Circuit breaker with Resilience4j in Spring Boot - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to annotate a method with a circuit breaker using Resilience4j.

Spring Boot

@CircuitBreaker(name = "[1]")
public String callExternalService() {
    // method implementation
}

Drag options to blanks, or click blank then click option'

AexternalService

BserviceCall

CcircuitBreaker

DbackendService

Attempts:

3 left

💡 Hint

Common Mistakes

Using a generic name that does not match configuration

Leaving the name blank

Using an invalid identifier

The circuit breaker name should match the configured backend service name, here 'backendService'.

2fill in blank

medium

Complete the code to configure the circuit breaker failure rate threshold in application.yml.

Spring Boot

resilience4j.circuitbreaker.instances.backendService.failureRateThreshold: [1]

Drag options to blanks, or click blank then click option'

A50

B100

C25

D75

Attempts:

3 left

💡 Hint

Common Mistakes

Setting failure rate threshold too high or too low

Using invalid numeric values

Forgetting to specify the correct instance name

The failure rate threshold is commonly set to 50%, meaning the circuit breaker opens if failures exceed 50%.

3fill in blank

hard

Fix the error in the annotation to enable fallback method on circuit breaker failure.

Spring Boot

@CircuitBreaker(name = "backendService", fallbackMethod = "[1]")
public String callExternalService() {
    // method implementation
}

Drag options to blanks, or click blank then click option'

AhandleFallback

BfallbackHandler

CfallbackMethod

Dfallback

Attempts:

3 left

💡 Hint

Common Mistakes

Using a generic or incorrect fallback method name

Not defining the fallback method at all

Mismatch in method parameters

The fallback method name must match the actual method defined to handle failures, here 'handleFallback'.

4fill in blank

hard

Fill both blanks to configure the wait duration and sliding window size for the circuit breaker.

Spring Boot

resilience4j.circuitbreaker.instances.backendService.waitDurationInOpenState: [1]
resilience4j.circuitbreaker.instances.backendService.slidingWindowSize: [2]

Drag options to blanks, or click blank then click option'

A30s

B60s

C100

D50

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing units between time and count

Setting too small or too large values

Using invalid formats like missing 's' for seconds

The wait duration is set to 60 seconds and sliding window size to 50 calls for balanced circuit breaker behavior.

5fill in blank

hard

Fill all three blanks to complete the method signature for a fallback method with exception parameter.

Spring Boot

public String [1](Throwable [2]) {
    return "Fallback response due to " + [3].getMessage();
}

Drag options to blanks, or click blank then click option'

AhandleFallback

Bex

Cexception

Derror

Attempts:

3 left

💡 Hint

Common Mistakes

Mismatch between fallback method name and annotation

Using different parameter names inconsistently

Not including Throwable parameter

The fallback method is named 'handleFallback' and uses 'ex' as the Throwable parameter to get the error message.

Practice

1. What is the primary purpose of using a circuit breaker like Resilience4j in a Spring Boot application?

easy

A. To automatically scale the application horizontally

B. To increase the speed of database queries

C. To encrypt data sent between microservices

D. To prevent repeated calls to a failing service and allow recovery

Solution

1. Step 1: Understand circuit breaker role

   A circuit breaker stops calls to a failing service temporarily to avoid cascading failures.

2. Step 2: Identify the main benefit

   This helps the system recover and prevents wasting resources on repeated failing calls.

3. Final Answer:

   To prevent repeated calls to a failing service and allow recovery -> Option D

4. Quick Check:

   Circuit breaker purpose = prevent repeated failures [OK]

Hint: Circuit breaker stops repeated failing calls [OK]

Common Mistakes:

• Confusing circuit breaker with scaling
• Thinking it speeds up database queries
• Assuming it encrypts data

2. Which annotation is used in Spring Boot to apply a Resilience4j circuit breaker on a method?

easy

A. @Transactional

B. @CircuitBreaker

C. @Cacheable

D. @Retryable

Solution

1. Step 1: Recall Resilience4j integration

   Resilience4j provides the @CircuitBreaker annotation to mark methods for circuit breaking.

