96 lines
6.5 KiB
ReStructuredText
96 lines
6.5 KiB
ReStructuredText
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.. _stream-io-java:
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#########################
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Working with streaming IO
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#########################
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Akka Streams provides a way of handling TCP connections with Streams.
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While the general approach is very similar to the `Actor based TCP handling`_ using Akka IO,
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by using Akka Streams you are freed of having to manually react to back-pressure signals,
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as the library does it transparently for you.
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.. _Actor based TCP handling: http://doc.akka.io/docs/akka/current/java/io-tcp.html
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Accepting connections: Echo Server
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==================================
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In order to implement a simple EchoServer we ``bind`` to a given address, which returns a ``Source[IncomingConnection]``,
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which will emit an :class:`IncomingConnection` element for each new connection that the Server should handle:
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.. includecode:: ../../../akka-samples/akka-docs-java-lambda/src/test/java/docs/stream/StreamTcpDocTest.java#echo-server-simple-bind
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Next, we simply handle *each* incoming connection using a :class:`Flow` which will be used as the processing stage
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to handle and emit ByteStrings from and to the TCP Socket. Since one :class:`ByteString` does not have to necessarily
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correspond to exactly one line of text (the client might be sending the line in chunks) we use the ``parseLines``
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recipe from the :ref:`cookbook-parse-lines-java` Akka Streams Cookbook recipe to chunk the inputs up into actual lines of text.
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In this example we simply add exclamation marks to each incoming text message and push it through the flow:
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.. includecode:: ../../../akka-samples/akka-docs-java-lambda/src/test/java/docs/stream/StreamTcpDocTest.java#echo-server-simple-handle
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Notice that while most building blocks in Akka Streams are reusable and freely shareable, this is *not* the case for the
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incoming connection Flow, since it directly corresponds to an existing, already accepted connection its handling can
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only ever be materialized *once*.
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Closing connections is possible by cancelling the *incoming connection* :class:`Flow` from your server logic (e.g. by
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connecting its downstream to an :class:`CancelledSink` and its upstream to a *completed* :class:`Source`).
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It is also possible to shut down the servers socket by cancelling the ``connections:Source[IncomingConnection]``.
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We can then test the TCP server by sending data to the TCP Socket using ``netcat``:
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::
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$ echo -n "Hello World" | netcat 127.0.0.1 8889
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Hello World!!!
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Connecting: REPL Client
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=======================
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In this example we implement a rather naive Read Evaluate Print Loop client over TCP.
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Let's say we know a server has exposed a simple command line interface over TCP,
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and would like to interact with it using Akka Streams over TCP. To open an outgoing connection socket we use
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the ``outgoingConnection`` method:
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.. includecode:: ../../../akka-samples/akka-docs-java-lambda/src/test/java/docs/stream/StreamTcpDocTest.java#repl-client
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The ``repl`` flow we use to handle the server interaction first prints the servers response, then awaits on input from
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the command line (this blocking call is used here just for the sake of simplicity) and converts it to a
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:class:`ByteString` which is then sent over the wire to the server. Then we simply connect the TCP pipeline to this
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processing stage–at this point it will be materialized and start processing data once the server responds with
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an *initial message*.
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A resilient REPL client would be more sophisticated than this, for example it should split out the input reading into
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a separate mapAsync step and have a way to let the server write more data than one ByteString chunk at any given time,
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these improvements however are left as exercise for the reader.
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Avoiding deadlocks and liveness issues in back-pressured cycles
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===============================================================
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When writing such end-to-end back-pressured systems you may sometimes end up in a situation of a loop,
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in which *either side is waiting for the other one to start the conversation*. One does not need to look far
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to find examples of such back-pressure loops. In the two examples shown previously, we always assumed that the side we
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are connecting to would start the conversation, which effectively means both sides are back-pressured and can not get
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the conversation started. There are multiple ways of dealing with this which are explained in depth in :ref:`graph-cycles-java`,
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however in client-server scenarios it is often the simplest to make either side simply send an initial message.
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.. note::
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In case of back-pressured cycles (which can occur even between different systems) sometimes you have to decide
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which of the sides has start the conversation in order to kick it off. This can be often done by injecting an
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initial message from one of the sides–a conversation starter.
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To break this back-pressure cycle we need to inject some initial message, a "conversation starter".
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First, we need to decide which side of the connection should remain passive and which active.
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Thankfully in most situations finding the right spot to start the conversation is rather simple, as it often is inherent
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to the protocol we are trying to implement using Streams. In chat-like applications, which our examples resemble,
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it makes sense to make the Server initiate the conversation by emitting a "hello" message:
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.. includecode:: ../../../akka-samples/akka-docs-java-lambda/src/test/java/docs/stream/StreamTcpDocTest.java#welcome-banner-chat-server
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The way we constructed a :class:`Flow` using a :class:`PartialFlowGraph` is explained in detail in
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:ref:`constructing-sources-sinks-flows-from-partial-graphs-java`, however the basic concepts is rather simple–
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we can encapsulate arbitrarily complex logic within a :class:`Flow` as long as it exposes the same interface, which means
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exposing exactly one :class:`UndefinedSink` and exactly one :class:`UndefinedSource` which will be connected to the TCP
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pipeline. In this example we use a :class:`Concat` graph processing stage to inject the initial message, and then
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continue with handling all incoming data using the echo handler. You should use this pattern of encapsulating complex
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logic in Flows and attaching those to :class:`StreamIO` in order to implement your custom and possibly sophisticated TCP servers.
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In this example both client and server may need to close the stream based on a parsed command command - ``BYE`` in the case
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of the server, and ``q`` in the case of the client. This is implemented by using a custom :class:`PushStage`
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(see :ref:`stream-using-push-pull-stage-java`) which completes the stream once it encounters such command.
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