[[java-rest-low-config]] == Common configuration As explained in <>, 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",subs="attributes,callouts,macros"] -------------------------------------------------- include-tagged::{doc-tests}/RestClientDocumentation.java[rest-client-config-timeouts] -------------------------------------------------- === 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",subs="attributes,callouts,macros"] -------------------------------------------------- include-tagged::{doc-tests}/RestClientDocumentation.java[rest-client-config-threads] -------------------------------------------------- === 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",subs="attributes,callouts,macros"] -------------------------------------------------- include-tagged::{doc-tests}/RestClientDocumentation.java[rest-client-config-basic-auth] -------------------------------------------------- Preemptive Authentication can be disabled, which means that every request will be sent without authorization headers to see if it is accepted and, upon receiving an 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",subs="attributes,callouts,macros"] -------------------------------------------------- include-tagged::{doc-tests}/RestClientDocumentation.java[rest-client-config-disable-preemptive-auth] -------------------------------------------------- <1> Disable preemptive authentication === 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",subs="attributes,callouts,macros"] -------------------------------------------------- include-tagged::{doc-tests}/RestClientDocumentation.java[rest-client-config-encrypted-communication] -------------------------------------------------- If no explicit configuration is provided, the http://docs.oracle.com/javase/7/docs/technotes/guides/security/jsse/JSSERefGuide.html#CustomizingStores[system default configuration] will be used. === Others For any other required configuration needed, the Apache HttpAsyncClient docs should be consulted: https://hc.apache.org/httpcomponents-asyncclient-4.1.x/ . NOTE: If your application runs under the security manager you might be subject to the JVM default policies of caching positive hostname resolutions indefinitely and negative hostname resolutions for ten seconds. If the resolved addresses of the hosts to which you are connecting the client to vary with time then you might want to modify the default JVM behavior. These can be modified by adding http://docs.oracle.com/javase/8/docs/technotes/guides/net/properties.html[`networkaddress.cache.ttl=`] and http://docs.oracle.com/javase/8/docs/technotes/guides/net/properties.html[`networkaddress.cache.negative.ttl=`] to your http://docs.oracle.com/javase/8/docs/technotes/guides/security/PolicyFiles.html[Java security policy]. === Node selector The client sends each request to one of the configured nodes in round-robin fashion. Nodes can optionally be filtered through a node selector that needs to be provided when initializing the client. This is useful when sniffing is enabled, in case only dedicated master nodes should be hit by HTTP requests. For each request the client will run the eventually configured node selector to filter the node candidates, then select the next one in the list out of the remaining ones. ["source","java",subs="attributes,callouts,macros"] -------------------------------------------------- include-tagged::{doc-tests}/RestClientDocumentation.java[rest-client-init-allocation-aware-selector] -------------------------------------------------- <1> Set an allocation aware node selector that allows to pick a node in the local rack if any available, otherwise go to any other node in any rack. It acts as a preference rather than a strict requirement, given that it goes to another rack if none of the local nodes are available, rather than returning no nodes in such case which would make the client forcibly revive a local node whenever none of the nodes from the preferred rack is available. WARNING: Node selectors that do not consistently select the same set of nodes will make round-robin behaviour unpredictable and possibly unfair. The preference example above is fine as it reasons about availability of nodes which already affects the predictability of round-robin. Node selection should not depend on other external factors or round-robin will not work properly.