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Spring / Spring Retry Interview Questions

What is the difference between retry and idempotency? Why does it matter?

Retry and idempotency are closely related but distinct concepts that must be understood together when designing resilient systems.

Retry is the mechanism of re-executing a failed operation. Idempotency is the property of an operation that guarantees the same result regardless of how many times it is executed with the same input.

The relationship matters because retrying a non-idempotent operation can cause unintended side effects:

  • Double charges: Retrying a payment API call that already succeeded on the server but timed out before returning a response will charge the customer twice.
  • Duplicate records: Retrying a database insert may create duplicate rows if the original insert succeeded but the acknowledgment was lost.
  • Double emails: Retrying a notification service can send the same email multiple times.

Safe operations to retry (naturally idempotent): HTTP GET, HTTP PUT with full resource replacement, database SELECT, idempotent queue dequeues with unique processing keys.

Risky to retry without idempotency guards: HTTP POST, payment debits, external notifications, non-idempotent database writes.

Solutions: Use idempotency keys (a unique request ID sent with each call so the server can detect and deduplicate retries), optimistic locking, or at-least-once processing with deduplication at the consumer side. When using Spring Retry, limit retries to operations that are safe to re-execute, or ensure the downstream system supports idempotent requests.

Which HTTP method is naturally idempotent and safe to retry without side effects?
What is an idempotency key used for in the context of retry?

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More Related questions...

What is Spring Retry and why is it used? How do you add Spring Retry to a Spring Boot project? What is the @Retryable annotation and what are its key attributes? What is the @Recover annotation and when is it invoked? What is the @Backoff annotation and how does it control retry delays? What is RetryTemplate and how does it differ from annotation-based retry? What are the different retry policies available in Spring Retry? What is a BackOffPolicy in Spring Retry and what implementations are available? What is a RetryContext and what information does it carry? How does the RetryListener interface work in Spring Retry? How does Spring Retry implement the circuit breaker pattern? What is stateful retry in Spring Retry and when should it be used? How do you configure Spring Retry using application properties in Spring Boot? What is the difference between include and exclude in @Retryable? How does Spring Retry integrate with Spring Batch? What common pitfalls should you avoid when using @Retryable? How do you write a unit test for a @Retryable method? How does Spring Retry handle exceptions that are not in the retryFor list? What is the RetryOperationsInterceptor and how is it used? How can you use Spring Retry with Spring WebClient or RestTemplate? How does the ExceptionClassifierRetryPolicy work? What is the RetryCallback interface in Spring Retry? How does Spring Retry differ from Resilience4j Retry? What is the RetryContextCache and when do you need a custom implementation? Can Spring Retry be used with Spring Cloud OpenFeign? How? How do you implement a custom RetryPolicy in Spring Retry? How do you use Spring Retry with Spring Kafka consumers? What is the proxyTargetClass attribute in @EnableRetry? How does the CompositeRetryPolicy work in Spring Retry? What is the difference between retry and idempotency? Why does it matter? How do you configure Spring Retry to use a fixed backoff programmatically? What is the maxAttemptsExpression attribute and how does it differ from maxAttempts? How does Spring Retry integrate with Spring's @Transactional? What is the use of the recover attribute in @Retryable? What are transient vs permanent failures and why does the distinction matter for retry? How do you implement retry with a custom exception condition using RetryPolicy? How does Spring Retry handle the case when @Recover returns void? What is exponential backoff with jitter and why is it preferred over plain exponential backoff? How can you monitor and observe Spring Retry behavior in production? What is the difference between RetryTemplate.execute() and RetryTemplate.executeWithLoadBalancer()?
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