private void checkAvailable() throws RestServiceException {
    if (!isUp) {
      throw new RestServiceException("FrontendRestRequestService unavailable", RestServiceErrorCode.ServiceUnavailable);
    }
  }
Checks if {@link FrontendRestRequestService} is available to serve requests.
@throws RestServiceException if {@link FrontendRestRequestService} is not available to serve requests.
ambry-frontend/src/main/java/com/github/ambry/frontend/FrontendRestRequestService.java