Cache stampede prevention in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

What is the main goal of cache stampede prevention?

Imagine many users request the same data at the same time, but the cache is empty or expired. What problem does cache stampede prevention solve?

A It duplicates cache data across multiple servers for backup.

B It speeds up the cache to store more data permanently.

C It deletes all cache entries regularly to keep data fresh.

D It prevents many requests from hitting the database simultaneously, avoiding overload.

💡 Hint

Think about what happens when many users ask for the same missing cache data at once.

Cache stampede prevention stops many requests from querying the database at the same time when cache is empty or expired. This avoids database overload and improves performance.

❓ query_result

intermediate

What is the output of this Redis Lua script for cache stampede prevention?

Given this Lua script run in Redis to set a lock key for cache rebuild: local lock_key = KEYS[1] local lock_ttl = tonumber(ARGV[1]) if redis.call('SETNX', lock_key, 'locked') == 1 then redis.call('PEXPIRE', lock_key, lock_ttl) return 'LOCK_ACQUIRED' else return 'LOCK_EXISTS' end What will the script return if the lock key does not exist?

Redis

local lock_key = KEYS[1]
local lock_ttl = tonumber(ARGV[1])

if redis.call('SETNX', lock_key, 'locked') == 1 then
  redis.call('PEXPIRE', lock_key, lock_ttl)
  return 'LOCK_ACQUIRED'
else
  return 'LOCK_EXISTS'
end

A 'LOCK_ACQUIRED'

B 'LOCK_EXISTS'

C nil

D Error: wrong number of arguments

💡 Hint

SETNX sets the key only if it does not exist.

If the lock key does not exist, SETNX sets it and returns 1, so the script returns 'LOCK_ACQUIRED'.

📝 Syntax

advanced

Which Redis command sequence correctly implements a cache stampede lock with expiration?

You want to set a lock key 'cache_lock' with a 5-second expiration only if it does not exist. Which option is correct?

A SET cache_lock locked EX 5 NX

B SET cache_lock locked NX PX 5000

C SETNX cache_lock locked EX 5

D SET cache_lock locked PX 5000

💡 Hint

Look for the command that sets the key only if it does not exist and sets expiration in milliseconds.

Option B uses SET with NX (only if not exists) and PX 5000 (expire in 5000 ms), which is the correct way to set a lock with expiration atomically.

❓ optimization

advanced

How can you reduce the chance of cache stampede when cache expiration times are the same for many keys?

If many cache keys expire simultaneously, many requests may hit the database at once. What is a good strategy to reduce this problem?

A Disable cache expiration and refresh data manually only.

B Set all cache keys to expire exactly at the same time for consistency.

C Add random jitter to cache expiration times so keys expire at different moments.

D Increase cache size to store more data permanently.

💡 Hint

Think about spreading out expiration times to avoid many keys expiring together.

Adding random jitter to expiration times staggers cache expiry, preventing many keys from expiring simultaneously and reducing database load.

🔧 Debug

expert

Why does this Redis Lua script cause a deadlock in cache stampede prevention?

Consider this Lua script: local lock_key = KEYS[1] local lock_ttl = tonumber(ARGV[1]) if redis.call('SETNX', lock_key, 'locked') == 1 then -- Missing expiration set here return 'LOCK_ACQUIRED' else return 'LOCK_EXISTS' end Why can this cause a deadlock?

A Because the lock key never expires, so other clients wait forever.

B Because SETNX is not atomic and can fail silently.

C Because the script returns wrong values causing client confusion.

D Because the lock key is deleted immediately after setting.

💡 Hint

Think about what happens if a lock is set but never removed or expired.

Without setting an expiration, the lock key remains forever, blocking other clients indefinitely and causing a deadlock.

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