2. Step 2: Differentiate from other annotations

   @Retryable is for retries, @Cacheable for caching, and @Transactional for DB transactions.

3. Final Answer:

   @CircuitBreaker -> Option B

4. Quick Check:

   Resilience4j circuit breaker annotation = @CircuitBreaker [OK]

Hint: Use @CircuitBreaker to enable circuit breaking [OK]

Common Mistakes:

• Confusing with @Retryable for retries
• Using caching or transaction annotations incorrectly
• Misspelling annotation names

3. Given this Spring Boot method with Resilience4j circuit breaker:

@CircuitBreaker(name = "serviceA", fallbackMethod = "fallback")
public String callService() {
    // calls external service
    throw new RuntimeException("Service down");
}

public String fallback(Throwable t) {
    return "Fallback response";
}

What will be the output when callService() is invoked?

medium

A. "Fallback response"

B. RuntimeException thrown

C. null

D. Method callService() returns normally

Solution

1. Step 1: Analyze method behavior with circuit breaker

   The method throws an exception, but the circuit breaker calls the fallback method.

2. Step 2: Understand fallback method output

   The fallback method returns the string "Fallback response" which is returned to the caller.

3. Final Answer:

   "Fallback response" -> Option A

4. Quick Check:

   Fallback method output = "Fallback response" [OK]

Hint: Exception triggers fallback method output [OK]

Common Mistakes:

• Expecting exception to propagate without fallback
• Assuming null is returned on failure
• Thinking method returns normally without fallback

4. Consider this incorrect Resilience4j usage in Spring Boot:

@CircuitBreaker(name = "serviceB", fallback = "fallbackMethod")
public String fetchData() {
    // call external API
}

What is the main issue with this code?

medium

A. The method fetchData must return void

B. Circuit breaker name cannot be "serviceB"

C. The fallback method name should be specified with fallbackMethod, not fallback

D. The annotation @CircuitBreaker cannot be used on public methods

Solution

1. Step 1: Check annotation attribute names

   The correct attribute for fallback method name is fallbackMethod, not fallback.

2. Step 2: Validate other conditions

   Method return type and visibility are valid; circuit breaker name can be any string.

3. Final Answer:

   The fallback method name should be specified with fallbackMethod, not fallback -> Option C

4. Quick Check:

   Correct fallback attribute = fallbackMethod [OK]

Hint: Use fallbackMethod attribute for fallback name [OK]

Common Mistakes:

• Using 'fallback' instead of 'fallbackMethod'
• Thinking method must be void
• Assuming circuit breaker name is restricted

5. You want to design a Spring Boot microservice that calls an unstable external payment API. Using Resilience4j circuit breaker, which approach best ensures system stability and graceful degradation?

hard

A. Apply @CircuitBreaker with a fallback method returning a cached payment status

B. Remove circuit breaker and rely on retries only

C. Use @CircuitBreaker without fallback to fail fast

D. Disable circuit breaker and increase thread pool size

Solution

1. Step 1: Understand system stability needs

   Using a circuit breaker prevents repeated calls to the unstable API, avoiding overload.

2. Step 2: Importance of fallback for graceful degradation

   A fallback method returning cached or default data keeps the system responsive during failures.

3. Step 3: Evaluate other options

   Retries alone can worsen overload; no fallback causes failures; increasing threads doesn't solve API instability.

4. Final Answer:

   Apply @CircuitBreaker with a fallback method returning a cached payment status -> Option A

5. Quick Check:

   Circuit breaker + fallback = stability + graceful degradation [OK]

Hint: Combine circuit breaker with fallback for best resilience [OK]

Common Mistakes:

• Relying only on retries without circuit breaker
• Not providing fallback causing failures
• Increasing threads without handling failures