public boolean createIndex(String indexName,
                               Mappings mappings,
                               Map<String, ?> settings) {
        indexName = indexNameConverter.apply(indexName);

        return es.get().index().create(indexName, mappings, settings);
    }
ElasticSearchClient connects to the ES server by using ES client APIs.
oap-server/server-library/library-client/src/main/java/org/apache/skywalking/oap/server/library/client/elasticsearch/ElasticSearchClient.java