From a7c384b9456c6bfe89d8e9e0a23a2bb57ba2a0a2 Mon Sep 17 00:00:00 2001
From: Richard Imaoka <richard.e.imaoka@gmail.com>
Date: Tue, 28 Nov 2017 17:03:13 +0900
Subject: [PATCH] Add stream-substream.md  #23461

---
 .../stream-substream-flatMapConcat1.png       | Bin 0 -> 4906 bytes
 .../stream-substream-flatMapConcat2.png       | Bin 0 -> 5143 bytes
 .../images/stream-substream-flatMapMerge.png  | Bin 0 -> 6813 bytes
 .../images/stream-substream-groupBy1.png      | Bin 0 -> 5029 bytes
 .../images/stream-substream-groupBy2.png      | Bin 0 -> 6445 bytes
 .../images/stream-substream-groupBy3.png      | Bin 0 -> 8282 bytes
 .../images/stream-substream-groupBy4.png      | Bin 0 -> 7091 bytes
 .../stream-substream-splitWhen-splitAfter.png | Bin 0 -> 7379 bytes
 akka-docs/src/main/paradox/stream/index.md    |   3 +-
 .../main/paradox/stream/stages-overview.md    |   3 +
 .../main/paradox/stream/stream-substream.md   | 145 ++++++++++++++++++
 .../java/jdocs/stream/SubstreamDocTest.java   | 115 ++++++++++++++
 .../scala/docs/stream/SubstreamDocSpec.scala  |  79 ++++++++++
 13 files changed, 344 insertions(+), 1 deletion(-)
 create mode 100644 akka-docs/src/main/paradox/images/stream-substream-flatMapConcat1.png
 create mode 100644 akka-docs/src/main/paradox/images/stream-substream-flatMapConcat2.png
 create mode 100644 akka-docs/src/main/paradox/images/stream-substream-flatMapMerge.png
 create mode 100644 akka-docs/src/main/paradox/images/stream-substream-groupBy1.png
 create mode 100644 akka-docs/src/main/paradox/images/stream-substream-groupBy2.png
 create mode 100644 akka-docs/src/main/paradox/images/stream-substream-groupBy3.png
 create mode 100644 akka-docs/src/main/paradox/images/stream-substream-groupBy4.png
 create mode 100644 akka-docs/src/main/paradox/images/stream-substream-splitWhen-splitAfter.png
 create mode 100644 akka-docs/src/main/paradox/stream/stream-substream.md
 create mode 100644 akka-docs/src/test/java/jdocs/stream/SubstreamDocTest.java
 create mode 100644 akka-docs/src/test/scala/docs/stream/SubstreamDocSpec.scala

diff --git a/akka-docs/src/main/paradox/images/stream-substream-flatMapConcat1.png b/akka-docs/src/main/paradox/images/stream-substream-flatMapConcat1.png
new file mode 100644
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diff --git a/akka-docs/src/main/paradox/stream/index.md b/akka-docs/src/main/paradox/stream/index.md
index f1fa4daddd..93be480114 100644
--- a/akka-docs/src/main/paradox/stream/index.md
+++ b/akka-docs/src/main/paradox/stream/index.md
@@ -19,7 +19,8 @@
 * [stream-parallelism](stream-parallelism.md)
 * [stream-testkit](stream-testkit.md)
 * [stages-overview](stages-overview.md)
+* [stream-substream](stream-substream.md)
 * [stream-cookbook](stream-cookbook.md)
 * [../general/stream/stream-configuration](../general/stream/stream-configuration.md)
 
-@@@
\ No newline at end of file
+@@@
diff --git a/akka-docs/src/main/paradox/stream/stages-overview.md b/akka-docs/src/main/paradox/stream/stages-overview.md
index b320f9a838..c4917a8fb3 100644
--- a/akka-docs/src/main/paradox/stream/stages-overview.md
+++ b/akka-docs/src/main/paradox/stream/stages-overview.md
@@ -1375,9 +1375,12 @@ the flow with a `BufferOverflowException`.
 
 ## Nesting and flattening stages
 
+
 These stages either take a stream and turn it into a stream of streams (nesting) or they take a stream that contains
 nested streams and turn them into a stream of elements instead (flattening).
 
