Flow.fromSinkAndSource
Creates a Flow
from a Sink
and a Source
where the Flow’s input will be sent to the Sink
and the Flow
’s output will come from the Source.
Flow operators composed of Sinks and Sources
Signature
Description
fromSinkAndSource
combines a separate Sink
and Source
into a Flow
.
Useful in many cases where an API requires a Flow
but you want to provide a Sink
and Source
whose flows of elements are decoupled.
Note that termination events, like completion and cancellation, are not automatically propagated through to the “other-side” of the such-composed Flow
. The Source
can complete and the sink will continue to accept elements.
Use fromSinkAndSourceCoupled if you want to couple termination of both of the ends.
Examples
One use case is constructing a TCP server where requests and responses do not map 1:1 (like it does in the Echo TCP server sample where every incoming test is echoed back) but allows separate flows of elements from the client to the server and from the server to the client.
This example cancel
s the incoming stream, not allowing the client to write more messages, switching the TCP connection to “half-closed”, but keeps streaming periodic output to the client:
- Scala
-
source
// close in immediately val sink = Sink.cancelled[ByteString] // periodic tick out val source = Source.tick(1.second, 1.second, "tick").map(_ => ByteString(System.currentTimeMillis().toString + "\n")) val serverFlow = Flow.fromSinkAndSource(sink, source) Tcp(system).bind("127.0.0.1", 9999, halfClose = true).runForeach { incomingConnection => incomingConnection.handleWith(serverFlow) }
- Java
With this server running you could use telnet 127.0.0.1 9999
to see a stream of timestamps being printed, one every second.
The following sample is a little bit more advanced and uses the MergeHub
to dynamically merge incoming messages to a single stream which is then fed into a BroadcastHub
which emits elements over a dynamic set of downstreams allowing us to create a simplistic little TCP chat server in which a text entered from one client is emitted to all connected clients.
- Scala
-
source
val (sink, source) = MergeHub.source[String].toMat(BroadcastHub.sink[String])(Keep.both).run() val framing = Framing.delimiter(ByteString("\n"), 1024) val sinkWithFraming = framing.map(bytes => bytes.utf8String).to(sink) val sourceWithFraming = source.map(text => ByteString(text + "\n")) val serverFlow = Flow.fromSinkAndSource(sinkWithFraming, sourceWithFraming) Tcp(system).bind("127.0.0.1", 9999).runForeach { incomingConnection => incomingConnection.handleWith(serverFlow) }
- Java
The same patterns can also be applied to Akka HTTP WebSockets which also have an API accepting a Flow
of messages.
If we would replace the fromSinkAndSource
here with fromSinkAndSourceCoupled
it would allow the client to close the connection by closing its outgoing stream.
fromSinkAndSource
can also be useful when testing a component that takes a Flow
allowing for complete separate control and assertion of incoming and outgoing elements using stream testkit test probes for sink and source:
- Scala
-
source
val inProbe = TestSubscriber.probe[String]() val outProbe = TestPublisher.probe[String]() val testFlow = Flow.fromSinkAndSource(Sink.fromSubscriber(inProbe), Source.fromPublisher(outProbe)) myApiThatTakesAFlow(testFlow) inProbe.expectNext("first") outProbe.expectRequest() outProbe.sendError(new RuntimeException("test error")) // ...
- Java
Reactive Streams semantics
emits when the Source
emits
backpressures when the Sink
backpressures
completes when the Source
has completed and the Sink
has cancelled.