=== Common configuration

The `RestClientBuilder` supports providing both a `RequestConfigCallback` and
an `HttpClientConfigCallback` which allow for any customization that the Apache
Async Http Client exposes. Those callbacks make it possible to modify some
specific behaviour of the client without overriding every other default
configuration that the `RestClient` is initialized with. This section
describes some common scenarios that require additional configuration for the
low-level Java REST Client.

==== Timeouts

Configuring requests timeouts can be done by providing an instance of
`RequestConfigCallback` while building the `RestClient` through its builder.
The interface has one method that receives an instance of
https://hc.apache.org/httpcomponents-client-ga/httpclient/apidocs/org/apache/http/client/config/RequestConfig.Builder.html[`org.apache.http.client.config.RequestConfig.Builder`]
 as an argument and has the same return type. The request config builder can
be modified and then returned. In the following example we increase the
connect timeout (defaults to 1 second) and the socket timeout (defaults to 30
seconds). Also we adjust the max retry timeout accordingly (defaults to 30
seconds too).

[source,java]
--------------------------------------------------
RestClient restClient = RestClient.builder(new HttpHost("localhost", 9200))
        .setRequestConfigCallback(new RestClientBuilder.RequestConfigCallback() {
            @Override
            public RequestConfig.Builder customizeRequestConfig(RequestConfig.Builder requestConfigBuilder) {
                return requestConfigBuilder.setConnectTimeout(5000)
                        .setSocketTimeout(60000);
            }
        })
        .setMaxRetryTimeoutMillis(60000)
        .build();
--------------------------------------------------

==== Number of threads

The Apache Http Async Client starts by default one dispatcher thread, and a
number of worker threads used by the connection manager, as many as the number
of locally detected processors (depending on what
`Runtime.getRuntime().availableProcessors()` returns). The number of threads
can be modified as follows:

[source,java]
--------------------------------------------------
RestClient restClient = RestClient.builder(new HttpHost("localhost", 9200))
        .setHttpClientConfigCallback(new RestClientBuilder.HttpClientConfigCallback() {
            @Override
            public HttpAsyncClientBuilder customizeHttpClient(HttpAsyncClientBuilder httpClientBuilder) {
                return httpClientBuilder.setDefaultIOReactorConfig(
                        IOReactorConfig.custom().setIoThreadCount(1).build());
            }
        })
        .build();
--------------------------------------------------

==== Basic authentication

Configuring basic authentication can be done by providing an
`HttpClientConfigCallback` while building the `RestClient` through its builder.
The interface has one method that receives an instance of
https://hc.apache.org/httpcomponents-asyncclient-dev/httpasyncclient/apidocs/org/apache/http/impl/nio/client/HttpAsyncClientBuilder.html[`org.apache.http.impl.nio.client.HttpAsyncClientBuilder`]
 as an argument and has the same return type. The http client builder can be
modified and then returned. In the following example we set a default
credentials provider that requires basic authentication.

[source,java]
--------------------------------------------------
final CredentialsProvider credentialsProvider = new BasicCredentialsProvider();
credentialsProvider.setCredentials(AuthScope.ANY,
        new UsernamePasswordCredentials("user", "password"));

RestClient restClient = RestClient.builder(new HttpHost("localhost", 9200))
        .setHttpClientConfigCallback(new RestClientBuilder.HttpClientConfigCallback() {
            @Override
            public HttpAsyncClientBuilder customizeHttpClient(HttpAsyncClientBuilder httpClientBuilder) {
                return httpClientBuilder.setDefaultCredentialsProvider(credentialsProvider);
            }
        })
        .build();
--------------------------------------------------

You can disable Preemptive Authentication, which means that every request will be sent without
authorization headers to see if it is accepted and, upon receiving a HTTP 401 response, it will
resend the exact same request with the basic authentication header. If you wish to do this, then
you can do so by disabling it via the `HttpAsyncClientBuilder`:

[source,java]
--------------------------------------------------
final CredentialsProvider credentialsProvider = new BasicCredentialsProvider();
credentialsProvider.setCredentials(AuthScope.ANY,
        new UsernamePasswordCredentials("user", "password"));

RestClient restClient = RestClient.builder(new HttpHost("localhost", 9200))
        .setHttpClientConfigCallback(new RestClientBuilder.HttpClientConfigCallback() {
            @Override
            public HttpAsyncClientBuilder customizeHttpClient(HttpAsyncClientBuilder httpClientBuilder) {
                // disable preemptive authentication
                httpClientBuilder.disableAuthCaching();
                return httpClientBuilder.setDefaultCredentialsProvider(credentialsProvider);
            }
        })
        .build();
--------------------------------------------------

==== Encrypted communication

Encrypted communication can also be configured through the
`HttpClientConfigCallback`. The
https://hc.apache.org/httpcomponents-asyncclient-dev/httpasyncclient/apidocs/org/apache/http/impl/nio/client/HttpAsyncClientBuilder.html[`org.apache.http.impl.nio.client.HttpAsyncClientBuilder`]
 received as an argument exposes multiple methods to configure encrypted
 communication: `setSSLContext`, `setSSLSessionStrategy` and
 `setConnectionManager`, in order of precedence from the least important.
 The following is an example:

[source,java]
--------------------------------------------------
KeyStore keystore = KeyStore.getInstance("jks");
try (InputStream is = Files.newInputStream(keyStorePath)) {
    keystore.load(is, keyStorePass.toCharArray());
}
RestClient restClient = RestClient.builder(new HttpHost("localhost", 9200))
        .setHttpClientConfigCallback(new RestClientBuilder.HttpClientConfigCallback() {
            @Override
            public HttpAsyncClientBuilder customizeHttpClient(HttpAsyncClientBuilder httpClientBuilder) {
                return httpClientBuilder.setSSLContext(sslcontext);
            }
        })
        .build();
--------------------------------------------------

==== Others

For any other required configuration needed, the Apache HttpAsyncClient docs
should be consulted: https://hc.apache.org/httpcomponents-asyncclient-4.1.x/ .