docs: public int reindex(String sourceIndex, String sourceType, String targetIndex) throws IOException {
          logger.debug("reindex [{}]/[{}] -> [{}]/[doc]", sourceIndex, sourceType, targetIndex);
  
          String reindexQuery = "{  \"source\": {\n" +
                  "    \"index\": \"" + sourceIndex + "\",\n" +
                  "    \"type\": \"" + sourceType + "\"\n" +
                  "  },\n" +
                  "  \"dest\": {\n" +
                  "    \"index\": \"" + targetIndex + "\",\n" +
                  "    \"type\": \"doc\"\n" +
                  "  }\n" +
                  "}\n";
  
          logger.trace("{}", reindexQuery);
  
          HttpEntity entity = EntityBuilder.create().setText(reindexQuery).setContentType(ContentType.APPLICATION_JSON).build();
          Response restResponse = lowLevelClient.performRequest("POST", "/_reindex", Collections.emptyMap(), entity);
          Map<String, Object> response = asMap(restResponse);
          logger.debug("reindex response: {}", response);
  
          return (response == null ? -1 : (int) response.get("total"));
      }