+sam #3925 Adding Supervision for Java with Lambda Support Activator template

akka-samples/akka-sample-supervision-java-lambda/COPYING

@@ -0,0 +1,121 @@
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akka-samples/akka-sample-supervision-java-lambda/LICENSE

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+Activator Template by Typesafe
+
+Licensed under Public Domain (CC0)
+
+To the extent possible under law, the person who associated CC0 with
+this Activator Tempate has waived all copyright and related or neighboring
+rights to this Activator Template.
+
+You should have received a copy of the CC0 legalcode along with this
+work.  If not, see <http://creativecommons.org/publicdomain/zero/1.0/>.

akka-samples/akka-sample-supervision-java-lambda/activator.properties

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+name=akka-supervision-java-lambda
+title=Akka Supervision in Java with Lambdas
+description=Illustrates supervision in Akka
+tags=akka,java,java8,sample

akka-samples/akka-sample-supervision-java-lambda/build.sbt

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+name := "akka-supervision-java-lambda"
+
+version := "1.0"
+
+scalaVersion := "2.10.3"
+
+javacOptions ++= Seq("-source", "1.8", "-target", "1.8", "-Xlint")
+
+testOptions += Tests.Argument(TestFrameworks.JUnit, "-v", "-a")
+
+libraryDependencies ++= Seq(
+  "com.typesafe.akka" %%      "akka-actor" % "2.3-SNAPSHOT",
+  "com.typesafe.akka" %%    "akka-testkit" % "2.3-SNAPSHOT" % "test",
+              "junit"  %           "junit" % "4.11"         % "test",
+       "com.novocode"  % "junit-interface" % "0.10"         % "test")

akka-samples/akka-sample-supervision-java-lambda/pom.xml

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+<project xmlns="http://maven.apache.org/POM/4.0.0"
+  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+  xsi:schemaLocation="http://maven.apache.org/POM/4.0.0
+                      http://maven.apache.org/xsd/maven-4.0.0.xsd">
+  <modelVersion>4.0.0</modelVersion>
+
+  <properties>
+    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
+  </properties>
+
+  <groupId>sample</groupId>
+  <artifactId>akka-supervision-java-lambda</artifactId>
+  <packaging>jar</packaging>
+  <version>1.0</version>
+
+  <dependencies>
+    <dependency>
+      <groupId>com.typesafe.akka</groupId>
+      <artifactId>akka-actor_2.10</artifactId>
+      <version>2.3-SNAPSHOT</version>
+    </dependency>
+    <dependency>
+      <groupId>com.typesafe.akka</groupId>
+      <artifactId>akka-testkit_2.10</artifactId>
+      <version>2.3-SNAPSHOT</version>
+    </dependency>
+    <dependency>
+      <groupId>junit</groupId>
+      <artifactId>junit</artifactId>
+      <version>4.11</version>
+      <scope>test</scope>
+    </dependency>
+  </dependencies>
+
+  <build>
+    <plugins>
+      <plugin>
+        <groupId>org.apache.maven.plugins</groupId>
+        <artifactId>maven-compiler-plugin</artifactId>
+        <version>3.1</version>
+        <configuration>
+          <source>1.8</source>
+          <target>1.8</target>
+          <fork>true</fork>
+          <compilerArgs>
+            <arg>-Xlint</arg>
+          </compilerArgs>
+        </configuration>
+      </plugin>
+    </plugins>
+  </build>
+
+</project>

akka-samples/akka-sample-supervision-java-lambda/project/build.properties

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+sbt.version=0.13.1

akka-samples/akka-sample-supervision-java-lambda/src/main/java/supervision/ArithmeticService.java

