Sorry, I was mistaken, it was actually a server which broke the connection 
after responding to the first request of the batch. Multiple requests to 
httpbin.org were pipelined just fine. But if 
akka.http.host-connection-pool.max-connections is greater than one, the 
default behaviour is to open multiple TCP connections instead of 
pipelining. Why is that? How do I do pipelining first and open new TCP 
connections in parallel only when other connections' pipelines are 
saturated?

On Tuesday, August 18, 2015 at 3:44:05 PM UTC+3, [email protected] wrote:
>
> Hi,
>
> I'm trying to get akka-http client to pipeline HTTP requests in one TCP 
> connection. I rewrote what was based on Http().singleRequest into this:
>
> (path("batch-geocode") & put & entity(as[Map[String, String]])) { br =>
>   val connectionPool = Http().cachedHostConnectionPool[String]("geocoder.ca")
>   val requests = br.mapValues(GeocodeRequest).toList.map(_.swap)
>   val ftrRs: Future[List[(String, StandardResponse)]] = Source(requests)
>     .via(connectionPool)
>     .mapAsync(4){ case (tryResponse, id) =>
>       Future.fromTry(tryResponse)
>         .flatMap(transformResponse).recover{
>           case t: Throwable => ErrorResponse(None, t.getMessage)
>         }
>         .map(id -> _)
>     }.runFold(List[(String, StandardResponse)]())((a,b) => b :: a)
>
>   onSuccess(ftrRs){ case rs =>
>     complete(OK -> rs.toMap)
>   }
> }
>
> (In case you were wondering, I have a JSON codec that knows how to work 
> with scala Maps.)
>
> my config has
>
> akka.http.host-connection-pool {
>   max-connections = 2
>   pipelining-limit = 4
> }
>
>
> Wireshark consistently shows two TCP connections established one by one 
> (initiating SYN packets are adjacent in the log). However, if I set 
> max-connections to 1, akka-http actually sends two HTTP requests at once, 
> but after the first response is received, it immediately sends FIN, ACK to 
> the server and then opens another TCP connection and sends the second 
> request again to receive a response. Am I missing something?
>

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