Calling another service with RestClient

Key points

• RestClient is the modern blocking client; RestTemplate is in maintenance and WebClient is reactive.
• Build it once as a @Bean with a base URL and shared headers, then inject it. Never build one per call.
• Always set connect and read timeouts. Unset means a thread waits forever on a peer that never answers.
• A 4xx or 5xx throws by default. Use onStatus to translate it into an exception of your own.
• An @HttpExchange interface gives you a typed client with no implementation to write or test.
• Log the correlation id on the way out so one request can be followed across every service.

Example

@Configuration
class RatesClientConfig {

    @Bean
    RestClient ratesRestClient(RestClient.Builder builder) {
        SimpleClientHttpRequestFactory factory = new SimpleClientHttpRequestFactory();
        factory.setConnectTimeout(Duration.ofSeconds(2));   // never leave these unset
        factory.setReadTimeout(Duration.ofSeconds(3));
        return builder
                .baseUrl("https://rates.example.com")
                .requestFactory(factory)
                .defaultStatusHandler(HttpStatusCode::isError, (req, res) -> {
                    throw new RateUnavailableException(res.getStatusCode());
                })
                .build();
    }
}

// Boot 3.4+ can set the same timeouts for every client from properties instead:
//   spring.http.client.connect-timeout: 2s
//   spring.http.client.read-timeout: 3s

// A typed client: declare the calls, Spring generates the implementation.
@HttpExchange("/rates")
interface RatesApi {

    @GetExchange("/{currency}")
    Rate byCurrency(@PathVariable String currency);
}

Every outbound call needs a timeout and an owner for its failure - the default is to wait forever.