type Operation uint8

const (
	// Add indicates a new address is added.
	Add Operation = iota
	// Delete indicates an exisiting address is deleted.
	Delete
)

type Resolver interface {
	// Resolve creates a Watcher for target.
	Resolve(target string) (Watcher, error)
}

type Update struct {
	// Op indicates the operation of the update.
	Op Operation
	// Addr is the updated address. It is empty string if there is no address update.
	Addr string
	// Metadata is the updated metadata. It is nil if there is no metadata update.
	// Metadata is not required for a custom naming implementation.
	Metadata interface{}
}

type Watcher interface {
	// Next blocks until an update or error happens. It may return one or more
	// updates. The first call should get the full set of the results. It should
	// return an error if and only if Watcher cannot recover.
	Next() ([]*Update, error)
	// Close closes the Watcher.
	Close()
}