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+package supervision;
+
+import akka.actor.*;
+import akka.japi.pf.DeciderBuilder;
+import akka.japi.pf.ReceiveBuilder;
+import scala.PartialFunction;
+import scala.runtime.BoxedUnit;
+
+import java.util.HashMap;
+import java.util.Map;
+
+import static  supervision.FlakyExpressionCalculator.FlakinessException;
+import static  supervision.FlakyExpressionCalculator.Result;
+import static  supervision.FlakyExpressionCalculator.Position.Left;
+
+// A very simple service that accepts arithmetic expressions and tries to
+// evaluate them. Since the calculation is dangerous (at least for the sake
+// of this example) it is delegated to a worker actor of type
+// FlakyExpressionCalculator.
+class ArithmeticService extends AbstractLoggingActor {
+
+  // Map of workers to the original actors requesting the calculation
+  Map<ActorRef, ActorRef> pendingWorkers = new HashMap<>();
+
+  private SupervisorStrategy strategy = new OneForOneStrategy(false, DeciderBuilder.
+    match(FlakinessException.class, e -> {
+      log().warning("Evaluation of a top level expression failed, restarting.");
+      return SupervisorStrategy.restart();
+    }).
+    match(ArithmeticException.class, e -> {
+      log().error("Evaluation failed because of: {}", e.getMessage());
+      notifyConsumerFailure(sender(), e);
+      return SupervisorStrategy.stop();
+    }).
+    match(Throwable.class, e -> {
+      log().error("Unexpected failure: {}", e.getMessage());
+      notifyConsumerFailure(sender(), e);
+      return SupervisorStrategy.stop();
+    }).build());
+  @Override
+  public SupervisorStrategy supervisorStrategy() {
+    return strategy;
+  }
+
+  private void notifyConsumerFailure(ActorRef worker, Throwable failure) {
+    // Status.Failure is a message type provided by the Akka library. The
+    // reason why it is used is because it is recognized by the "ask" pattern
+    // and the Future returned by ask will fail with the provided exception.
+    ActorRef pending = pendingWorkers.get(worker);
+    if (pending != null) {
+      pending.tell(new Status.Failure(failure), self());
+      pendingWorkers.remove(worker);
+    }
+  }
+
+  private void notifyConsumerSuccess(ActorRef worker, Integer result) {
+    ActorRef pending = pendingWorkers.get(worker);
+    if (pending != null) {
+      pending.tell(result, self());
+      pendingWorkers.remove(worker);
+    }
+  }
+
+  @Override
+  public PartialFunction<Object, BoxedUnit> receive() {
+    return ReceiveBuilder.
+      match(Expression.class, expr -> {
+        // We delegate the dangerous task of calculation to a worker, passing the
+        // expression as a constructor argument to the actor.
+        ActorRef worker = context().actorOf(FlakyExpressionCalculator.props(expr, Left));
+        pendingWorkers.put(worker, sender());
+      }).
+      match(Result.class, r -> notifyConsumerSuccess(sender(), r.getValue())).build();
+  }
+}

akka-samples/akka-sample-supervision-java-lambda/src/main/java/supervision/Expression.java

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+package supervision;
+
+// Represents an arithmetic expression involving integer numbers
+public interface Expression {
+
+  public Expression getLeft();
+
+  public Expression getRight();
+
+  // Basic arithmetic operations that are supported by the ArithmeticService. Every
+  // operation except the constant value has a left and right side. For example
+  // the addition in (3 * 2) + (6 * 6) has the left side (3 * 2) and the right
+  // side (6 * 6).
+  public static abstract class AbstractExpression implements Expression {
+    private final Expression left;
+    private final Expression right;
+    private final String operator;
+
+    protected AbstractExpression(Expression left, Expression right, String operator) {
+      this.left = left;
+      this.right = right;
+      this.operator = operator;
+    }
+
+    public Expression getLeft() {
+      return left;
+    }
+
+    public Expression getRight() {
+      return right;
+    }
+
+    @Override
+    public boolean equals(Object o) {
+      if (this == o) return true;
+      if (!(o instanceof AbstractExpression)) return false;
+
+      AbstractExpression that = (AbstractExpression) o;
+
+      if (!left.equals(that.left)) return false;
+      if (!operator.equals(that.operator)) return false;
+      if (!right.equals(that.right)) return false;
+
+      return true;
+    }
+
+    @Override
+    public int hashCode() {
+      int result = left.hashCode();
+      result = 31 * result + right.hashCode();
+      result = 31 * result + operator.hashCode();
+      return result;
+    }
+
+    @Override
+    public String toString() {
+      return "(" + getLeft() + " " + operator + " " + getRight() + ")";
+    }
+  }
+
+  public static final class Add extends AbstractExpression {
+    public Add(Expression left, Expression right) {
+      super(left, right, "+");
+    }
+  }
+
+  public static final class Multiply extends AbstractExpression {
+    public Multiply(Expression left, Expression right) {
+      super(left, right, "*");
+    }
+  }
+
+  public static final class Divide extends AbstractExpression {
+    public Divide(Expression left, Expression right) {
+      super(left, right, "/");
+    }
+  }
+
+  public static final class Const implements Expression{
+    private final int value;
+
+    public Const(int value) {
+      this.value = value;
+    }
+
+    @Override
+    public Expression getLeft() {
+      return this;
+    }
+
+    @Override
+    public Expression getRight() {
+      return this;
+    }
+
+    public int getValue() {
+      return value;
+    }
+
+    @Override
+    public boolean equals(Object o) {
+      if (this == o) return true;
+      if (!(o instanceof Const)) return false;
+
+      Const aConst = (Const) o;
+
+      if (value != aConst.value) return false;
+
+      return true;
+    }
+
+    @Override
+    public int hashCode() {
+      return value;
+    }
+
+    @Override
+    public String toString() {
+      return String.valueOf(value);
+    }
+  }
+}