+See the [Substreams](stream-substream.md) page for more detail and code samples.
+
 ---------------------------------------------------------------
 
 ### prefixAndTail
diff --git a/akka-docs/src/main/paradox/stream/stream-substream.md b/akka-docs/src/main/paradox/stream/stream-substream.md
new file mode 100644
index 0000000000..fd84bb467c
--- /dev/null
+++ b/akka-docs/src/main/paradox/stream/stream-substream.md
@@ -0,0 +1,145 @@
+# Substreams
+
+Substreams are represented as `SubSource` or `SubFlow` instances, on which you can multiplex a single `Source` or `Flow`
+into a stream of streams.
+
+SubFlows cannot contribute to the super-flow’s materialized value since they are materialized later,
+during the runtime of the flow graph processing.
+
+Stages that create substreams are listed on @ref:[Nesting and flattening stages](stages-overview.md#nesting-and-flattening-stages)
+
+## Nesting stages
+
+### groupBy
+
+A typical operation that generates substreams is `groupBy`.
+
+Scala
+:   @@snip [SubstreamDocSpec.scala]($code$/scala/docs/stream/SubstreamDocSpec.scala) { #groupBy1 }
+
+Java
+:   @@snip [SubstreamDocTest.java]($code$/java/jdocs/stream/SubstreamDocTest.java) { #groupBy1 }
+
+![stream-substream-groupBy1.png](../../images/stream-substream-groupBy1.png)
+
+This operation splits the incoming stream into separate output
+streams, one for each element key. The key is computed for each element
+using the given function, which is `f` in the above diagram. When a new key is encountered for the first time
+a new substream is opened and subsequently fed with all elements belonging to that key.
+
+If you add a `Sink` or `Flow` right after the `groupBy` stage,
+all transformations are applied to all encountered substreams in the same fashion.
+So, if you add the following `Sink`, that is added to each of the substreams as in the below diagram.
+
+Scala
+:   @@snip [SubstreamDocSpec.scala]($code$/scala/docs/stream/SubstreamDocSpec.scala) { #groupBy2 }
+
+Java
+:   @@snip [SubstreamDocTest.java]($code$/java/jdocs/stream/SubstreamDocTest.java) { #groupBy2 }
+
+![stream-substream-groupBy2.png](../../images/stream-substream-groupBy2.png)
+
+Also substreams, more precisely, `SubFlow` and `SubSource` have methods that allow you to
+merge or concat substreams into the master stream again.
+
+The `mergeSubstreams` method merges an unbounded number of substreams back to the master stream.
+
+Scala
+:   @@snip [SubstreamDocSpec.scala]($code$/scala/docs/stream/SubstreamDocSpec.scala) { #groupBy3 }
+
+Java
+:   @@snip [SubstreamDocTest.java]($code$/java/jdocs/stream/SubstreamDocTest.java) { #groupBy3 }
+
+![stream-substream-groupBy3.png](../../images/stream-substream-groupBy3.png)
+
+You can limit the number of active substreams running and being merged at a time,
+with either the `mergeSubstreamsWithParallelism` or `concatSubstreams` method.
+
+Scala
+:   @@snip [SubstreamDocSpec.scala]($code$/scala/docs/stream/SubstreamDocSpec.scala) { #groupBy4 }
+
+Java
+:   @@snip [SubstreamDocTest.java]($code$/java/jdocs/stream/SubstreamDocTest.java) { #groupBy4 }
+
+However, since the number of running (i.e. not yet completed) substreams is capped,
+be careful so that these methods do not cause deadlocks with back pressure like in the below diagram.
+
+Element one and two leads to two created substreams, but since the number of substreams are capped to 2 
+when element 3 comes in it cannot lead to creation of a new substream until one of the previous two are completed 
+and this leads to the stream being deadlocked.
+
+![stream-substream-groupBy4.png](../../images/stream-substream-groupBy4.png)
+
+### splitWhen and splitAfter
+
+`splitWhen` and `splitAfter` are two other operations which generate substreams.
+
+The difference from `groupBy` is that, if the predicate for `splitWhen` and `splitAfter` returns true,
+a new substream is generated, and the succeeding elements after split will flow into the new substream.
+
+`splitWhen` flows the element on which the predicate returned true to a new substream,
+ whereas `splitAfter` flows the next element to the new substream after the element on which predicate returned true.
+
+Scala
+:   @@snip [SubstreamDocSpec.scala]($code$/scala/docs/stream/SubstreamDocSpec.scala) { #splitWhenAfter }
