Akka（29）： Http：Server-Side-Api，Low-Level-Api

  Akka-http针对Connection的两头都提供了方便编程的Api，分别是Server-Side-Api和Client-Side-Api。通过这两个Api可以大大提高编程效率。当然，上期我们提到过，Http-Server是Akka-http的核心部分，所有系统集成功能都是在Server-Side实现的。Akka-http-Server-Side-Api可以说是最先进的Http-Server编程工具，支持：

Full support for HTTP persistent connections
Full support for HTTP pipelining
Full support for asynchronous HTTP streaming including “chunked” transfer encoding accessible through an idiomatic API
Optional SSL/TLS encryption
WebSocket support

Server-Side-Api又分两个层次：Low-level-Server-Side-Api和High-level-Server-Side-Api。Low-level-server-api支持HTTP/1.1Server所有功能，包括：

Connection management
Parsing and rendering of messages and headers
Timeout management (for requests and connections)
Response ordering (for transparent pipelining support)
其它Server功能如请求解析request routing，文件服务file serving，数据压缩compression等都放在了High-level-server-api里。Akka-http是基于Akka-stream编写的，所以我们需要从Akka-stream运算模式来理解Akka-http的类型表现形式。

一个Http-Server是绑定在一个Socket上来接收客户端上传的request进行相关的服务提供的。Server对Socket的绑定在Akka-http里的可以Stream形式来表现：

val serverSource: Source[Http.IncomingConnection, Future[Http.ServerBinding]] =
Http().bind(interface = "localhost", port = 8080)

Server-Side Socket绑定实际上就是一个Akka-stream-source：Source[IncomingConnection]：

/**
* Creates a [[akka.stream.scaladsl.Source]] of [[akka.http.scaladsl.Http.IncomingConnection]] instances which represents a prospective HTTP server binding
* on the given `endpoint`.
*
* If the given port is 0 the resulting source can be materialized several times. Each materialization will
* then be assigned a new local port by the operating system, which can then be retrieved by the materialized
* [[akka.http.scaladsl.Http.ServerBinding]].
*
* If the given port is non-zero subsequent materialization attempts of the produced source will immediately
* fail, unless the first materialization has already been unbound. Unbinding can be triggered via the materialized
* [[akka.http.scaladsl.Http.ServerBinding]].
*
* If an [[ConnectionContext]] is given it will be used for setting up TLS encryption on the binding.
* Otherwise the binding will be unencrypted.
*
* If no `port` is explicitly given (or the port value is negative) the protocol's default port will be used,
* which is 80 for HTTP and 443 for HTTPS.
*
* To configure additional settings for a server started using this method,
* use the `akka.http.server` config section or pass in a [[akka.http.scaladsl.settings.ServerSettings]] explicitly.
*/
def bind(interface: String, port: Int = DefaultPortForProtocol,
connectionContext: ConnectionContext = defaultServerHttpContext,
settings:          ServerSettings    = ServerSettings(system),
log:               LoggingAdapter    = system.log)(implicit fm: Materializer): Source[Http.IncomingConnection, Future[ServerBinding]] = {
val fullLayer = fuseServerBidiFlow(settings, connectionContext, log)

tcpBind(interface, choosePort(port, connectionContext), settings)
.map(incoming ⇒ {
val serverFlow = fullLayer.addAttributes(prepareAttributes(settings, incoming)) join incoming.flow
IncomingConnection(incoming.localAddress, incoming.remoteAddress, serverFlow)
})
.mapMaterializedValue(materializeTcpBind)
}

run这个Source[IncomingConnection]产生一串连接Connection：

/**
* Represents one accepted incoming HTTP connection.
*/
final case class IncomingConnection(
localAddress:  InetSocketAddress,
remoteAddress: InetSocketAddress,
flow:          Flow[HttpResponse, HttpRequest, NotUsed]) {

/**
* Handles the connection with the given flow, which is materialized exactly once
* and the respective materialization result returned.
*/
def handleWith[Mat](handler: Flow[HttpRequest, HttpResponse, Mat])(implicit fm: Materializer): Mat =
flow.joinMat(handler)(Keep.right).run()

/**
* Handles the connection with the given handler function.
*/
def handleWithSyncHandler(handler: HttpRequest ⇒ HttpResponse)(implicit fm: Materializer): Unit =
handleWith(Flow[HttpRequest].map(handler))

/**
* Handles the connection with the given handler function.
*/
def handleWithAsyncHandler(handler: HttpRequest ⇒ Future[HttpResponse], parallelism: Int = 1)(implicit fm: Materializer): Unit =
handleWith(Flow[HttpRequest].mapAsync(parallelism)(handler))
}

IncomingConnection类型提供了个handleWith这样的streaming函数进行request到response的转换。用户可以下面的方式提供自定义的转换方法：

