private void start() {
      restRequest.getMetricsTracker()
          .injectMetrics(frontendMetrics.putBlobMetricsGroup.getRestRequestMetrics(restRequest.isSslUsed(), false));
      // Start the callback chain by parsing blob info headers and performing request security processing.
      securityService.processRequest(restRequest, securityProcessRequestCallback());
    }
Start the chain by calling {@link SecurityService#processRequest}.
ambry-frontend/src/main/java/com/github/ambry/frontend/s3/S3MultipartUploadPartHandler.java