+
+Java
+:   @@snip [SubstreamDocTest.java]($code$/java/jdocs/stream/SubstreamDocTest.java) { #splitWhenAfter }
+
+These are useful when you scanned over something and you don't need to care about anything behind it.
+A typical example is counting the number of characters for each line like below.
+
+Scala
+:   @@snip [SubstreamDocSpec.scala]($code$/scala/docs/stream/SubstreamDocSpec.scala) { #wordCount }
+
+Java
+:   @@snip [SubstreamDocTest.java]($code$/java/jdocs/stream/SubstreamDocTest.java) { #wordCount }
+
+This prints out the following output.
+
+```
+23
+16
+26
+``` 
+
+![stream-substream-splitWhen-splitAfter.png](../../images/stream-substream-splitWhen-splitAfter.png)
+
+## Flattening stages
+
+### flatMapConcat
+
+`flatMapConcat` and `flatMapMerge` are substream operations different from `groupBy` and `splitWhen/After`.
+
+`flatMapConcat` takes a function, which is `f` in the following diagram.
+The function `f` of `flatMapConcat` transforms each input element into a `Source` that is then flattened
+into the output stream by concatenation.
+
+Scala
+:   @@snip [SubstreamDocSpec.scala]($code$/scala/docs/stream/SubstreamDocSpec.scala) { #flatMapConcat }
+
+Java
+:   @@snip [SubstreamDocTest.java]($code$/java/jdocs/stream/SubstreamDocTest.java) { #flatMapConcat }
+
+![stream-substream-flatMapConcat1.png](../../images/stream-substream-flatMapConcat1.png)
+
+Like the `concat` operation on `Flow`, it fully consumes one `Source` after the other.
+So, there is only one substream actively running at a given time.
+
+Then once the active substream is fully consumed, the next substream can start running.
+Elements from all the substreams are concatnated to the sink.
+
+![stream-substream-flatMapConcat2.png](../../images/stream-substream-flatMapConcat2.png)
+
+### flatMapMerge
+
+`flatMapMerge` is similar to `flatMapConcat`, but it doesn't wait for one `Source` to be fully consumed.
+ Instead, up to `breadth` number of streams emit elements at any given time.
+
+Scala
+:   @@snip [SubstreamDocSpec.scala]($code$/scala/docs/stream/SubstreamDocSpec.scala) { #flatMapMerge }
+
+Java
+:   @@snip [SubstreamDocTest.java]($code$/java/jdocs/stream/SubstreamDocTest.java) { #flatMapMerge }
+
+![stream-substream-flatMapMerge.png](../../images/stream-substream-flatMapMerge.png)
diff --git a/akka-docs/src/test/java/jdocs/stream/SubstreamDocTest.java b/akka-docs/src/test/java/jdocs/stream/SubstreamDocTest.java
new file mode 100644
index 0000000000..7e20e7ea16
--- /dev/null
+++ b/akka-docs/src/test/java/jdocs/stream/SubstreamDocTest.java
@@ -0,0 +1,115 @@
+/**
+ *  Copyright (C) 2015-2017 Lightbend Inc. <http://www.lightbend.com/>
+ */
+package jdocs.stream;
+
+import akka.actor.ActorSystem;
+import akka.stream.ActorMaterializer;
+import akka.stream.Materializer;
+import akka.stream.javadsl.Sink;
+import akka.stream.javadsl.Source;
+import akka.testkit.javadsl.TestKit;
+import jdocs.AbstractJavaTest;
+import org.junit.AfterClass;
+import org.junit.BeforeClass;
+import org.junit.Test;
+
+import java.util.Arrays;
+import java.util.Collections;
+import java.util.List;
+
+public class SubstreamDocTest  extends AbstractJavaTest {
+
+    static ActorSystem system;
+    static Materializer mat;
+
+    @BeforeClass
+    public static void setup() {
+        system = ActorSystem.create("FlowDocTest");
+        mat = ActorMaterializer.create(system);
+    }
+
+    @AfterClass
+    public static void tearDown() {
+        TestKit.shutdownActorSystem(system);
+        system = null;
+        mat = null;
+    }
+
+    @Test
+    public void demonstrateGroupBy() throws Exception {
+        //#groupBy1
+        Source.from(Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10))
+          .groupBy(3, elem -> elem % 3);
+        //#groupBy1
+
+        //#groupBy2
+        Source.from(Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10))
+          .groupBy(3, elem -> elem % 3)
+          .to(Sink.ignore())
+          .run(mat);
+        //#groupBy2
+
+        //#groupBy3
+        Source.from(Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10))
+          .groupBy(3, elem -> elem % 3)
+          .mergeSubstreams()
+          .runWith(Sink.ignore(), mat);
