In the Akka docs
<http://doc.akka.io/docs/akka/2.5/scala/stream/stream-io.html>, in the
section titled "Working with streaming IO", there is an example TCP server:
connections.runForeach { connection =>
// server logic, parses incoming commands
val commandParser = Flow[String].takeWhile(_ != "BYE").map(_ + "!")
import connection._
val welcomeMsg = s"Welcome to: $localAddress, you are: $remoteAddress!"
val welcome = Source.single(welcomeMsg)
val serverLogic = Flow[ByteString]
.via(Framing.delimiter(
ByteString("\n"),
maximumFrameLength = 256,
allowTruncation = true))
.map(_.utf8String)
.via(commandParser)
// merge in the initial banner after parser
.merge(welcome)
.map(_ + "\n")
.map(ByteString(_))
connection.handleWith(serverLogic)
}
and REPL client:
val connection = Tcp().outgoingConnection("127.0.0.1", 8888)
val replParser =
Flow[String].takeWhile(_ != "q")
.concat(Source.single("BYE"))
.map(elem => ByteString(s"$elem\n"))
val repl = Flow[ByteString]
.via(Framing.delimiter(
ByteString("\n"),
maximumFrameLength = 256,
allowTruncation = true))
.map(_.utf8String)
.map(text => println("Server: " + text))
.map(_ => readLine("> "))
.via(replParser)
connection.join(repl).run()
In my case, the server does not send the welcome message at the start,
which is needed to get the flow started.
I thought I could work around this in the client by inserting this as the
first line in the flow:
.merge(Source.single(ByteString("Welcome\r\n:")))
This sort of worked, but now it is not possible to have two clients running
at once, unless they each send a message to the server right away on
starting.
(Otherwise I get the error:
[INFO] [09/17/2017 00:25:07.490] [default-akka.actor.default-dispatcher-2] [
akka://default/system/IO-TCP/selectors/$a/0] Message [akka.io.Tcp$Register]
from Actor[akka://default/user/StreamSupervisor-0/$$a#-1654072721] to
Actor[akka://default/system/IO-TCP/selectors/$a/0#1152954885] was not
delivered. [1] dead letters encountered. This logging can be turned off or
adjusted with configuration settings 'akka.log-dead-letters' and
'akka.log-dead-letters-during-shutdown'.
[INFO] [09/17/2017 00:25:07.493] [default-akka.actor.default-dispatcher-2] [
akka://default/system/IO-TCP/selectors/$a/0] Message [akka.io.Tcp$Write]
from Actor[akka://default/user/StreamSupervisor-0/$$a#-1654072721] to
Actor[akka://default/system/IO-TCP/selectors/$a/0#1152954885] was not
delivered. [2] dead letters encountered. This logging can be turned off or
adjusted with configuration settings 'akka.log-dead-letters' and
'akka.log-dead-letters-during-shutdown'.
I assume this is because there was no initial message from the server to
the client, which causes the connection to be initialized.
What is the correct way to handle this situation?
I also noticed that when you type 'q' in the client REPL, the process does
not exit. What is missing there?
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