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