Cache invalidation strategies in HLD - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding Cache Invalidation Types

Which cache invalidation strategy immediately removes or updates cached data when the original data changes?

A Lazy invalidation

B Manual cache clearing

C Time-to-live (TTL) expiration

D Write-through caching

💡 Hint

Think about which strategy keeps cache and database synchronized instantly.

Write-through caching updates the cache at the same time as the database, ensuring immediate consistency.

❓ Architecture

intermediate

Choosing Cache Invalidation for a News Website

A news website updates articles frequently but can tolerate slight delays in showing updates. Which cache invalidation strategy best balances freshness and performance?

A Time-to-live (TTL) expiration

B Write-back caching

C Write-through caching

D Manual cache clearing

💡 Hint

Consider a strategy that automatically expires cache after a set time.

TTL expiration allows cached data to live for a set time, balancing freshness and reducing load.

❓ scaling

advanced

Scaling Cache Invalidation in Distributed Systems

In a distributed system with multiple cache nodes, which cache invalidation approach helps maintain consistency without excessive network overhead?

A Each cache node polling the database for changes every second

B No cache invalidation, relying on TTL only

C Using a centralized cache invalidation service with asynchronous notifications

D Broadcasting invalidation messages to all cache nodes synchronously

💡 Hint

Think about reducing network chatter while keeping caches updated.

A centralized invalidation service with async notifications reduces network load and keeps caches consistent efficiently.

❓ tradeoff

advanced

Tradeoffs in Cache Invalidation Strategies

Which statement correctly describes a tradeoff when using lazy invalidation compared to write-through caching?

A Lazy invalidation reduces write latency but risks serving stale data until invalidation occurs.

B Write-through caching may serve stale data longer than lazy invalidation.

C Lazy invalidation guarantees immediate cache consistency but increases write latency.

D Write-through caching reduces read latency by avoiding cache updates on writes.

💡 Hint

Consider how each strategy affects latency and data freshness.

Lazy invalidation delays cache updates, lowering write latency but risking stale reads until invalidation.

❓ estimation

expert

Estimating Cache Expiration Time for Optimal Performance

You manage a cache for a product catalog updated on average every 10 minutes. Cache misses cost 100ms extra latency, and stale data causes 1% user dissatisfaction. Which TTL expiration time balances latency and freshness best?

A 1 minute TTL

B 5 minutes TTL

C 20 minutes TTL

D 10 minutes TTL

💡 Hint

Think about reducing stale data while minimizing cache misses.

5 minutes TTL reduces stale data exposure and limits cache misses, balancing user experience and performance.

Practice

1. Which cache invalidation strategy automatically removes cached data after a set time period?

easy

A. Time-to-Live (TTL)

B. Write-through caching

C. Explicit invalidation

D. Cache-aside

Solution

1. Step 1: Understand TTL concept

   TTL means cached data expires automatically after a fixed time, ensuring freshness.

2. Step 2: Compare with other strategies

   Write-through updates cache on writes, explicit invalidation requires manual removal, cache-aside loads on demand.

3. Final Answer:

   Time-to-Live (TTL) -> Option A

4. Quick Check:

   Automatic expiry = TTL [OK]

Hint: TTL means cache expires after fixed time automatically [OK]

Common Mistakes:

• Confusing write-through with automatic expiry
• Thinking explicit invalidation is automatic
• Mixing cache-aside with TTL

2. Which of the following is the correct description of the write-through cache invalidation strategy?

easy

A. Cache is cleared manually by the application

B. Cache is updated only when data is read

C. Cache is updated immediately when data is written

D. Cache data expires after a fixed time

Solution

1. Step 1: Define write-through caching

   Write-through means every write to the database also updates the cache immediately.

2. Step 2: Eliminate other options

   Cache is updated only when data is read describes cache-aside, C is explicit invalidation, D is TTL.

3. Final Answer:

   Cache is updated immediately when data is written -> Option C

4. Quick Check:

   Write-through = update cache on write [OK]

Hint: Write-through updates cache on every write instantly [OK]

Common Mistakes:

• Confusing read update with write update
• Mixing explicit invalidation with write-through
• Assuming TTL applies to write-through

3. Consider a cache-aside strategy where the cache is empty initially. If a client requests data key K and the cache misses, what happens next?

medium

A. The system returns an error immediately

B. The system fetches data from the database and updates the cache

C. The system waits for explicit invalidation before fetching

D. The system uses stale data from cache

Solution

1. Step 1: Understand cache-aside behavior on miss

   Cache-aside loads data from the database on a cache miss and updates the cache.

2. Step 2: Check other options

   The system returns an error immediately is wrong because system does not error on miss, C is incorrect as explicit invalidation is unrelated, D is wrong because cache is empty.

3. Final Answer:

   The system fetches data from the database and updates the cache -> Option B

4. Quick Check:

   Cache miss triggers DB fetch and cache update [OK]

Hint: Cache-aside loads from DB on miss, then caches data [OK]

Common Mistakes:

• Assuming cache returns error on miss
• Confusing explicit invalidation with cache miss handling
• Thinking stale data is returned on miss

4. A system uses explicit invalidation but sometimes serves stale data. What is the most likely cause?

medium

A. Cache entries expire too quickly

B. Cache updates on every write immediately

C. Cache-aside strategy is used incorrectly

D. Invalidation messages are delayed or lost

Solution

1. Step 1: Analyze explicit invalidation issues

   Explicit invalidation depends on timely invalidation messages to remove stale cache entries.

2. Step 2: Identify cause of stale data

   If invalidation messages are delayed or lost, stale data remains in cache and is served.

3. Final Answer:

   Invalidation messages are delayed or lost -> Option D

4. Quick Check:

   Delayed invalidation causes stale cache [OK]

Hint: Delayed invalidation messages cause stale cache data [OK]

Common Mistakes:

• Thinking cache expiry too fast causes stale data
• Confusing write-through with explicit invalidation
• Assuming cache-aside errors cause stale data here

5. You design a high-traffic e-commerce system where product prices update rarely but must be accurate. Which cache invalidation strategy best balances freshness and performance?

hard

A. Use explicit invalidation triggered by price update events

B. Use write-through caching to update cache on every price change

C. Use cache-aside with manual cache clearing

D. Use a short TTL to expire cache frequently

Solution

1. Step 1: Analyze system requirements

   Prices update rarely but must be accurate, so cache should update only on changes, not frequently expire.

2. Step 2: Evaluate strategies

   Short TTL causes unnecessary reloads, write-through may be costly if writes are rare, cache-aside with manual clearing is error-prone.

3. Step 3: Choose explicit invalidation

   Explicit invalidation triggered by price update events ensures cache is fresh only when needed, balancing performance and accuracy.

4. Final Answer:

   Use explicit invalidation triggered by price update events -> Option A

5. Quick Check:

   Rare updates + accuracy = explicit invalidation [OK]

Hint: Rare updates + accuracy = explicit invalidation best [OK]

Common Mistakes:

• Choosing short TTL causing frequent reloads
• Assuming write-through is best for rare updates
• Ignoring event-driven invalidation benefits