The Java REST client uses the same logging library that the Apache Async Http
Client uses: Apache Commons Logging,
which comes with support for a number of popular logging implementations. The
java packages to enable logging for are
org.elasticsearch.client for the
client itself and
org.elasticsearch.client.sniffer for the sniffer.
The request tracer logging can also be enabled to log every request and
corresponding response in curl format. That comes handy when debugging, for
instance in case a request needs to be manually executed to check whether it
still yields the same response as it did. Enable trace logging for the
package to have such log lines printed out. Do note that this type of logging is
expensive and should not be enabled at all times in production environments,
but rather temporarily used only when needed.