CPS-989: Replace RestTemplate with WebClient Analisys

1 References
2 Issues & Decisions
3 Overview
- 3.1 Blocking vs Non-Blocking Client
4 Affected classes
5 Example configuration
- - 5.1.1 Configuration Example
- 5.2 Sync call example
  - 5.2.1 Sync Call
- 5.3 Async call example with subscription
  - 5.3.1 declaration
  - 5.3.2 subscribe

References

CPS-989: Replace RestTemplate with WebClientClosed
Medium article

Issues & Decisions

	Issue	Notes	Decision

	Issue	Notes	Decision
1

Overview

As specified in the Jira ticket the replacement of RestTemplate became neccessary due the fact that it is not under development anymore and also because it does not support basic function required in the modern dev world such as async calls.

Blocking vs Non-Blocking Client

RestTemplate is a blocking clinet. Which means that an HTTP request will use a thread as long as the request is pending/finishes (thread-per-request model).

This means that a thread will use an extra amount of memory and CPU resource that could lead to performance degradation.

WebClient at the other hand is based on the Spring Reactive framework which provides capability to non-blocking async calls thanks to it's event-driven architecture. Because of this WebClient uses less resources (threads, memory).

Affected classes

Apart from the *Spec files the RestTemplate is used only in 2 classes: NcmpConfiguration and DmiRestClient.

The first class configures the client while the second performs requests with RestClient.

These two classes have to be rewritten/refactored to use the WebClient.

Example configuration

Configuration Example

@Slf4j
@Configuration
@AllArgsConstructor
public class WebClientConfiguration {

    public static final int TIMEOUT = 2000;
    private final DmiConfiguration.SdncProperties sdncProperties;

    @Bean
    public WebClient webClient() {
        final var httpClient = HttpClient.create()
                .option(ChannelOption.CONNECT_TIMEOUT_MILLIS, TIMEOUT) // millis
                .doOnConnected(connection ->
                        connection
                                .addHandlerLast(new ReadTimeoutHandler(TIMEOUT, TimeUnit.MILLISECONDS)) // millis
                                .addHandlerLast(new WriteTimeoutHandler(TIMEOUT, TimeUnit.MILLISECONDS))); //millis

        return WebClient.builder()
                .baseUrl(sdncProperties.getBaseUrl())
                .defaultHeaders(header -> header.set(HttpHeaders.CONTENT_TYPE, MediaType.APPLICATION_JSON_VALUE))
                .defaultHeaders(header -> header.setBasicAuth(sdncProperties.getAuthUsername(), sdncProperties.getAuthPassword()))
                .clientConnector(new ReactorClientHttpConnector(httpClient))
                .filter(logRequest())
                .filter(logResponse())
                .build();
    }

    private ExchangeFilterFunction logRequest() {
        return (clientRequest, next) -> {
            log.info("Request: {} {}", clientRequest.method(), clientRequest.url());
            log.info("--- Http Headers: ---");
            clientRequest.headers().forEach(this::logHeader);
            log.info("--- Http Cookies: ---");
            clientRequest.cookies().forEach(this::logHeader);
            return next.exchange(clientRequest);
        };
    }

    private ExchangeFilterFunction logResponse() {
        return ExchangeFilterFunction.ofResponseProcessor(clientResponse -> {
            log.info("Response: {}", clientResponse.statusCode());
            clientResponse.headers().asHttpHeaders()
                    .forEach((name, values) -> values.forEach(value -> log.info("{}={}", name, value)));
            return Mono.just(clientResponse);
        });
    }

    private void logHeader(String name, List values) {
        values.forEach(value -> log.info("{}={}", name, value));
    }
}

Sync call example

Sync Call

String response = webClient
 .get()
 .uri(resourceUrl)
 .header(HttpHeaders.CONTENT_TYPE, MediaType.APPLICATION_JSON_VALUE)
 .retrieve()
 .onStatus(HttpStatus::is4xxClientError,
 error -> Mono.error(new RuntimeException("API not found")))
 .onStatus(HttpStatus::is5xxServerError,
 error -> Mono.error(new RuntimeException("Server is not responding")))
 .bodyToMono(String.class)
 .block();
return ResponseEntity.ok(response);

Async call example with subscription

declaration

public Mono<String> httpOperationWithJsonData(final HttpMethod httpMethod, 
												final String resourceUrl, 
												final String jsonData,
												final HttpHeaders httpHeaders) {

return webClient
 .get()
 .uri(resourceUrl)
 .header(HttpHeaders.CONTENT_TYPE, MediaType.APPLICATION_JSON_VALUE)
 .retrieve()
 .onStatus(HttpStatus::is4xxClientError,
 error -> Mono.error(new RuntimeException("API not found")))
 .onStatus(HttpStatus::is5xxServerError,
 error -> Mono.error(new RuntimeException("Server is not responding")))
 .bodyToMono(String.class);
}