akka-samples/akka-sample-supervision-java-lambda/src/main/java/supervision/FlakyExpressionCalculator.java

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+package supervision;
+
+import akka.actor.*;
+import akka.japi.pf.DeciderBuilder;
+import akka.japi.pf.ReceiveBuilder;
+import scala.PartialFunction;
+import scala.runtime.BoxedUnit;
+
+import java.util.HashMap;
+import java.util.Map;
+import java.util.Set;
+import java.util.concurrent.ThreadLocalRandom;
+import java.util.stream.Collectors;
+import java.util.stream.Stream;
+
+import static supervision.Expression.*;
+import static supervision.FlakyExpressionCalculator.Position.*;
+
+public class FlakyExpressionCalculator extends AbstractLoggingActor {
+
+  public static Props props(Expression expr, Position position) {
+    return Props.create(FlakyExpressionCalculator.class, expr, position);
+  }
+
+  // Encodes the original position of a sub-expression in its parent expression
+  // Example: (4 / 2) has position Left in the original expression (4 / 2) * 3
+  public static enum Position {
+    Left, Right
+  }
+
+  public static class Result {
+    private final Expression originalExpression;
+    private final Integer value;
+    private final Position position;
+
+    public Result(Expression originalExpression, Integer value, Position position) {
+      this.originalExpression = originalExpression;
+      this.value = value;
+      this.position = position;
+    }
+
+    public Expression getOriginalExpression() {
+      return originalExpression;
+    }
+
+    public Integer getValue() {
+      return value;
+    }
+
+    public Position getPosition() {
+      return position;
+    }
+  }
+
+  public static class FlakinessException extends RuntimeException {
+    static final long serialVersionUID = 1;
+
+    public FlakinessException() {
+      super("Flakiness");
+    }
+  }
+
+  // This actor has the sole purpose of calculating a given expression and
+  // return the result to its parent. It takes an additional argument,
+  // myPosition, which is used to signal the parent which side of its
+  // expression has been calculated.
+  private final Expression expr;
+  private final Position myPosition;
+
+  public FlakyExpressionCalculator(Expression expr, Position myPosition) {
+    this.expr = expr;
+    this.myPosition = myPosition;
+  }
+
+  private Expression getExpr() {
+    return expr;
+  }
+
+  private SupervisorStrategy strategy = new OneForOneStrategy(false, DeciderBuilder.
+    match(FlakinessException.class, e -> {
+      log().warning("Evaluation of {} failed, restarting.", getExpr());
+      return SupervisorStrategy.restart();
+    }).
+    matchAny(e -> SupervisorStrategy.escalate()).build());
+  @Override
+  public SupervisorStrategy supervisorStrategy() {
+    return strategy;
+  }
+
+  // The value of these variables will be reinitialized after every restart.
+  // The only stable data the actor has during restarts is those embedded in
+  // the Props when it was created. In this case expr, and myPosition.
+  Map<Position, Integer> results  = new HashMap<>();
+  Set<Position> expected = Stream.of(Left, Right).collect(Collectors.toSet());
+
+  @Override
+  public void preStart() {
+    if (expr instanceof Const) {
+      Integer value = ((Const) expr).getValue();
+      context().parent().tell(new Result(expr, value, myPosition), self());
+      // Don't forget to stop the actor after it has nothing more to do
+      context().stop(self());
+    }
+    else {
+      context().actorOf(FlakyExpressionCalculator.props(expr.getLeft(), Left), "left");
+      context().actorOf(FlakyExpressionCalculator.props(expr.getRight(), Right), "right");
+    }
+  }
+
+  @Override
+  public PartialFunction<Object, BoxedUnit> receive() {
+    return ReceiveBuilder.
+      match(Result.class, r -> expected.contains(r.getPosition()), r -> {
+        expected.remove(r.getPosition());
+        results.put(r.getPosition(), r.getValue());
+        if (results.size() == 2) {
+          // Sometimes we fail to calculate
+          flakiness();
+          Integer result = evaluate(expr, results.get(Left),results.get(Right));
+          log().info("Evaluated expression {} to value {}", expr, result);
+          context().parent().tell(new Result(expr, result, myPosition), self());
+          // Don't forget to stop the actor after it has nothing more to do
+          context().stop(self());
+        }
+      }).match(Result.class, r -> {
+      throw new IllegalStateException("Expected results for positions " +
+        expected.stream().map(Object::toString).collect(Collectors.joining(", ")) +
+        " but got position " + r.getPosition());
+    }).build();
+  }
+
+  private Integer evaluate(Expression expr, Integer left, Integer right) {
+    if (expr instanceof Add) {
+      return left + right;
+    } else if( expr instanceof Multiply) {
+      return left * right;
+    } else if (expr instanceof Divide) {
+      return left / right;
+    } else {
+      throw new IllegalStateException("Unknown expression type " + expr.getClass());
+    }
+  }
+
+  private void flakiness() throws FlakinessException {
+    if (ThreadLocalRandom.current().nextDouble() < 0.2)
+      throw new FlakinessException();
+  }
+}