+        //#groupBy3
+
+        //#groupBy4
+        Source.from(Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10))
+          .groupBy(3, elem -> elem % 3)
+          .mergeSubstreamsWithParallelism(2)
+          .runWith(Sink.ignore(), mat);
+        //concatSubstreams is equivalent to mergeSubstreamsWithParallelism(1)
+        Source.from(Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10))
+          .groupBy(3, elem -> elem % 3)
+          .concatSubstreams()
+          .runWith(Sink.ignore(), mat);
+        //#groupBy4
+    }
+
+    @Test
+    public void demonstrateSplitWhenAfter() throws Exception {
+        //#splitWhenAfter
+        Source.from(Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10))
+          .splitWhen(elem -> elem == 3);
+
+        Source.from(Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10))
+          .splitAfter(elem -> elem == 3);
+        //#splitWhenAfter
+
+        //#wordCount
+        String text =
+          "This is the first line.\n" +
+          "The second line.\n" +
+          "There is also the 3rd line\n";
+
+        Source.from(Arrays.asList(text.split("")))
+          .map(x -> x.charAt(0))
+          .splitAfter(x -> x == '\n')
+          .filter(x -> x != '\n')
+          .map(x -> 1)
+          .reduce((x,y) -> x + y)
+          .to(Sink.foreach(x -> System.out.println(x)))
+          .run(mat);
+        //#wordCount
+        Thread.sleep(1000);
+    }
+
+    @Test
+    public void demonstrateflatMapConcatMerge() throws Exception {
+        //#flatMapConcat
+        Source.from(Arrays.asList(1, 2))
+          .flatMapConcat(i -> Source.from(Arrays.asList(i, i, i)))
+          .runWith(Sink.ignore(), mat);
+        //#flatMapConcat
+
+        //#flatMapMerge
+        Source.from(Arrays.asList(1, 2))
+          .flatMapMerge(2, i -> Source.from(Arrays.asList(i, i, i)))
+          .runWith(Sink.ignore(), mat);
+        //#flatMapMerge
+    }
+}
diff --git a/akka-docs/src/test/scala/docs/stream/SubstreamDocSpec.scala b/akka-docs/src/test/scala/docs/stream/SubstreamDocSpec.scala
new file mode 100644
index 0000000000..ec458e640f
--- /dev/null
+++ b/akka-docs/src/test/scala/docs/stream/SubstreamDocSpec.scala
@@ -0,0 +1,79 @@
+/**
+ * Copyright (C) 2015-2017 Lightbend Inc. <http://www.lightbend.com>
+ */
+package docs.stream
+
+import akka.stream.scaladsl.{ Sink, Source }
+import akka.stream.{ ActorMaterializer, SubstreamCancelStrategy }
+import akka.testkit.AkkaSpec
+
+class SubstreamDocSpec extends AkkaSpec {
+  implicit val materializer = ActorMaterializer()
+
+  "generate substreams by groupBy" in {
+    //#groupBy1
+    val source = Source(1 to 10).groupBy(3, _ % 3)
+    //#groupBy1
+
+    //#groupBy2
+    Source(1 to 10).groupBy(3, _ % 3).to(Sink.ignore).run()
+    //#groupBy2
+
+    //#groupBy3
+    Source(1 to 10)
+      .groupBy(3, _ % 3)
+      .mergeSubstreams
+      .runWith(Sink.ignore)
+    //#groupBy3
+
+    //#groupBy4
+    Source(1 to 10)
+      .groupBy(3, _ % 3)
+      .mergeSubstreamsWithParallelism(2)
+      .runWith(Sink.ignore)
+
+    //concatSubstreams is equivalent to mergeSubstreamsWithParallelism(1)
+    Source(1 to 10)
+      .groupBy(3, _ % 3)
+      .concatSubstreams
+      .runWith(Sink.ignore)
+    //#groupBy4
+  }
+
+  "generate substreams by splitWhen and splitAfter" in {
+    //#splitWhenAfter
+    Source(1 to 10).splitWhen(SubstreamCancelStrategy.drain)(_ == 3)
+
+    Source(1 to 10).splitAfter(SubstreamCancelStrategy.drain)(_ == 3)
+    //#splitWhenAfter
+
+    //#wordCount
+    val text =
+      "This is the first line.\n" +
+        "The second line.\n" +
+        "There is also the 3rd line\n"
+
+    val charCount = Source(text.toList)
+      .splitAfter { _ == '\n' }
+      .filter(_ != '\n')
+      .map(_ ⇒ 1)
+      .reduce(_ + _)
+      .to(Sink.foreach(println))
+      .run()
+    //#wordCount
+  }
+
+  "generate substreams by flatMapConcat and flatMapMerge" in {
+    //#flatMapConcat
+    Source(1 to 2)
+      .flatMapConcat(i ⇒ Source(List.fill(3)(i)))
+      .runWith(Sink.ignore)
+    //#flatMapConcat
+
+    //#flatMapMerge
+    Source(1 to 2)
+      .flatMapMerge(2, i ⇒ Source(List.fill(3)(i)))
+      .runWith(Sink.ignore)
+    //#flatMapMerge
+  }
+}