private void start() {
      RestRequestMetrics requestMetrics =
          frontendMetrics.postDatasetsMetricsGroup.getRestRequestMetrics(restRequest.isSslUsed(), false);
      restRequest.getMetricsTracker().injectMetrics(requestMetrics);
      RetainingAsyncWritableChannel channel = new RetainingAsyncWritableChannel(frontendConfig.maxJsonRequestSizeBytes);
      restRequest.readInto(channel, fetchDatasetUpdateBodyCallback(channel));
    }
Start the chain by calling {@link SecurityService#processRequest}.
ambry-frontend/src/main/java/com/github/ambry/frontend/PostDatasetsHandler.java