How would you call another service in the microservice architecture?

Question

Accepted Answer

Services in a microservice architecture typically call each other either synchronously over HTTP REST APIs or asynchronously through a messaging queue, depending on whether an immediate response is needed.

Key Points:
• Synchronous calls use REST (or gRPC) over HTTP, where the caller sends a request and waits for a response, usually carrying JSON payloads.
• Asynchronous calls publish a message to a queue or topic, and the caller continues without waiting for the receiving service to process it.
• REST calls are simpler to reason about but couple the caller to the callee's availability at request time.
• Messaging decouples services in time, improving resilience if the downstream service is temporarily unavailable, at the cost of added complexity.
• A service client library or a discovery-aware HTTP client is often used so the caller doesn't hardcode target URLs.

Example:
An Order service might call a Payment service synchronously via a REST POST to charge a card, then publish an OrderPlaced event asynchronously to a queue that the Notification service consumes to send a confirmation email.

Code Example:

```
@Service
public class PaymentClient {
    private final RestTemplate restTemplate;

public PaymentResponse charge(PaymentRequest request) {
        return restTemplate.postForObject(
            "http://payment-service/api/payments",
            request,
            PaymentResponse.class
        );
    }
}
```

Interview Tip:
A concise interview answer is:

"I'd use synchronous REST calls when the caller needs an immediate response, like charging a payment, and asynchronous messaging through a queue when the caller just needs to notify other services of something that happened. Messaging decouples services and improves resilience, at the cost of eventual rather than immediate consistency."