// - on success, start a background thread to maintain the connection, with exp. backoff.
func New(discoveryAddr string, opts *ADSConfig) (*ADSC, error) {
	if opts == nil {
		opts = &ADSConfig{}
	}
	opts.Config = setDefaultConfig(&opts.Config)
	opts.Address = discoveryAddr
	adsc := &ADSC{
		Updates:     make(chan string, 100),
		XDSUpdates:  make(chan *discovery.DiscoveryResponse, 100),
		VersionInfo: map[string]string{},

func NewDelta(discoveryAddr string, config *DeltaADSConfig, opts ...Option) *Client {
	if config == nil {
		config = &DeltaADSConfig{}
	}
	config.Address = discoveryAddr
	config.Config = setDefaultConfig(&config.Config)
	c := &Client{
		cfg:             config,
		handlers:        map[string]HandlerFunc{},
		tree:            map[resourceKey]resourceNode{},
		errChan:         make(chan error, 10),