akka-samples/akka-sample-supervision-java-lambda/src/main/java/supervision/Main.java

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+package supervision;
+
+import akka.actor.ActorRef;
+import akka.actor.Props;
+import akka.actor.ActorSystem;
+import akka.util.Timeout;
+import scala.concurrent.Await;
+import scala.concurrent.duration.Duration;
+import scala.concurrent.duration.FiniteDuration;
+import java.util.concurrent.TimeUnit;
+
+import static supervision.Expression.*;
+import static akka.pattern.Patterns.ask;
+import static akka.japi.Util.classTag;
+
+public class Main {
+
+  public static void main(String[] args) throws Exception {
+    ActorSystem system = ActorSystem.create("calculator-system");
+    ActorRef calculatorService =
+      system.actorOf(Props.create(ArithmeticService.class), "arithmetic-service");
+
+    // (3 + 5) / (2 * (1 + 1))
+    Expression task = new Divide(
+      new Add(new Const(3), new Const(5)),
+      new Multiply(
+        new Const(2),
+        new Add(new Const(1), new Const(1))
+      )
+    );
+
+    FiniteDuration duration = Duration.create(1, TimeUnit.SECONDS);
+    Integer result = Await.result(ask(calculatorService, task, new Timeout(duration)).mapTo(classTag(Integer.class)), duration);
+    System.out.println("Got result: " + result);
+
+    system.shutdown();
+    system.awaitTermination();
+  }
+}

akka-samples/akka-sample-supervision-java-lambda/src/test/java/supervision/ArithmeticServiceTest.java

