code: func (src DBSource) Response(req *ocsp.Request) ([]byte, http.Header, error) {
  	if req == nil {
  		return nil, nil, errors.New("called with nil request")
  	}
  
  	aki := hex.EncodeToString(req.IssuerKeyHash)
  	sn := req.SerialNumber
  
  	if sn == nil {
  		return nil, nil, errors.New("request contains no serial")
  	}
  	strSN := sn.String()
  
  	if src.Accessor == nil {
  		log.Errorf("No DB Accessor")
  		return nil, nil, errors.New("called with nil DB accessor")
  	}
  	records, err := src.Accessor.GetOCSP(strSN, aki)
  
  	// Response() logs when there are errors obtaining the OCSP response
  	// and returns nil, false.
  	if err != nil {
  		log.Errorf("Error obtaining OCSP response: %s", err)
  		return nil, nil, fmt.Errorf("failed to obtain OCSP response: %s", err)
  	}
  
  	if len(records) == 0 {
  		return nil, nil, ErrNotFound
  	}
  
  	// Response() finds the OCSPRecord with the expiration date furthest in the future.
  	cur := records[0]
  	for _, rec := range records {
  		if rec.Expiry.After(cur.Expiry) {
  			cur = rec
  		}
  	}
  	return []byte(cur.Body), nil, nil
  }
docs: // Response implements cfssl.ocsp.responder.Source, which returns the
  // OCSP response in the Database for the given request with the expiration
  // date furthest in the future.