type Transport interface {
	// Append converts the span to the wire representation and adds it
	// to sender's internal buffer.  If the buffer exceeds its designated
	// size, the transport should call Flush() and return the number of spans
	// flushed, otherwise return 0. If error is returned, the returned number
	// of spans is treated as failed span, and reported to metrics accordingly.
	Append(span *zipkincore.Span) (int, error)

	// Flush submits the internal buffer to the remote server. It returns the
	// number of spans flushed. If error is returned, the returned number of
	// spans is treated as failed span, and reported to metrics accordingly.
	Flush() (int, error)

	io.Closer
}