Imagine you are designing a Spring Boot application that interfaces with multiple external APIs. How would you handle API rate limits and failures?

Question

Accepted Answer

Handling rate limits and failures when calling multiple external APIs requires a combination of resilience patterns, circuit breakers, retries, rate limiting, and caching, so a slow or failing dependency doesn't cascade into the whole application.

Key Points:
• A circuit breaker, such as Resilience4j's CircuitBreaker, stops calling a failing API temporarily and fails fast instead of piling up timeouts.
• Client-side rate limiting throttles outgoing requests to stay under each API's published quota.
• A retry mechanism with exponential backoff handles transient failures without hammering the external service.
• Caching frequently requested responses reduces the total number of calls needed in the first place.
• Bulkheads isolate calls to different external APIs so one slow dependency can't exhaust the thread pool used by others.

Example:
When a weather API starts returning 429 Too Many Requests, Resilience4j's rate limiter throttles outgoing calls, the circuit breaker opens after repeated failures to stop wasting requests, and cached responses continue serving users until the API recovers.

Code Example:
```
@CircuitBreaker(name = "weatherApi", fallbackMethod = "fallbackWeather")
@Retry(name = "weatherApi")
public WeatherResponse getWeather(String city) {
    return weatherClient.fetch(city);
}
```

Interview Tip:
A concise interview answer is:

"I'd wrap external API calls with a circuit breaker so repeated failures fail fast instead of piling up, add retries with exponential backoff for transient errors, apply client-side rate limiting to respect quotas, and cache responses where possible to reduce call volume. Together that keeps the app resilient even when a dependency misbehaves."