Cache invalidation strategies in HLD - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is cache invalidation in system design?

Cache invalidation is the process of removing or updating cached data when the original data changes, ensuring that users get fresh and accurate information.

beginner

Explain the 'Time-to-Live (TTL)' cache invalidation strategy.

TTL sets a fixed time duration for cached data. After this time expires, the cache entry is automatically removed or refreshed, ensuring data does not become too old.

intermediate

What is the difference between 'Write-through' and 'Write-back' cache invalidation?

Write-through updates the cache and the main data store simultaneously, keeping them in sync. Write-back updates only the cache first and writes to the main store later, which can cause stale data if not managed carefully.

intermediate

Describe the 'Explicit Invalidation' strategy.

Explicit invalidation means the system actively removes or updates cache entries when data changes, often triggered by events or notifications, ensuring immediate consistency.

advanced

Why is cache invalidation considered one of the hardest problems in computer science?

Because it requires balancing data freshness, system performance, and complexity. Incorrect invalidation can lead to stale data or excessive overhead, making it tricky to design efficient and reliable systems.

Which cache invalidation strategy removes cache entries after a fixed time?

A Write-back

B Explicit Invalidation

C Time-to-Live (TTL)

D Lazy Loading

TTL sets a fixed expiration time for cache entries, after which they are invalidated.

In which strategy is the cache updated immediately along with the main data store?

A Write-back

B Write-through

C Lazy Invalidation

D TTL

Write-through updates both cache and main store at the same time to keep data consistent.

What is a risk of using write-back cache invalidation?

A Stale data if main store update is delayed

B Automatic cache refresh

C Cache entries never expire

D Cache and main store always in sync

Write-back delays main store update, which can cause stale data if not handled properly.

Which strategy requires the system to actively remove cache entries when data changes?

A Lazy Loading

B TTL

C Write-back

D Explicit Invalidation

Explicit invalidation means actively removing or updating cache entries on data change.

Why is cache invalidation challenging in system design?

A Because it balances freshness, performance, and complexity

B Because it never affects system performance

C Because cache is always fresh

D Because it only applies to databases

Cache invalidation is hard because it must balance data freshness, system speed, and design complexity.

Explain the main cache invalidation strategies and when to use each.

Think about how and when cached data gets updated or removed.

Describe why cache invalidation is important and what problems it solves in system design.

Consider what happens if cache is not updated properly.

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