Using DEL command for locking

3. Given this Redis Lua script snippet for cache stampede prevention:

local lock = redis.call('SET', KEYS[1], '1', 'NX', 'PX', 5000)
if not lock then
  return redis.error_reply('Lock not acquired')
end
-- refresh cache here
redis.call('DEL', KEYS[1])
return 'Cache refreshed'

What will be the output if the lock key already exists?

medium

A. Lock not acquired

B. nil

C. Syntax error

D. 'Cache refreshed'

Solution

1. Step 1: Understand SET with NX behavior

   SET with NX returns OK if key set, nil if key exists already.

2. Step 2: Check script logic when lock not acquired

   If lock is nil (not acquired), script returns redis.error_reply('Lock not acquired').

3. Final Answer:

   Lock not acquired -> Option A

4. Quick Check:

   Lock exists -> error reply 'Lock not acquired' [OK]

Hint: If SET NX fails, script returns 'Lock not acquired' error [OK]

Common Mistakes:

• Assuming script returns 'Cache refreshed' always
• Expecting nil instead of error reply
• Thinking script has syntax error

4. You wrote this Redis command to prevent cache stampede:

SET lock_key 1 PX NX 5000

But it does not work as expected. What is the problem?

medium

A. The value '1' is invalid for locking

B. The order of PX and NX options is incorrect

C. SET command does not support NX option

D. PX option requires seconds, not milliseconds

Solution

1. Step 1: Check Redis SET command option order

   Redis requires SET options in order: NX before PX or PX before NX is accepted, but best practice is NX PX.

2. Step 2: Identify issue with given command

   Given command uses PX before NX, which Redis does not accept; correct order is NX PX.

3. Final Answer:

   The order of PX and NX options is incorrect -> Option B

4. Quick Check:

   Correct option order is NX PX [OK]

Hint: Use NX before PX in SET command options [OK]

Common Mistakes:

• Thinking value '1' is invalid
• Believing SET does not support NX
• Confusing PX milliseconds with seconds

5. You want to prevent cache stampede using Redis locks. Which approach correctly handles the scenario where the cache refresh takes longer than the lock expiration time?

hard

A. Do not use PX expiration and rely on manual DEL of lock key only

B. Set a very short PX expiration to release lock quickly and retry immediately

C. Set a longer PX expiration than the expected refresh time and renew the lock if needed

D. Use GETSET command to overwrite lock without expiration

Solution

1. Step 1: Understand lock expiration vs refresh time

   If lock expires before refresh finishes, another process may acquire lock causing duplicate refresh.

2. Step 2: Choose approach to avoid premature lock expiration

   Setting PX longer than refresh time and renewing lock prevents other processes from acquiring lock too early.

3. Final Answer:

   Set a longer PX expiration than the expected refresh time and renew the lock if needed -> Option C

4. Quick Check:

   Lock expiry > refresh time + renew if needed [OK]

Hint: Lock expiry must cover refresh duration; renew lock if needed [OK]

Common Mistakes:

• Setting too short PX causing multiple refreshes
• Not using PX expiration causing stale locks
• Using GETSET which overwrites lock unsafely