@Override
  public void start() throws InstantiationException {
    if (responseHandler == null) {
      throw new InstantiationException("ResponseHandler is not set.");
    }
    serviceRunning = true;
  }
Determines the blob ID desired by the request.
@param restRequest a {@link RestRequest} that represents the request.
@return the blob ID desired by the request.
ambry-api/src/test/java/com/github/ambry/rest/MockRestRequestService.java