This selector returns connections in a round-robin fashion. Node #1 is selected on the first request, Node #2 on the second request, etc. This ensures an even load of traffic across your cluster. Round-robin’ing happens on a per-request basis (e.g. sequential requests go to different nodes).

The RoundRobinSelector is default, but if you wish to explicitily configure it you can do:

$client = ClientBuilder::create()
            ->setSelector('\Elasticsearch\ConnectionPool\Selectors\RoundRobinSelector')
            ->build();

Note that the implementation is specified via a namespace path to the class.