调用handleWith传入Flow[HttpRequest,HttpResponse,_]，如：

def req2Resp: HttpRequest => HttpResponse = _ => HttpResponse(entity=
HttpEntity(ContentTypes.`text/html(UTF-8)`,"<h> Hello World! </h>"))
val flow = Flow.fromFunction(req2Resp)

提供HttpRequest=>HttpResponse函数传人handleWithSyncHandler：

def syncHandler: HttpRequest => HttpResponse = {
case HttpRequest(HttpMethods.GET,Uri.Path("/"),_headers,_entiy,_protocol) =>
HttpResponse(entity=
HttpEntity(ContentTypes.`text/html(UTF-8)`,"<h> Hello World! </h>"))

case req: HttpRequest =>
req.discardEntityBytes() // important to drain incoming HTTP Entity stream
HttpResponse(404, entity = "Unknown resource!")
}

提供HttpRequest=>Future[HttpResponse]函数传人handleWithASyncHandler：

def asyncHandler: HttpRequest => Future[HttpResponse] = {
case HttpRequest(HttpMethods.GET,Uri.Path("/"),_headers,_entiy,_protocol) => Future {
HttpResponse(entity=
HttpEntity(ContentTypes.`text/html(UTF-8)`,"<h> Hello World! </h>")) }

case req: HttpRequest => Future {
req.discardEntityBytes() // important to drain incoming HTTP Entity stream
HttpResponse(404, entity = "Unknown resource!")
}
}

run Source[IncomingConnection,Future[ServerBinding]]返回结果为Future[ServerBinding]：

val futBinding: Future[Http.ServerBinding] =
connSource.to { Sink.foreach{ connection =>
println(s"client address ${connection.remoteAddress}")
//      connection handleWith flow
//    connection handleWithSyncHandler syncHandler
connection handleWithAsyncHandler asyncHandler
}}.run()

我们可以通过ServerBinding来释放绑定的Socket：

/**
* Represents a prospective HTTP server binding.
*
* @param localAddress  The local address of the endpoint bound by the materialization of the `connections` [[akka.stream.scaladsl.Source]]
*
*/
final case class ServerBinding(localAddress: InetSocketAddress)(private val unbindAction: () ⇒ Future[Unit]) {

/**
* Asynchronously triggers the unbinding of the port that was bound by the materialization of the `connections`
* [[akka.stream.scaladsl.Source]]
*
* The produced [[scala.concurrent.Future]] is fulfilled when the unbinding has been completed.
*/
def unbind(): Future[Unit] = unbindAction()
}

我们可以调用这个unbind()：

futBinding.flatMap(_.unbind())

整个示范源代码如下：

import akka.actor._
import akka.stream._
import akka.stream.scaladsl._
import akka.http.scaladsl.Http
import akka.http.scaladsl.model._
import scala.concurrent._

object LowLevelServerApi extends App {
implicit val httpSys = ActorSystem("actorSystem")
implicit val httpMat = ActorMaterializer()
implicit val httpEc = httpSys.dispatcher

val (interface,port) = ("localhost",8088)
val connSource: Source[Http.IncomingConnection,Future[Http.ServerBinding]] =
Http().bind(interface,port)

def req2Resp: HttpRequest => HttpResponse = _ => HttpResponse(entity=
HttpEntity(ContentTypes.`text/html(UTF-8)`,"<h> Hello World! </h>"))
val flow = Flow.fromFunction(req2Resp)

def syncHandler: HttpRequest => HttpResponse = {
case HttpRequest(HttpMethods.GET,Uri.Path("/"),_headers,_entiy,_protocol) =>
HttpResponse(entity=
HttpEntity(ContentTypes.`text/html(UTF-8)`,"<h> Hello World! </h>"))

case req: HttpRequest =>
req.discardEntityBytes() // important to drain incoming HTTP Entity stream
HttpResponse(404, entity = "Unknown resource!")
}

def asyncHandler: HttpRequest => Future[HttpResponse] = {
case HttpRequest(HttpMethods.GET,Uri.Path("/"),_headers,_entiy,_protocol) => Future {
HttpResponse(entity=
HttpEntity(ContentTypes.`text/html(UTF-8)`,"<h> Hello World! </h>")) }

case req: HttpRequest => Future {
req.discardEntityBytes() // important to drain incoming HTTP Entity stream
HttpResponse(404, entity = "Unknown resource!")
}
}

val futBinding: Future[Http.ServerBinding] =
connSource.to { Sink.foreach{ connection =>
println(s"client address ${connection.remoteAddress}")
//      connection handleWith flow
//    connection handleWithSyncHandler syncHandler
connection handleWithAsyncHandler asyncHandler
}}.run()

println(s"Server running at $interface $port. Press any key to exit ...")

scala.io.StdIn.readLine()

futBinding.flatMap(_.unbind())
.onComplete(_ => httpSys.terminate())

}