Redis as cache provider in Spring Boot - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when a cached value expires in Redis?

Consider a Spring Boot application using Redis as a cache provider with a TTL (time-to-live) set on cached entries.

What will happen when the TTL expires for a cached key?

A The cached key is moved to a backup Redis database automatically.

B The cached key remains in Redis but returns a null value until manually deleted.

C Redis throws an error when trying to access an expired key.

D The cached key is automatically removed from Redis, so the next request will fetch fresh data and cache it again.

💡 Hint

Think about how Redis handles keys with expiration times.

Redis automatically deletes keys when their TTL expires. This means the key no longer exists, so the application will fetch fresh data and cache it again.

📝 Syntax

intermediate

Which annotation enables Redis caching in Spring Boot?

In a Spring Boot application, you want to enable Redis caching support. Which annotation should you add to your main application class?

A @RedisCacheable

B @EnableCaching

C @EnableRedisCache

D @CacheConfig

💡 Hint

Look for the general Spring annotation that activates caching.

@EnableCaching is the standard Spring annotation to enable caching abstraction, which works with Redis when configured.

🔧 Debug

advanced

Why does this Redis cache not store values correctly?

Given this Spring Boot method with Redis caching:

@Cacheable(value = "items")
public List getItems() {
    return List.of("apple", "banana");
}

But the cache never stores the list, and the method runs every time. What is the likely cause?

A Redis does not support caching lists.

B The @Cacheable annotation requires a key attribute to work.

C The Redis cache manager is not configured properly or missing in the application context.

D The method must be static to be cached.

💡 Hint

Check if the caching infrastructure is set up correctly.

If Redis cache manager is not configured or missing, caching annotations have no effect and methods run every time.

🧠 Conceptual

advanced

What is the effect of using @CachePut instead of @Cacheable with Redis?

In Spring Boot Redis caching, what is the main difference between @CachePut and @Cacheable annotations on a method?

A @CachePut always executes the method and updates the cache, while @Cacheable skips method execution if cache exists.

B @CachePut caches only null values, @Cacheable caches non-null values.

C @CachePut disables Redis caching, @Cacheable enables it.

D @CachePut caches the method result only on exceptions, @Cacheable caches on success.

💡 Hint

Think about when the method runs and when the cache updates.

@CachePut always runs the method and updates the cache with the result. @Cacheable skips method execution if the cache has a value.

❓ state_output

expert

What is the output after these Redis cache operations in Spring Boot?

Consider this Spring Boot service using Redis cache:

@Cacheable(value = "numbers", key = "#num")
public int square(int num) {
    return num * num;
}

// Calls:
int a = square(3);
int b = square(3);
int c = square(4);

What are the values of a, b, and c, and how many times is the method body executed?

A a=9, b=9, c=16; method executed 2 times

B a=9, b=9, c=16; method executed 3 times

C a=9, b=0, c=16; method executed 3 times

D a=9, b=9, c=16; method executed 1 time

💡 Hint

Remember how @Cacheable caches results by key.

The first call with 3 computes and caches 9. The second call with 3 returns cached 9 without running method. The call with 4 computes and caches 16. So method runs twice.