RestClient advanced configuration

[yanivnahoum]

In Spring Boot 3.5.x, when using RestClient with Apache HttpClient5, there are typically some configurations required as the defaults are not production-ready:

1. Setting connect/read timeouts
2. Setting the connection pool size: max-per-route/max-total (defaults are only 5/25)

Now in case I have 2 (or more) http clients in my app, I can configure my timeouts globally, as well as my factory.

spring:
  http:
    client:
      factory: http-components
      connect-timeout: 5s
      read-timeout: 10s

In order to customize the Apache HttpClient5 connection pool, I can expose a ClientHttpRequestFactoryBuilderCustomizer bean, or a HttpComponentsClientHttpRequestFactoryBuilder bean:

@Bean
ClientHttpRequestFactoryBuilderCustomizer<HttpComponentsClientHttpRequestFactoryBuilder> customizer() {
    return builder -> builder.withConnectionManagerCustomizer(this::customizeConnectionPool);
}

private void customizeConnectionPool(PoolingHttpClientConnectionManagerBuilder pool) {
    pool.setMaxConnPerRoute(100);
    pool.setMaxConnTotal(400);
}

I would inject a RestClient.Builder into the constructor of each of my 2 client classes, and configure each one according to its special needs:

# In SomeClient1.java
private final RestClient restClient;

public SomeClient1(RestClient.Builder builder, SomeClient1Properties properties) {
    this.restClient = builder.baseUrl(properties.baseUrl()).build();
}

# In SomeClient2.java
private final RestClient restClient;

public SomeClient2(RestClient.Builder builder, SomeClient2Properties properties) {
    this.restClient = builder.baseUrl(properties.baseUrl()).build();
}

But now say SomeClient2 needs a different read timeout since the global default is too short for the specific service it's calling. I can adjust it as follows:

# In SomeClient2.java
private final RestClient restClient;

public SomeClient2(RestClient.Builder builder, SomeClient2Properties properties) {
    ClientHttpRequestFactorySettings settings = ClientHttpRequestFactorySettings.withReadTimeout(Duration.ofMinutes(1));
    ClientHttpRequestFactory requestFactory = ClientHttpRequestFactoryBuilder.httpComponents().build(settings);
    this.restClient = builder.baseUrl(properties.baseUrl())
            .requestFactory(requestFactory)
            .build();
}

But this change is not additive - it replaces the ClientHttpRequestFactory created by the auto-configured ClientHttpRequestFactoryBuilder that included my ClientHttpRequestFactoryBuilderCustomizer.

Finally the question: was the EAGER connection between the ClientHttpRequestFactorySettings and the ClientHttpRequestFactoryBuilder done intentionally?

Could the 2 concerns have been separated and used only in the RestClient.build() phase to allow overriding one but not the other?

private final RestClient restClient;

public SomeClient2(RestClient.Builder builder, SomeClient2Properties properties) {
    ClientHttpRequestFactorySettings settings = ClientHttpRequestFactorySettings.withReadTimeout(Duration.ofMinutes(1));
    // pseudo-code - override settings but use auto-configured ClientHttpRequestFactoryBuilder
    this.restClient = builder.baseUrl(properties.baseUrl())
            .requestSettings(settings)
            .build();
}

[philwebb]

Could the 2 concerns have been separated and used only in the RestClient.build() phase to allow overriding one but not the other?

It would be nice, but unfortunately it's not possible. The RestClient.Builder is part of Spring Framework but ClientHttpRequestFactorySettings is part of Spring Boot.

Would injecting the ClientHttpRequestFactoryBuilder work for you? That should apply the customizers. Something like:

public SomeClient2(RestClient.Builder builder, ClientHttpRequestFactoryBuilder factoryBuilder, ClientHttpRequestFactorySettings settings, SomeClient2Properties properties) {
    ClientHttpRequestFactory requestFactory = factoryBuilder.build(settings.withReadTimeout(Duration.ofMinutes(1)));
    this.restClient = builder.baseUrl(properties.baseUrl())
            .requestFactory(requestFactory)
            .build();
}

(warning, I've not actually tested that code)

[yanivnahoum]

The RestClient.Builder is part of Spring Framework but ClientHttpRequestFactorySettings is part of Spring Boot.

That's a fair point :-)
Thanks @philwebb, we'll go with injecting the ClientHttpRequestFactoryBuilder (and ClientHttpRequestFactorySettings if needed)