@@ -0,0 +1,97 @@
+package supervision;
+
+import akka.actor.ActorRef;
+import akka.actor.ActorSystem;
+import akka.actor.Props;
+import akka.actor.Status;
+import akka.testkit.JavaTestKit;
+import org.junit.AfterClass;
+import org.junit.BeforeClass;
+import org.junit.Test;
+
+import java.util.stream.IntStream;
+import static supervision.Expression.*;
+
+public class ArithmeticServiceTest {
+  static ActorSystem system;
+
+  @BeforeClass
+  public static void setup() {
+    system = ActorSystem.create("BuncherTest");
+  }
+
+  @AfterClass
+  public static void tearDown() {
+    JavaTestKit.shutdownActorSystem(system);
+    system = null;
+  }
+
+  @Test
+  public void TheArithmeticServiceShouldCalculateConstantExpressionsProperly(){
+    new JavaTestKit(system) {{
+      final ActorRef service =
+        system.actorOf(Props.create(ArithmeticService.class));
+      final ActorRef probe = getRef();
+      IntStream.range(-2, 3).forEach(x -> {
+        service.tell(new Const(x), probe);
+        expectMsgEquals(x);
+      });
+    }};
+  }
+
+  @Test
+  public void TheArithmeticServiceShouldCalculateAdditionProperly(){
+    new JavaTestKit(system) {{
+      final ActorRef service =
+        system.actorOf(Props.create(ArithmeticService.class));
+      final ActorRef probe = getRef();
+      IntStream.range(-2, 3).forEach(x ->
+        IntStream.range(-2, 3).forEach(y -> {
+          service.tell(new Add(new Const(x), new Const(y)), probe);
+          expectMsgEquals(x + y);
+        })
+      );
+    }};
+  }
+
+  @Test
+  public void TheArithmeticServiceShouldCalculateMultiplicationAndDivisionProperly(){
+    new JavaTestKit(system) {{
+      final ActorRef service =
+        system.actorOf(Props.create(ArithmeticService.class));
+      final ActorRef probe = getRef();
+      IntStream.range(-2, 3).forEach(x ->
+        IntStream.range(-2, 3).forEach(y -> {
+          service.tell(new Multiply(new Const(x), new Const(y)), probe);
+          expectMsgEquals(x * y);
+        })
+      );
+
+      // Skip zero in the second parameter
+      IntStream.range(-2, 3).forEach(x ->
+        IntStream.of(-2, -1, 1, 2).forEach(y -> {
+          service.tell(new Divide(new Const(x), new Const(y)), probe);
+          expectMsgEquals(x / y);
+        })
+      );
+    }};
+  }
+
+  @Test
+  public void TheArithmeticServiceShouldSurviveIllegalExpressions(){
+    new JavaTestKit(system) {{
+      final ActorRef service =
+        system.actorOf(Props.create(ArithmeticService.class));
+      final ActorRef probe = getRef();
+
+      service.tell(new Divide(new Const(1), new Const(0)), probe);
+      expectMsgClass(Status.Failure.class);
+
+      service.tell(new Add(null, new Const(0)), probe);
+      expectMsgClass(Status.Failure.class);
+
+      service.tell(new Add(new Const(1), new Const(0)), probe);
+      expectMsgEquals(1);
+    }};
+  }
+}

