Cache stampede prevention in HLD - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a cache stampede?

A cache stampede happens when many requests try to fetch data from the cache at the same time, but the cache is empty or expired. This causes all requests to hit the database or backend, leading to overload.

intermediate

Explain the 'lock' or 'mutex' method for cache stampede prevention.

This method uses a lock to allow only one request to fetch and update the cache while others wait. It prevents multiple requests from hitting the backend simultaneously.

intermediate

What is 'early recomputation' in cache stampede prevention?

Early recomputation means refreshing the cache before it expires. This way, requests get fresh data and avoid many requests triggering backend calls at once.

advanced

Describe the 'request coalescing' technique.

Request coalescing groups multiple requests for the same data into one backend call. The result is shared among all waiting requests, reducing load.

intermediate

Why is random expiration time useful in cache stampede prevention?

Random expiration times spread out cache expiry moments. This avoids many caches expiring simultaneously, which can cause a stampede.

What problem does cache stampede prevention solve?

A Cache data being outdated

B Too many requests hitting backend when cache expires

C Cache memory running out

D Slow network connections

Cache stampede prevention stops many requests from hitting the backend at once when cache expires.

Which method allows only one request to update the cache while others wait?

A Lock or mutex

B Early recomputation

C Random expiration

D Request coalescing

Lock or mutex ensures only one request updates the cache to prevent stampede.

What does early recomputation do?

A Deletes cache early

B Groups requests

C Refreshes cache before expiry

D Locks cache access

Early recomputation refreshes cache before it expires to avoid many backend hits.

How does random expiration help prevent stampede?

A By making cache expire at the same time

B By deleting cache

C By locking cache

D By spreading cache expiry times

Random expiration spreads expiry times to avoid many caches expiring simultaneously.

What is request coalescing?

A Grouping requests to one backend call

B Making multiple backend calls

C Deleting cache early

D Locking cache access

Request coalescing groups multiple requests into one backend call to reduce load.

Explain what causes a cache stampede and describe two methods to prevent it.

Think about what happens when cache is empty and how to control request flow.

Describe how request coalescing works and why it helps in cache stampede prevention.

Imagine many friends asking the same question and one person answering for all.

Practice

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

easy

A. To prevent many requests from hitting the database simultaneously

B. To increase the size of the cache storage

C. To reduce the number of cache servers

D. To encrypt cached data for security

Solution

1. Step 1: Understand cache stampede problem

   Cache stampede happens when many requests try to fetch data from the database because the cache expired or missed.

2. Step 2: Identify the goal of prevention

   The goal is to stop all these requests from hitting the database at once to avoid overload.

3. Final Answer:

   To prevent many requests from hitting the database simultaneously -> Option A

4. Quick Check:

   Cache stampede prevention = prevent database overload [OK]

Hint: Focus on reducing database hits during cache misses [OK]

Common Mistakes:

• Confusing cache size with stampede prevention
• Thinking encryption is related to stampede
• Assuming reducing cache servers helps stampede

2. Which of the following is a common technique used to prevent cache stampede?

easy

A. Storing data only in the database

B. Increasing cache TTL to infinite

C. Disabling cache for all requests

D. Using distributed locks around cache refresh

Solution

1. Step 1: Review common stampede prevention methods

   Techniques include locking, early expiration, and request coalescing to control cache refresh.

2. Step 2: Match technique to options

   Distributed locks prevent multiple processes from refreshing cache simultaneously, stopping stampede.

3. Final Answer:

   Using distributed locks around cache refresh -> Option D

4. Quick Check:

   Locking = stampede prevention [OK]

Hint: Look for locking or coordination methods in options [OK]

Common Mistakes:

• Choosing infinite TTL which can cause stale data
• Disabling cache which increases load
• Ignoring locking as a prevention method

3. Consider this pseudo-code for cache stampede prevention:

if cache miss:
  acquire lock
  if cache still miss:
    data = fetch from DB
    update cache
  release lock
return cached data

What happens if the lock is not used here?

medium

A. Multiple requests fetch from DB simultaneously causing overload

B. Cache never updates and returns stale data

C. Requests wait indefinitely causing deadlock

D. Cache size increases exponentially

Solution

1. Step 1: Analyze the role of the lock

   The lock ensures only one request fetches data from DB and updates cache on miss.

2. Step 2: Consequence of missing lock

   Without lock, many requests detect cache miss and all fetch from DB, causing overload.

3. Final Answer:

   Multiple requests fetch from DB simultaneously causing overload -> Option A

4. Quick Check:

   No lock = DB overload [OK]

Hint: Lock prevents simultaneous DB fetch on cache miss [OK]

Common Mistakes:

• Thinking cache never updates without lock
• Confusing deadlock with missing lock
• Assuming cache size grows without lock

4. A system uses early expiration to prevent cache stampede by setting cache TTL shorter than data TTL. However, users report stale data sometimes. What is the likely cause?

medium

A. Cache TTL is longer than data TTL causing stale reads

B. Cache is not refreshed after early expiration due to missing lock

C. Database is down causing stale cache

D. Cache size is too small causing evictions

Solution

1. Step 1: Understand early expiration technique

   Early expiration refreshes cache before data TTL ends to avoid stampede.

2. Step 2: Identify cause of stale data

   If cache refresh is not coordinated (missing lock), cache may not update properly, causing stale data.

3. Final Answer:

   Cache is not refreshed after early expiration due to missing lock -> Option B

4. Quick Check:

   Missing lock = stale cache after early expiration [OK]

Hint: Check if locking coordinates cache refresh [OK]

Common Mistakes:

• Assuming TTL mismatch causes stale data
• Blaming database downtime without evidence
• Confusing cache size with freshness

5. You design a high-traffic API with cache stampede prevention. Which combination best prevents stampede while ensuring fresh data and minimal DB load?

hard

A. Disable cache and use database connection pooling

B. Use long cache TTL with no locking and rely on DB scaling

C. Use short cache TTL with distributed locking and request coalescing

D. Use infinite cache TTL and manual cache invalidation

Solution

1. Step 1: Evaluate each option for stampede prevention

   Use long cache TTL with no locking and rely on DB scaling risks DB overload without locking; C disables cache increasing DB load; D risks stale data.

2. Step 2: Identify best practice combination

   Short TTL triggers refresh often; distributed locking prevents multiple refreshes; request coalescing merges requests to reduce DB hits.

3. Final Answer:

   Use short cache TTL with distributed locking and request coalescing -> Option C

4. Quick Check:

   Locking + coalescing + short TTL = best stampede prevention [OK]

Hint: Combine locking and coalescing with short TTL for best results [OK]

Common Mistakes:

• Ignoring locking and causing DB overload
• Disabling cache increases latency and load
• Using infinite TTL causes stale data