Node class, Client interface and RestHighLevelClient

import org.elasticsearch.node.Node;
import org.elasticsearch.client.Client;

Settings settings = Settings.builder().loadFromPath(Paths.get("config/elasticsearch.yml").toAbsolutePath()).build();
Path path = Paths.get("config/elasticsearch.yml").toAbsolutePath();
Environment environment = new Environment(settings, path);
Node nodeTmp = new Node(environment);
clientTmp = nodeTmp.client();

How do I gracefully replace this with the new RestHighLevelClient class?
Is there a way to convert RestHighLevelClient into a Client object to store it in Map<String, Client> instance?

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Have a look at https://www.elastic.co/guide/en/elasticsearch/client/java-rest/current/java-rest-high-getting-started-initialization.html

Why would you do this? It's threadsafe so you need only one instance I believe.

This is because the application had a few types of Client for different purposes. For example, TransportClient (implements Client interface), and Client interface provided by Node.client() method. They are stored in a Map<String, Client> object.

It seems like RestHighLevelClient is not meant to be related to other old Client classes and interfaces at all.
Does it sound smarter to use RestHighLevelClient for all connections from 7.8 onward?

Yes. That's the way to go.