akka-samples/akka-sample-supervision-java-lambda/tutorial/index.html

@@ -0,0 +1,254 @@
+<html>
+<head>
+    <title>Actor Supervision Java with Lambda Support</title>
+</head>
+<body>
+<div>
+    <h2>Quick Overview</h2>
+
+    <p>Congratulations! You have just created your first fault-resilient Akka
+        application, nice job!</p>
+
+    <p>Let's start with an overview and discuss the problem we want
+        to solve. This tutorial application demonstrates the use of Akka
+        supervision hierarchies to implement reliable systems. This particular
+        example demonstrates a calculator service that calculates arithmetic
+        expressions. We will visit each of the components shortly, but you might
+        want to take a quick look at the components before we move on.</p>
+
+    <ul>
+        <li><a href="#code/src/main/java/supervision/Expression.java" class="shortcut">Expression.java</a>
+            contains our "domain model", a very simple representation of
+            arithmetic expressions
+        </li>
+        <li><a href="#code/src/main/java/supervision/ArithmeticService.java"
+               class="shortcut">ArithmeticService.java</a> is the entry point
+            for our calculation service
+        </li>
+        <li><a href="#code/src/main/java/supervision/FlakyExpressionCalculator.java"
+               class="shortcut">FlakyExpressionCalculator.java</a> is our
+            heavy-lifter, a worker actor that can evaluate an expression
+            concurrently
+        </li>
+        <li><a href="#code/src/main/java/supervision/Main.java" class="shortcut">Main.java</a>
+            example code that starts up the calculator service and sends a few
+            jobs to it
+        </li>
+    </ul>
+</div>
+<div>
+    <h2>The Expression Model</h2>
+
+    <p>Our service deals with arithmetic expressions on integers involving
+        addition, multiplication and (integer) division. In
+        <a href="#code/src/main/java/supervision/Expression.java" class="shortcut">Expression.java</a>
+        you can see a very simple model of these kind of expressions.</p>
+
+    <p>Any arithmetic expression is a descendant of <code>Expression</code>, and
+        have a left and right side (<code>Const</code> is the only exception)
+        which is also an <code>Expression</code>.</p>
+
+    <p>For example, the expression (3 + 5) / (2 * (1 + 1)) could be constructed
+        as:</p>
+
+
+<code><pre>
+new Divide(
+  new Add(
+    new Const(3),
+    new Const(5)
+  ),        // (3 + 5)
+  new Multiply(
+    new Const(2),
+    new Add(
+      new Const(1),
+      new Const(1)
+    )       // (1 + 1)
+  )         // (2 * (1 + 1))
+);          // (3 + 5) / (2 * (1 + 1))
+</pre></code>
+
+    <p>Apart from the encoding of an expression and some pretty printing, our
+        model does not provide other services, so lets move on, and see how we
+        can calculate the result of such expressions.</p>
+</div>
+<div>
+    <h2>Arithmetic Service</h2>
+
+    <p>Our entry point is the <a
+            href="#code/src/main/java/supervision/ArithmeticService.java"
+            class="shortcut">ArithmeticService</a> actor that accepts arithmetic
+        expressions, calculates them and returns the result to the original
+        sender of the <code>Expression</code>.This
+        logic is implemented in the <code>receive</code> block. The actor
+        handles <code>Expression</code> messages and starts a worker for them,
+        carefully recording which worker belongs to which requester in the
+        <code>pendingWorkers</code> map.
+    </p>
+
+    <p>Who calculates
+        the expression? As you see, on the reception of an
+        <code>Expression</code> message we create a <code>FlakyExpressionCalculator</code>
+        actor and pass the expression as a parameter to its <code>Props</code>.
+        What happens here is that we delegate the calculation work to a worker
+        actor because the work can be "dangerous". After the worker
+        finishes its job, it replies to its parent (in this
+        case <code>ArithmeticService</code>) with a <code>Result</code>
+        message. At this point the top level service actor looks up which
+        actor it needs to send the final result to, and forwards it the value
+        of the computation.</p>
+</div>
+
+<div>
+    <h2>The Dangers of Arithmetic</h2>
+
+    <p>At first, it might feel strange that we don't calculate the result
+        directly but we delegate it to a new actor. The reason for that, is that
+        we want to treat the calculation as a dangerous task and isolate its
+        execution in a different actor to keep the top level service safe.</p>
+
+    <p>In our example we will see two kinds of failures</p>
+    <ul>
+        <li><code>FlakinessException</code> is a dummy exception that we throw
+            randomly to simulate transient failures. We will assume that
+            flakiness is temporary, and retrying the calculation is enough to
+            eventually get rid of the failure.
+        </li>
+        <li>Fatal failures, like <code>ArithmeticException</code> that will not
+            go away no matter how many times we retry the task. Division by zero
+            is a good example, since it indicates that the expression is
+            invalid, and no amount of attempts to calculate it again will fix
+            it.
+        </li>
+    </ul>
+
+    <p>To handle these kind of failure modes differently we customized the
+        supervisor strategy of <a
+                href="#code/src/main/java/supervision/ArithmeticService.java"
+                class="shortcut">ArithmeticService</a>. Our strategy here
+        is to restart the child when a recoverable error is detected (in our
