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.

Practice

1. What is the main purpose of using Redis as a cache provider in a Spring Boot application?

easy

A. To store frequently accessed data in memory for faster retrieval

B. To replace the database completely

C. To handle user authentication

D. To serve static web pages

Solution

1. Step 1: Understand Redis caching role

   Redis stores data in memory, which is much faster than disk storage.

2. Step 2: Identify caching benefit in Spring Boot

   Caching reduces database calls by storing frequently used data temporarily.

3. Final Answer:

   To store frequently accessed data in memory for faster retrieval -> Option A

4. Quick Check:

   Redis cache = fast data access [OK]

Hint: Redis cache speeds up data access by storing in memory [OK]

Common Mistakes:

• Thinking Redis replaces the database
• Confusing caching with authentication
• Assuming Redis serves static files

2. Which annotation is used in Spring Boot to enable caching support when using Redis?

easy

A. @RedisCache

B. @Cacheable

C. @EnableCaching

D. @CacheConfig

Solution

1. Step 1: Identify the annotation to activate caching

   @EnableCaching is used on a configuration class to turn on caching support.

2. Step 2: Differentiate from other annotations

   @Cacheable marks methods to cache results, but does not enable caching globally.

3. Final Answer:

   @EnableCaching -> Option C

4. Quick Check:

   Enable caching = @EnableCaching [OK]

Hint: Use @EnableCaching to activate caching in Spring Boot [OK]

Common Mistakes:

• Confusing @Cacheable with enabling caching
• Using non-existent @RedisCache annotation
• Mistaking @CacheConfig for enabling caching

3. Given the following Spring Boot method with Redis caching:

@Cacheable("books")
public Book findBookById(Long id) {
    System.out.println("Fetching from DB");
    return bookRepository.findById(id).orElse(null);
}

What will be printed if findBookById(1L) is called twice in a row?

medium

A. Fetching from DB

B. No output

C. Fetching from DB
Fetching from DB

D. Error at runtime

Solution

1. Step 1: Understand @Cacheable behavior

   The first call caches the result and prints "Fetching from DB".

2. Step 2: Analyze second call output

   The second call returns cached data without executing method body, so no print.

3. Final Answer:

   Fetching from DB -> Option A

4. Quick Check:

   Cache avoids repeated DB fetch print [OK]

Hint: First call prints, second uses cache silently [OK]

Common Mistakes:

• Expecting print on every call
• Thinking cache disables method
• Assuming runtime error due to caching

4. Consider this Spring Boot Redis cache configuration snippet:

@Bean
public RedisCacheManager cacheManager(RedisConnectionFactory factory) {
    return RedisCacheManager.builder(factory).build();
}

If the Redis server is down, what will happen when the application tries to cache data?

medium

A. Application falls back to file-based cache

B. Application throws a runtime exception and crashes

C. Data is cached in memory without Redis

D. Caching silently fails but application continues

Solution

1. Step 1: Understand RedisCacheManager dependency

   RedisCacheManager requires a running Redis server to store cache data.

2. Step 2: Analyze behavior if Redis is down

   Without Redis, cache operations typically fail silently and the application continues unless explicitly configured to fail.

3. Final Answer:

   Caching silently fails but application continues -> Option D

4. Quick Check:

   Redis down usually does not crash app by default [OK]

Hint: Redis down causes silent cache failure unless handled [OK]

Common Mistakes:

• Assuming fallback to memory cache
• Thinking caching silently fails
• Expecting file-based fallback

5. You want to cache the result of a method that returns a list of users filtered by role. Which is the best way to use Redis caching in Spring Boot to avoid caching empty or null results?

public List<User> getUsersByRole(String role) {
    // fetch users
}

hard

A. Use @CachePut to always update cache

B. Use @Cacheable with condition = "#result != null && !#result.isEmpty()"

C. Manually check and cache inside the method

D. Use @Cacheable without any condition

Solution

1. Step 1: Understand caching empty results issue

   Caching empty or null results wastes cache space and may cause stale data.

2. Step 2: Use @Cacheable condition attribute

   Adding condition "#result != null && !#result.isEmpty()" caches only meaningful results.

3. Final Answer:

   Use @Cacheable with condition = "#result != null && !#result.isEmpty()" -> Option B

4. Quick Check:

   Condition filters empty cache entries [OK]

Hint: Use @Cacheable condition to skip empty results [OK]

Common Mistakes:

• Caching all results including empty lists
• Manually caching inside method instead of annotation
• Using @CachePut which always updates cache