public Collection<String> retrievalIndexByAliases(String alias) {
        alias = indexNameConverter.apply(alias);

        return es.get().alias().indices(alias).keySet();
    }
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