+        case the dummy <code>FlakinessException</code>), but when arithmetic
+        errors happen &mdash; like division by zero &mdash; we have no hope to recover
+        and therefore we stop the worker. In addition,
+        we have to notify the original requester of the calculation job
+        about the failure.</p>
+
+    <p>We used <code>OneForOneStrategy</code>, since we only want to act on the
+        failing child, not on all of our children at the same time.</p>
+
+    <p>We set <code>loggingEnabled</code> to false, since we wanted to use our
+        custom logging instead of the built-in reporting.</p>
+
+
+
+</div>
+
+<div>
+
+    <h2>The Joy of Calculation</h2>
+
+    <p>We have now seen our <code>Expression</code> model, our fault modes
+        and how we deal with them at the top level, delegating the dangerous
+        work to child workers to isolate the failure, and setting
+        <code>Stop</code> or <code>Restart</code> directives depending on the
+        nature of the failure (fatal or transient). Now it's time to
+        calculate and visit <a href="#code/src/main/java/supervision/FlakyExpressionCalculator.java"
+                               class="shortcut">FlakyExpressionCalculator.java</a>!
+    </p>
+
+    <p>Let's review first our evaluation strategy. When we are facing an
+        expression like ((4 * 4) / (3 + 1)) we might be tempted to calculate (4
+        * 4) first, then (3 + 1), and then the final division. We can do better:
+        Let's calculate the two sides of the division in parallel!</p>
+
+    <p>To achieve this, our worker delegates the calculation of the left and
+        right side of the expression it has been given to two child workers of
+        the same type (except in the case of constant, where it just sends its
+        value as <code>Result</code> to its parent.
+        This logic is in <code>preStart()</code>
+        since this is the code that will be executed when an actor starts (and
+        during restarts if the <code>postRestart()</code> is not
+        overridden).</p>
+
+    <p>Since any of the sides of the original expression can finish before the
+        other, we have to indicate somehow which side has been calculated, that
+        is why we pass a <code>Position</code> as an argument to workers which
+        they will put in their <code>Result</code> which they send after the
+        calculation finished successfully.</p>
+
+</div>
+<div>
+
+
+    <h2>Failing Calculations</h2>
+
+    <p>As you might have observed, we added a method called
+        <code>flakiness()</code> that sometimes just misbehaves
+        (throws a <code>FlakinessException</code>).
+        This simulates a transient failure. Let's see how our
+        FlakyExpressionCalculator deals with failure situations.</p>
+
+    <p>A supervisor strategy is applied to the children of an actor. Since our
+        children are actually workers for calculating the left and right side of
+        our subexpression, we have to think what different failures mean for
+        us.</p>
+
+    <p>If we encounter a <code>FlakinessException</code> it indicates that one
+        of our workers
+        just made a hiccup and failed to calculate the answer. Since we know
+        this failure is recoverable, we just restart the responsible worker.</p>
+
+    <p>In case of fatal failures we cannot really do anything ourselves. First
+        of all, it indicates that the expression is invalid so restart does not
+        help, second, we are not necessarily the top level worker for the
+        expression. When an unknown failure is encountered it
+        is escalated to the parent. The parent of this actor is either another
+        <code>FlakyExpressionCalculator</code> or the
+        <code>ArithmeticService </code>. Since the calculators all escalate, no
+        matter how deep the failure happened, the <code>ArithmeticService</code>
+        will decide on the fate of the job (in our case, stop it).</p>
+</div>
+<div>
+    <h2>When to Split Work? A Small Detour.</h2>
+
+    <p>In our example we split expressions recursively and calculated the left
+       and right sides of each of the expressions. The question naturally
+       arises: do we gain anything here regarding performance?</p>
+
+    <p>In this example more probably not. There is an additional overhead of
+       splitting up tasks and collecting results, and this case the actual
+       subtasks consist of simple arithmetic operations which are very fast.
+       To really gain in performance in practice, the actual subtasks have to
+       be more heavyweight than this &mdash; but the pattern will be the
+       same.</p>
+
+</div>
+<div>
+    <h2>Where to go from here?</h2>
+
+    <p>After getting comfortable with the code, you can test your
+        understanding by trying to solve the following small exercises:</p>
+    <ul>
+        <li>Add <code>flakiness()</code> to various places in the calculator and
+            see what happens
+        </li>
+        <li>Try devising more calculation intensive nested jobs instead of
+            arithmetic expressions (for example transformations of a text
+            document) where parallelism improves performance
+        </li>
+    </ul>
+
+    <p>You should also visit</p>
+    <ul>
+        <li><a href="http://doc.akka.io/docs/akka/2.3-SNAPSHOT/java.html"
+               target="_blank">The Akka documentation</a></li>
+        <li><a
+               href="http://doc.akka.io/docs/akka/2.3-SNAPSHOT/java/lambda-fault-tolerance.html"
+               target="_blank">Documentation of supervision</a></li>
+        <li><a href="http://letitcrash.com" target="_blank">The Akka Team blog</a></li>
+    </ul>
+</div>
+
+
+</body>
+</html>