public Optional<Index> getIndex(String indexName) {
        if (StringUtil.isBlank(indexName)) {
            return Optional.empty();
        }
        indexName = indexNameConverter.apply(indexName);
        return es.get().index().get(indexName);
    }
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