Apart from completing requests with simple HttpResponse instances spray-routing also supports asynchronous response streaming. If you run spray-routing on top of the spray-can HTTP Server a response stream can be rendered as an HTTP/1.1 chunked response or, if chunkless-streaming is enabled, as a single response, whose entity body is sent in several parts, one by one, across the network.
When running spray-routing on top of spray-servlet the exact interpretation of the individual response chunks depends on the servlet container implementation (see the spray-servlet docs for more info on this).
A streaming response is started by sending a ChunkedResponseStart message to the responder of the RequestContext. Afterwards the responder is ready to receive a number of MessageChunk messages. A streaming response is terminated with a ChunkedMessageEnd message.
In order to not flood the network with chunks that it might not be able to currently digest it’s always a good idea to not send out another chunk before having received a “ACK” confirmation message from the underlying layer (see ACKed Sends in the spray-can documentation).
The Complete Examples both contain sample code, which shows how to send a streaming response that is “pulled” by the network via send confirmation messages.