private void start() {
      RestRequestMetrics requestMetrics =
          metrics.updateBlobTtlMetricsGroup.getRestRequestMetrics(restRequest.isSslUsed(), false);
      restRequest.getMetricsTracker().injectMetrics(requestMetrics);
      restRequest.setArg(RestUtils.InternalKeys.KEEP_ALIVE_ON_ERROR_HINT, true);
      securityService.processRequest(restRequest, securityProcessRequestCallback());
    }
Start the chain by calling {@link SecurityService#processRequest}.
ambry-frontend/src/main/java/com/github/ambry/frontend/TtlUpdateHandler.java