Cache stampede prevention in Redis - Time & Space Complexity

Time Complexity: Cache stampede prevention

O(n)

Understanding Time Complexity

When using Redis to prevent cache stampedes, we want to see how the time to handle requests changes as more users ask for data.

We ask: How does the system handle many requests trying to update the cache at once?

Scenario Under Consideration

Analyze the time complexity of the following Redis commands used to prevent cache stampedes.

# Try to get cached data
GET cache_key

# If missing, try to set a lock
SET lock_key "1" NX PX 5000

# If lock acquired, fetch data from DB and update cache
SET cache_key data PX 60000
DEL lock_key
    

This code tries to get data from cache. If missing, it sets a lock to let only one process update the cache, avoiding many simultaneous DB calls.

Identify Repeating Operations

Look for repeated actions that affect time.

• Primary operation: Multiple clients try GET and SET commands concurrently.
• How many times: Once per client request, but only one client sets the lock and updates cache.

How Execution Grows With Input

As more clients request data, most do a quick GET. Only one client does the heavier update work.

Input Size (n)	Approx. Operations
10	10 GETs, 1 SET lock + 1 SET cache + 1 DEL lock
100	100 GETs, 1 SET lock + 1 SET cache + 1 DEL lock
1000	1000 GETs, 1 SET lock + 1 SET cache + 1 DEL lock

Pattern observation: Most work stays the same no matter how many clients ask, because only one updates the cache.

Final Time Complexity

Time Complexity: O(n)

This means the total time grows linearly with the number of requests, but the expensive cache update happens only once.

Common Mistake

[X] Wrong: "Every request will update the cache, so time grows very fast."

[OK] Correct: The lock ensures only one request updates the cache, so others just read quickly.

Interview Connect

Understanding how to control work in Redis when many users ask at once shows you can design systems that stay fast and reliable under pressure.

Self-Check

"What if we removed the lock and let all clients update the cache? How would the time complexity change?"

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